WO2018071081A2 - Fuel-cooled neutron reflector - Google Patents

Abstract

A nuclear reactor is provided that uses a liquid fuel and includes a neutron reflector that reflects neutrons in towards a core of the reactor. The neutron reflector can include louvers that create channels or passages through the neutron reflector such that the liquid fuel tends to flow through neutron reflector and remove heat from the neutron reflector. The liquid fuel can carry heat away from the neutron reflector, through the core of the reactor, and onwards to an intended heat exchange mechanism of the reactor. The louvers and passages can help to channel the moving fluid throughout the volume of the reactor and to prevent stagnant eddies in which undisturbed pockets of fuel can otherwise turn into unwanted hot spots.

Description

FUEL-COOLED NEUTRON REFLECTOR

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No. 62/362,861, filed July 15, 2016, entitled "Fuel-Cooled Neutron Reflector," the entirety of which is incorporated by reference.

BACKGROUND

[0002] The demand for energy is largely fed by fossil fuels, typically by taking hydrocarbons out of the Earth and burning them. However, those hydrocarbons are in limited supply and their use produces carbon dioxide. According to the U.S. Environmental Protection Agency, more than 9 trillion metric tons of carbon is released into the atmosphere each year. Nuclear power is appealing due to possibilities of abundant fuel and carbon-neutral energy production.

[0003] The predominant design of commercial nuclear reactor for electricity production is the light water reactor (LWR). LWR's have significant drawbacks however. They use solid fuel with long radioactive half-lives and have relatively inefficient fuel utilization. As a result, LWR's can produce dangerous and long-lived waste products. The fuel can also be vulnerable to extreme accidents or conversion to nuclear weapons.

[0004] To improve on LWR technologies, molten salt reactors (MSRs) have been researched since the 1950s. MSRs are a class of nuclear fission reactors in which the primary coolant, or even the fuel itself, is a molten salt mixture such as fluoride or chloride salt. Compared to LWRs, MSRs can offer projected lower per-kilowatt hour (kWh) levelized cost,

comparatively benign fuel and waste inventory composition, highly efficient fuel utilization, and a combination of much higher accident resistance with a much lower worst-case accident severity.

[0005] Early development of MSRs primarily occurred in the 1950s and 1960s.

Development of MSRs since the 1970s has taken a back seat, while the U.S. and other nations focused on the development of LWRs up until recent years. As LWR maintenance and upgrade costs continue to rise, older LWRs continue to shut down. And, as the world seeks more environmentally friendly, carbon-free energy, there is a significantly renewed interest in MSRs given the advantages over LWRs.

SUMMARY

[0006] Molten salt reactors face technical challenges. One particular challenge is heat. The liquid fuel, by design, can generate intense heat. However, this heat can have adverse effects on parts of the reactor itself. Even though molten salt reactors can include a heat exchanger to transfer heat into a downstream system such as a steam turbine, parts of the reactor core can undergo material fatigue or decay due to their continual proximity to the liquid fuel, a potential source of extreme heat energy.

[0007] Liquid-cooled neutron reflectors can provide for reflection of neutrons within the core of a liquid- fueled reactor by use of an inner barrier that is cooled by circulating fuel and that directs fuel flow within the reactor vessel.

[0008] A liquid fuel nuclear reactor is provided and can include a neutron reflector that reflects neutrons in towards a core of the reactor. The neutron reflector can include louvers that create channels or passages through the neutron reflector such that the liquid fuel tends to flow through neutron reflector and remove heat from the neutron reflector. The liquid fuel can carry the heat away from the neutron reflector, through the core of the reactor, and onwards to the reactor's intended heat exchange mechanism. The louvers and passages can help to channel the moving fluid throughout the volume of the reactor and prevent any stagnant eddies in which undisturbed pockets of fuel can otherwise turn into unwanted hot spots. Excessive heat can be avoided, as having flow throughout the reactor can efficiently bring heat energy to an intended location such as the heat exchanger. Additionally, the passages through the neutron reflector can give the reflector a good surface-area to volume ratio and help to ensure that substantially no portion of reflector is too deep from a surface with cooling liquid fuel flowing against it to carry heat away. Thus the neutron reflector itself, being cooled by the liquid fuel, does not overheat.

[0009] Since excessive heating can challenge the integrity of reflector structures, the cooling effects of the disclosed embodiments can help to ensure long-lived reflectors and safe operation. Because the neutron reflector can reflect free neutrons to the reactor core, core chemistry and criticality can be maintained only in the desired locations. Thus a neutron reflector that includes louvers defining passages through the reflector can contribute to the efficient, safe, and long-lived operation of liquid fuel reactors such as molten salt reactors. In fact, such a neutron reflector can have particular applicability in reactors characterized by highly corrosive fuels or materials-taxing neutronic energy properties. Thus, embodiments of the fuel-cooled neutron reflectors disclosed herein can have particular benefits in the context of a fast-spectrum, chloride salt molten salt reactor.

[0010] In various embodiments, the disclosed reactors can include an approximately cylindrical reflector with a louver set including louvers or vanes of a neutron-reflective material separated by gaps or channels through which fuel flows. The cylindrical reflector can be set within an approximately cylindrical reactor vessel so as to leave a space for fuel flow between the outer surface of the louver and the inner surface of the vessel, as well as a central, unobstructed approximately cylindrical core within the cylindrical louver. The cylindrical louver can acts as a neutron reflector, primarily confining neutrons to the core. Hot fuel can ascend within the core, be extracted from the top of the reactor, be cooled by harvesting heat with a loop of secondary working fluid (e.g., water), and it can be reintroduced into the reactor vessel in a manner that causes it to flow downward in the space between the outer surface of the reflector and the inner surface of the reactor vessel. The down- flowing fuel is herein referred to a "downcomer." Similarly, up-flowing fuel is herein referred to as an "upcomer". Some fraction of the relatively cool downcomer fuel can flow inward between the louvers of the reflecting louver surrounding the core, cooling the louvers and entering the core volume. In a nether region of the vessel, that portion of the downcomer which not already been diverted into the core through the permeable reflector can be directed inward toward the axis of the core, where it can close the flow loop by joining the upward movement of hot fuel within the core. Other embodiments depart from some aspects of the geometry just described, as shall be shown and described below with reference to illustrative embodiments.

[0011] In certain aspects, embodiments of the disclosure provide a reactor system and it can include a reactor vessel and a neutron reflector. The neutron reflector can include a plurality of louvers defining a shell with passages therethrough. The neutron reflector can be disposed within and surround a reactor core of the reactor vessel.

[0012] In an embodiment, the plurality of louvres can consist essentially of a stainless steel. Examples can include austenitic stainless steels, such as CF8C-Plus or 316FR. [0013] Embodiments of the plurality of louvers can have a variety of configurations. In one aspect, the plurality of louvers can be connected by at least one connecting member. In another aspect, the at least one connecting member can define a substantially circular rail.

[0014] In an embodiment, the shell can substantially define a cylindrical shape.

[0015] In an embodiment, the reactor system can further include a molten fuel salt in the reactor vessel, where the fuel salt surrounds and fills at least a portion of the neutron reflector and the passages. The fuel salt can include a fast-spectrum molten chloride salt.

[0016] In an embodiment, the reactor system can include a pumping system configured to move the molten salt upwards from the core. When the pumping system is operating, the fuel salt can flow upwards through the core, downwards on the outside of the neutron reflector, and through the passages. Flowing of the molten salt through the passages can remove heat from the neutron reflector.

[0017] In another embodiment, a method of operating a reactor system is provided. The method can include pumping a fuel salt from the core of a reactor vessel and into a heat exchanger; flowing the fuel salt from the heat exchanger into an area between the outside of a neutron reflector and an inner wall of the reactor vessel; allowing the fuel salt to return from the area to the core via passages through the neutron reflector and through a lower portion of the reactor; and removing heat from the neutron reflector by means of the fuel salt flowing through the passages.

[0018] In an embodiment, the neutron reflector can include a plurality of louvers defining a shell with the passages therethrough. The neutron reflector can surround the core of the reactor vessel.

[0019] In an embodiment, the method can include adding heat to a secondary fluid by the heat exchanger and flowing the secondary fluid to a downstream process to transfer the added heat to the downstream process.

[0020] In an embodiment, the downstream process can include the operation of a turbine to generate electricity.

[0021] In an embodiment, the shell can substantially define a cylindrical shape. [0022] In an embodiment, the fuel salt can include a fast-spectrum molten chloride salt.

[0023] In another embodiment, an apparatus for use as a neutron reflector and it can include a plurality of louvers connected together by at least one connecting member to define a shell surrounding a core, wherein the louvers define openings through the shell.

[0024] In an embodiment, the at least one connecting member can include a substantially circular rail from which the plurality of louvers extend.

[0025] Embodiments of the plurality of louvers can have a variety of configurations. In one aspect, each of the louvers can be movably connected to the at least one connecting member. In another aspect, the plurality of louvers can be formed from steel. As an example, the plurality of louvers and the at least one connecting member can essentially of stainless steel.

[0026] In an embodiment, any straight line from the core, through the shell, to a space outside of the shell can extend through at least two of the louvers.

[0027] In an embodiment, the shell can be substantially cylindrical in shape.

[0028] In an embodiment, the surfaces of the louvers can be curved and they can have portions that are not parallel to, or normal to, an idealized cylinder concentric with the shell.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] These and other features will be more readily understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

[0030] FIG. 1 show a reactor system for the generation of energy.

[0031] FIG. 2 shows portions of a fuel-conditioning plant.

[0032] FIG. 3 is a cross-section through a reactor system with a sleeve-type neutron reflector. [0033] FIG. 4 is a downward-looking cross-section through a liquid-fuel reactor system. [0034] FIG. 5 is an angled cross-sectional view through a neutron reflector. [0035] FIG. 6 is a partial cross-sectional view of the neutron reflector. [0036] FIG. 7 is a cross-sectional view through a tapered-louver reflector. [0037] FIG. 8 is a cross section of the tapered-louver reflector.

[0038] FIG. 9 is a cross-sectional through a portion of a contoured-louver reflector.

[0039] FIG. 10 is an angled cross-sectional view of the contoured-louver reflector.

[0040] FIG. 11 is a cross-section of the contoured-louver reflector.

[0041] FIG. 12 diagrams a reactor system.

[0042] FIG. 13 shows the reactor system with additional aspects indicated for clarity.

[0043] FIG. 14 is a cross-sectional view through a spheroid reactor system.

[0044] FIG. 15 is a cutaway view through a neutron reflector with annular louvers.

[0045] FIG. 16 gives a cross-sectional view through the neutron reflector with annular louvers.

[0046] FIG. 17 shows a liquid-fuel reactor with a reflector with annular louvers.

[0047] FIG. 18 shows a portion of a singly-curved louver.

[0048] FIG. 19 shows a portion of a doubly non-uniform louver.

[0049] FIG. 20 is a cutaway view through a reactor system.

[0050] FIG. 21 diagrams steps of a method of operating a reactor system

[0051] It is noted that the drawings are not necessarily to scale. The drawings are intended to depict only typical aspects of the subject matter disclosed herein, and therefore should not be considered as limiting the scope of the disclosure.

DETAILED DESCRIPTION

[0052] One insight of the disclosure is that an MSR or of any other "fast-spectrum" fluid-fuel reactor (e.g., a reactor that is dependent primarily on fast neutrons for perpetuating criticality in the core, rather than on relatively slow, "thermal" neutrons) should mitigate the loss of fast neutrons from the core. Neutrons that exit the core without either causing fissions or transmuting fertile nuclides (e.g., uranium-238) to fissile nuclides (e.g., plutonium-239) can be both wasted with regard to the productive processes of the reactor (heat production and fuel breeding) and deleterious to surrounding structural materials. Reflecting neutrons back into the core is therefore desirable. A neutron reflector decreases the required critical mass of a body of fissile material, while also reducing neutron flux out of a core that can cause radiation damage to surrounding structures. This damage can otherwise shorten the lifetimes of such structures and/or mandate a greater bulk of shielding to mitigate such damage.

Therefore, both to achieve criticality within a lower mass liquid reactor core and to mitigate the deleterious effects of high energy neutron radiation from the core, in fast- spectrum liquid- fuel reactors the core can be surrounded by a reflector consisting primarily of some substance whose relatively heavy nuclei tend to backscatter neutrons with small loss of energy and low neutron absorption cross sections. Substances used for high energy neutron reflection include but are not limited to graphite, Magnesium Oxide, Zircaloy, steel, and lead.

[0053] A neutron reflector with louvers benefits heat flow within the reactor. Removal of heat from the core can, in general, be accomplished either by (1) confining the liquid fuel within the core (e.g., within rods) and circulating a coolant through the core or (2) circulating the liquid fuel itself outside the core, e.g., using the fuel simultaneously as a fuel and a heat- transport fluid. A louvered neutron reflector helps to circulate fuel without requiring provisions for managing multiple fluids in the core. Moreover, the louvered neutron reflector assures that stagnant regions or stagnancies (e.g., small vortices), which mix relatively slowly with the bulk of fuel in the reactor, do not form and persist long enough to be deleterious. Stagnancies tend to become hot relative to surrounding fuel because they do not mix efficiently with cooled fuel recirculated to the core. Stagnancies tend to occur in the vicinity of reactor components, and local excessive heating that can shorten the lifespans of such components or deform structures. In short, the thermal, nuclear, and fluidic behaviors of fuel in a liquid-fuel reactor can be mutually influential, and the thorough, continuous mixing of the fuel provided by the louvered neutron reflector can be desirable. In fuel-circulating reactors, there can be a need to control the overall circulation of fuel within the core, e.g., by arranging for down-flowing regions of relatively cool fuel and up-flowing regions of relatively hot fuel.

[0054] Moreover, incorporation of reflectors in liquid- fuel reactors can be complicated by an inherent trade-off: namely, a reflector must be proximate to the core to perform its primary function of increasing the neutron flux within the core, but radiation from the core heats the reflector material in a volumetric manner, so that the bulkier the reflector and the closer to the core it is, the hotter the reflector tends to get. Such heat must be removed via thermal & hydraulic design elements to maintain the integrity of reflector structures, constrain reflector geometries, and reflector lifespans. Also, radiation (e.g., neutrons) can cause swelling and weakening of solid reflector materials such as graphite and steel, due in part to accumulation of radiogenic hydrogen and helium in the materials.

[0055] The disclosed neutron reflection structures give designers control over patterns of circulation of fuel within the core, to prevent the occurrence of stagnancies and to guide downcoming and upflowing circulations. Additionally, the louvered neutron reflectors can be tolerant of radiogenic swelling and damage.

[0056] FIG. 1 shows a molten salt reactor system 100 configured for the generation of electrical energy from nuclear fission. The molten salt reactor system 100 can include a molten salt reactor core 102 containing the molten fuel salt 104 (e.g., a mixture of chloride and fluoride salts). The molten fuel salt 104 can include fissile materials, fertile materials, and combinations thereof. Examples of fissile materials can include, but are not limited to, thorium (Th), protactinium (Pa), uranium (U), neptunium (Np), plutonium (Pu), americium (Am), curium (Cm). In certain embodiments, the fissile materials can include one or more of the following isotopes, in any combination: Th-225, Th-227, Th-229, Pa-228, Pa-230, Pa- 232, U-231, U-233, U-235, Np-234, Np-236, Np-238, Pu-237, Pu-239, Pu-241, Am-240, Am-242, Am-244, Cm-243, Cm-245, and Cm-247). Examples of fertile materials can include, but are not limited to, ²³²ThCl₄, ²³⁸UCl₃ and ²³⁸UC1₄. In an embodiment, the molten fuel salt 104 can include a mixture of fissile materials including 233 235 233

UCI₃, UCI₃, UC1₄, ²³⁵UC1₄, and ²³⁹PuCl₃; and carrier salts including sodium chloride (NaCl), potassium chloride (KC1), and/or calcium chloride (CaC^).

[0057] Upon absorbing neutrons, nuclear fission can be initiated and sustained in the molten fuel salt 104 by chain-reaction in the fuel salt 104 within the core 102, generating heat that elevates the temperature of the molten fuel salt 104 (e.g., to about 650°C or about 1,200°F). The heated the molten fuel salt 104 can be transported from the molten salt reactor core 102 to a heat exchange unit 106. The heat exchange unit 106 can be configured to transfer the heat generated by the nuclear fission from the molten fuel salt 104. [0058] The heat exchange unit 106 can be provided in a variety of configurations. In various embodiments, the heat exchange unit 106 can be either internal or external to a reactor vessel 108 that contains the core 102. In additional embodiments, the system 100 can be configured such that first-stage heat exchange (e.g., heat exchange from the fuel salt 104 to a different fluid) can occur both internally and externally to the reactor vessel 108. In other

embodiments, the system 100 can be provided such that the functions of nuclear fission and first-stage heat exchange can be integral to the core 102. That is, heat exchange fluids can be passed through the reactor core 102. In the embodiment of the system 100 of FIG. 1, the heat exchange unit 106 is internal to the reactor vessel 108.

[0059] In general, fluids of three types can be contained in and/or circulated through the reactor vessel 108, namely fuel, coolant, and moderator (e.g., any substance that slows neutrons). Various fluids can perform one or more of the fuel, coolant, and moderator functions simultaneously. One or more fluids, including more than one fluid of each functional type, can be contained within or circulated through the core 102. Examples of fluids contained within or circulated through the core 102 in various embodiments can include, but are not limited to, liquid metals, molten salts, supercritical H₂O, supercritical CO₂, and supercritical N₂O.

[0060] The transfer of heat from the molten fuel salt 104 can be realized in various ways. For example, the heat exchange unit 106 can include a pipe 110, through which the heated molten fuel salt 104 travels, and a secondary fluid 112 (e.g., a coolant salt) that surrounds the pipe 110 and absorbs heat from the molten fuel salt 104. Upon heat transfer, the temperature of the molten fuel salt 104 can be reduced in the heat exchange unit 106 and the molten fuel salt 104 can be transported from the heat exchange unit 106 back to the molten salt reactor core 102.

[0061] The system 100 can also include a secondary heat exchange unit 114 configured to transfer heat from the secondary fluid 112 to a tertiary fluid 116 (e.g., water). As shown in FIG. 1, the secondary fluid 112 can be circulated through secondary heat exchange unit 114 via a pipe 118.

[0062] Additionally or alternatively, in another embodiment (not shown), heat exchange can occur within the core 102 prior to heat exchange within the secondary heat exchange unit 114. As an example, heat from the molten fuel salt 104 can pass to a solid moderator, then to a liquid coolant circulating through the core 102. Subsequently, the liquid coolant circulating through the core 102 can be transported to the secondary heat exchange unit 114. As required by basic thermodynamics, after one or more stages of exchange, heat can be finally delivered to an ultimate heat sink, e.g., the overall environment (not shown).

[0063] Heat received from the molten fuel salt 104 can be used to generate power (e.g., electric power) using any suitable technology. For example, when the tertiary fluid 116 in the secondary heat exchange unit 114 is water, it can be heated to a steam and transported to a turbine 120. The turbine 120 can be turned by the steam and drive an electrical generator 122 to produce electricity. Steam from the turbine 120 can be conditioned by an ancillary gear 124 (e.g., a compressor, a heat sink, a pre-cooler, and a recuperator) and it can be transported back to the secondary heat exchange unit 114.

[0064] Additionally, or alternatively, the heat received from the molten fuel salt 104 can be used in other applications such as nuclear propulsion (e.g., marine propulsion), desalination, domestic or industrial heating, hydrogen production, or combinations thereof.

[0065] During the operation of the molten salt reactor core 102, fission products can be generated in the molten fuel salt 104. The fission products can include a range of elements. The fission products can include, but are not limited to, rubidium (Rb), strontium (Sr), cesium (Cs), and barium (Ba), an element selected from lanthanides, palladium (Pd), ruthenium (Ru), silver (Ag), molybdenum (Mo), niobium (Nb), antimony (Sb), technetium (Tc), xenon (Xe), or krypton (Kr).

[0066] The buildup of fission products (e.g., radioactive noble metals and radioactive noble gases) in the molten fuel salt 104 can impede or interfere with the nuclear fission in the molten salt reactor core 102 by poisoning the nuclear fission. For example, xenon- 135 and samarium- 149 can have a high neutron absorption capacity, and can lower the reactivity of the molten salt. Fission products can also reduce the useful lifetime of the molten salt reactor core 102 by clogging components, such as heat exchangers or piping.

[0067] Therefore, it can be desirable to keep concentrations of fission products in the molten fuel salt 104 below certain thresholds to maintain proper functioning of the molten salt reactor core 102. This goal can be accomplished by a chemical processing plant 126 configured to remove at least a portion of fission products generated in the molten fuel salt 104 during nuclear fission. During operation, molten fuel salt 104 can be transported from the molten salt reactor core 102 to the chemical processing plant 126, which can process the molten fuel salt 104 so that the molten salt reactor core 102 functions without loss of efficiency or degradation of components. As shown in FIG. 1, the chemical processing plant 126 can be contained within the reactor vessel 108 along with the reactor core 102 and the heat exchange unit 106. However, in alternative embodiments, at least one of the heat exchange unit 106 and the fuel-conditioning plant can be external to the reactor vessel 108.

[0068] In certain embodiments, the system 100 can also include an actively cooled freeze plug 130. The freeze plug 130 can be in fluid communication with the molten salt reactor core 102 and it can be configured to allow the molten fuel salt 104 to flow into a set of emergency dump tanks 132 in case of power failure and/or on active command.

[0069] FIG. 2 illustrates additional detail of the chemical processing plant 126. During a typical state of reactor operation, the molten fuel salt 104 can be circulated continuously or near-continuously from the molten salt reactor core 102 through one or more of the functional sub-units of the chemical processing plant 126 (e.g., using a pump 202). As discussed below, examples of the sub-units can include, but are not limited to, a corrosion reduction unit 204, a froth floatation unit 206, and a salt exchange unit 208. In various embodiments,

[0070] In an embodiment, the corrosion reduction unit 204 can be configured to prevent or mitigate corrosion of the molten salt reactor core 102 by the molten fuel salt 104. At least a portion of the molten salt reactor core 102 can be constructed of metallic alloy including one or more of the following elements: iron (Fe), nickel (Ni), chromium (Cr), manganese (Mn), carbon (C), silicon (Si), niobium (Nb), titanium (Ti), vanadium (V), phosphorus (P), sulfur (S), molybdenum (Mo), nitrogen (N), any of the cermet alloys, or a variant thereof, stainless steels (austenitic stainless steel), or a variant thereof, zirconium alloy, or a variant thereof, or tungsten alloy, or a variant thereof. When the molten fuel salt 104 includes uranium tetrachloride (UC1₄), it can corrode portions of the system 100 by oxidizing chromium according to:

Cr→ Crⁱ⁺ + 3e^~

Cr³⁺ + 3UCl₄→ CrCl₃ + 3UCl₃ [0071] During reactor operation, the molten fuel salt 104 can be transported from the molten salt reactor core 102 to the corrosion reduction unit 204 and from the corrosion reduction unit 204 back to the molten salt reactor core 102. The transportation of the molten fuel salt 104 can be driven by the pump 202, which can be configured to adjust the rate of transportation. The corrosion reduction unit 204 can be configured to process the molten fuel salt 104 to maintain an oxidation reduction (redox) ratio, E(o)/E(r), of the molten fuel salt 104 in the molten salt reactor core 102 (and elsewhere throughout the system 100) at a substantially constant level, where E(o) is an element (E) at a higher oxidation state (o) and E(r) is that element (E) at a lower oxidation state (r).

[0072] In one embodiment, the element (E) can be an actinide (e.g., uranium, U), E(o) can be U(IV) and E(r) can be U(III). In this embodiment, U(IV) can be in the form of uranium tetrachloride (UC1₄), U(III) can be in the form of uranium trichloride (UCI₃), and the redox ratio can be a ratio E(o)/E(r) of UCI₄/UCI₃. Although UCI₄ can corrode the molten salt reactor core 102, the existence of UCI₄ can reduce the melting point of the molten fuel salt 104. Therefore, the level of the redox ratio, UCI₄/UCI₃, can be selected based on at least one of a desired corrosion reduction and a desired melting point of the molten fuel salt 104. For example, the redox ratio can be substantially constant and selected between about 1/50 to about 1/2000. More specifically, the redox ratio can be at a substantially constant level of about 1/2000.

[0073] The froth flotation unit 206 can be configured to remove at least part of insoluble fission products and/or dissolved gas fission products from the molten fuel salt 104.

Examples of insoluble fission products can include one or more of the following in any combination: krypton (Kr), xenon (Xe), palladium (Pd), ruthenium (Ru), silver (Ag), molybdenum (Mo), niobium (Nb), antimony (Sb), and technetium (Tc). Examples of gas fission products can include one or more of xenon (Xe) and krypton (Kr). As an example, the froth flotation unit 206 can generate froth from the molten fuel salt 104 that includes the insoluble fission products and/or the dissolved gas fission products. The dissolved gas fission products can be removed from the froth, and at least a portion of the insoluble fission products can be removed by filtration.

[0074] The salt exchange unit 208 can be configured to remove at least a portion of the soluble fission products dissolved in the molten fuel salt 104. Examples of the soluble fission products can include one or more of the following, in any combination: rubidium (Rb), strontium (Sr), cesium (Cs), barium (Ba), and lanthanides. The removal of soluble fission products can be realized through various mechanisms.

[0075] Comprehensive lists of fission products applicable to various embodiments to the present disclosure are provided below. A person skilled in the art will appreciate that these lists are illustrative and not meant to be exhaustive.

[0076] Fission products sufficiently noble to maintain a reduced and insoluble state in a fluid fuel of the present disclosure where the fluid fuel is a molten salt:

• Germanium - 72, 73, 74, 76

• Arsenic - 75

• Selenium - 77, 78, 79, 80, 82

• Yttrium - 89

• Zirconium - 90 to 96

• Niobium - 95

• Molybdenum - 95, 97, 98, 100

• Technetium - 99

• Ruthenium - 101 to 106

• Rhodium - 103

•Palladium - 105 to 110

• Silver - 109

• Cadmium - 111 to 116

• Indium - 115

•Tin - 117 to 126

• Antimony - 121, 123, 124, 125

•Tellurium - 125 to 132

[0077] Fission products that can form gaseous products at the typical operating temperature of embodiments the present disclosure:

• Bromine - 81

•Iodine - 127, 129, 131

•Xenon - 131 to 136

• Krypton - 83, 84, 85, 86 [0078] Fission products that can remain in the fluid fuel, where the fuel is a molten salt, as chloride compounds in addition to actinide chlorides (Th, Pa, U, Np, Pu, Am, Cm) and carrier salt chlorides (Na, K, Ca) in connection with the present disclosure:

• Rubidium - 85, 87

• Strontium - 88, 89, 90

•Cesium - 133, 134, 135, 137

•Barium - 138, 139, 140

• Lanthanides

o Lanthanum - 139

o Cerium - 140 to 144

o Praseodymium - 141, 143

o Neodymium - 142 to 146, 148, 150

o Promethium - 147

o Samarium - 149, 151, 152, 154

o Europium - 153, 154, 155, 156

o Gadolinium - 155 to 160

o Terbium - 159, 161

o Dysprosium - 161

[0079] Neutron reflection has been explored to decrease the reactor core mass needed for criticality. As an example, reactors have been developed with neutron reflecting features that employ circulation of multiple fluids (e.g., liquid fuel and liquid lead) to cool a reflector.

[0080] FIG. 3 illustrates a top-down cross-sectional view through a fluid-fuel reactor system 300 including fuel-cooled neutron reflectors according to some embodiments. The reactor system 300 can include a reactor vessel 302, a first or inner neutron reflector 304, and a second or outer neutron reflector 306. A lumen of the inner reflector 304 can be occupied by a liquid core 308. Fluid fuels (e.g., molten fuel salts), as well as other non-fuel liquids (e.g., coolants, moderators, etc.) can circulate through the liquid core 308 in various embodiments not depicted herein. In other embodiments, solid components, such as bodies of a moderator material (not shown), can be positioned within the volume of the liquid core 308.

[0081] As shown in FIG. 3, each of the reflectors 304, 306 can be sleeve-type reflectors, formed in a generally cylindrical shape and concentrically disposed within the reactor vessel 302. The reflector 304 can be positioned radially inward of the second neutron reflector 306. In certain embodiments, the inner and outer reflectors 304, 306 can be formed from an austenitic stainless steel (e.g., CF8C-Plus, 316FR, etc.). Steel can be a good reflector material for fast-spectrum reactors and can be solid at reactor operating temperatures (unlike lead).

[0082] During normal operation of the reactor system 300, the average temperature of the liquid core 308 can be relatively hot (e.g., approximately 650°C or approximately 1200°F). In general, the maximum temperature of the components of the reactor system 300 during operating conditions should be kept well below the thermal tolerance of their constituent materials. As discussed in detail below, the reflectors 304 and 306 of the reactor system 300, as well as the reflectors of various other embodiments, can be configured to contain cooling liquid flows within the reactor vessel 302.

[0083] In general, the maximum internal temperature of each reflector 304, or 306 can bea function of the rate of heat generation within the bulk of each reflector 304 or 306 and of the rate of heat removal from the surface of each reflector 304 or 306. Heat can be added or removed from the reflectors 304, 306 by radiative, conductive, and convective mechanisms.

[0084] Cooling of steel reflectors can be desirable. The inner reflector 304 can undergo heating at a higher rate than the outer reflector 306. In general, radiative heating of reflector material can be greater nearer the liquid core 308 because most radiation produced by the reactor system 300 originates at points within the liquid core 308 where most of the fissile material can be located and maintained in criticality. Furthermore, the inner surface of the inner reflector 304 can be in direct contact with the relatively hot liquid core 308. This combination of radiation and conduction can heat the inner reflector 304 at higher average rate than it heats the outer reflector 306.

[0085] Flows of liquid fuel can be employed to cool the reflectors 304, 306 by conduction. As shown in FIG. 3, a downcomer 310 of liquid fuel can be configured to flow between the outer reflector 306 and the reactor vessel 302, while an upcomer 316 can be configured to flow between the inner reflector 304 and the outer reflector 306. The downcomer 310 can flow downwards (into the page) as indicated by the circled-X symbol 312 and the upcomer 316 can flow upwards (out of the page) as indicated by the circled dot symbol 314. The temperature of each of the downcomer 310 and upcomer 316 can be lower than that of the liquid core 308 in order to cool the reflectors 304, 306 by conduction. The temperature of the downcomer 310 can be significantly lower than that of the liquid core 308 and the upcomer 316 to promote conductive cooling of the inner reflector 304. As an example, the

temperature of the downcomer 310 can be approximately 450°C or approximately 840°F. The temperature of the upcomer 316 can be intermediate to the liquid core 308 and the downcomer 310.

[0086] The balance between cooling and radiative heating of each of the inner and outer reflectors 304, 306 can also constrain their thickness. In general, with exposure a given neutron flux, a thicker reflector can generate more heat within its bulk, present a smaller area (per unit volume) from which heat can be removed, and achieve a higher equilibrium temperature in its interior. Because the inner reflector 394 can be exposed to a higher radiation flux than outer reflector 306, the inner reflector 304 can be thinner than the outer reflector 306.

[0087] However, the reactor system 300, as well as similar tubular-reflector embodiments (such as those with three or more concentric reflectors, and those with various dispositions of up and down fluid flow between the reflectors 304, 306), can have limitations. In one aspect, as discussed above, constraints on the design due to reflector material limitations can arise due to radiation flux and temperature of the liquid core 308. In another aspect, flows within the downcomer 310, the upcomer 316, and liquid core 308 can be essentially uncontrolled bulk flows, with no local or zonal determination of flow patterns. Stagnancies, striations, and other undesirable flow patterns can therefore difficult to predict and mitigate in such a design. That is, circulation of multiple fluids in and around a reactor core can be inherently more complex than circulation of fuel alone.

[0088] In a further aspect, the reflectors 304, 306 can contribute a positive term to the reactivity coefficient of the liquid core 308. If radiation from the liquid core 308 increases, radiogenic heating of the reflectors 304, 306 can increase, causing the reflectors 304, 306 to expand. This expansion can increase the volume of the liquid core 308, which in turn can increase the core reactivity and thus core radiation output. This cycle, whereby increased core radiation output tends to increase core radiation output, constitutes an undesirable positive feedback. [0089] To address these limitations, reflectors formed from graphite have been proposed for use. Graphite reflectors can dispense with the need for reflector cooling due to graphite's high thermal tolerance. However, graphite has drawbacks as a reflector. For example, graphite reflectors can exhibit one or more of poor breeder properties, limited geometry, and brittleness.

[0090] Embodiments of the present disclosure present reactor designs that include neutron reflectors formed from steel (e.g., formed essentially from stainless steel) but address the limitations of discussed above. As discussed in detail below, embodiments of the disclosed reactors can cool the reflectors with reactor fuel, rather than an additional fuel and can employ a single structure to perform both neutron reflection and manage hydrodynamics of the reactor core.

[0091] FIG. 4 is a top-down, cross-sectional view through an embodiment of a liquid-fuel reactor system 400. The reactor system 400 can include a reactor vessel 402, a louver neutron reflector 404, and a reactor core 406. In certain embodiments, as shown in FIG. 4, each can be formed in an approximately cylindrical shape. The cylindrical louver neutron reflector 404 can surround the reactor core 406, which can be configured to contain a critical mass of liquid fuel. A downcomer 408, composed of reactor fuel, can flow between the louver reflector 404 and the reactor vessel 402 and mix with the reactor core 406 in a nether region of the reactor vessel 402 (not depicted).

[0092] The louver neutron reflector 404 can include a plurality of essentially identical, vertically oriented louvers 410, e.g., slats or vanes. Channels 414 can be positioned between adjacent louvers 410 for fluid flow therebetween. As an example, in a closed (e.g., circular) arrangement of N louvers, there are N channels.

[0093] In certain embodiments, the louvers 410 can be formed in a curved shape. So configured, neutrons emanating from the reactor core 406 cannot pass through the louver reflector 404 without passing through the substance of at least one louver 410. This arrangement can resulting in greater efficiency of neutron reflection back to the reactor core 406 and a desirable reduction in spatial variation in reflectivity.

[0094] In contrast, if the louvers 410 are simply planar and radial, the channels can also be planar and radial. Thus, neutrons could pass out of the reactor core 406, through the channels, with a low probability reflection back into the core. That is, less efficient overall reflection of neutrons into the core. Such an arrangement could also result in a vertically barred pattern of irradiation of the downcomer 408 and structures beyond (e.g., the reactor vessel 402), with corresponding spatial variations in nuclear processes in the downcomer 408 and radiative wear on structures outside the core, which can be undesirable.

[0095] In horizontal cross-section, the louvers 410 of reactor system 400 can also be singly curved. As an example, the louvers 410 can have an uninflected radius of curvature. In other embodiments, the louver neutron reflector can include vertical louvers or vanes having surfaces that are singly curved and/or multiply curved. As an example, the louvers or vanes can be formed with radii of curvature having one or more inflections. All variations in cross- sectional curvature are contemplated and within the scope of the disclosure.

[0096] While the overall flow of fuel in the downcomer 408 can be downward, with or without a tangential component, some fuel can flow through each reflector channel and into the reactor core 406 provided that the pressure in the downcomer 408 exceeds the pressure in the reactor core 406. In general, such a pressure difference can be assured by the action of a pump in the loop that introduces downcomer 408 to an upper region of the reactor vessel 402. Flow through reflector channels, driven by downcomer/core pressure differences, can be radial and inward but can also tend to have a vertical component, as vertical flow can be permitted in the curviplanar channels. In general, the local specifics of flow patterns throughout an embodiment such as the reactor system 400, or in various other embodiments, can vary with the specifics of design geometry and state of system operation and can be predicted using numerical methods (e.g., finite-element analysis).

[0097] In the circumstance where the downcomer fuel is cooler than core fuel, inward (coreward) flow of downcomer 408through the channels of the louver reflector 404 can cool the louvers 410 of the louver reflector 404. In particular, radiogenic heat originating within each louver 410 can be removed by coreward flows of the relatively cool downcomer 408 along the lateral surfaces of each louver 410. Thus, the louver reflector 404, as well as the reflectors of various other embodiments, can be fuel-cooled.

[0098] FIG. 5 is an angled, cross-sectional view through the louver reflector 404. As shown, the louver reflector 404 can include three vertically stacked, louver tiers 416, 418, 420. In certain embodiments the louver tiers 416, 418, 420 can be approximately identical. As an example, each tier can be approximately 1 meter high and approximately 30 cm thick. In various other embodiments the number of tiers can be varied from 1 to any larger integer and the tiers need not be essentially identical. Advantages of employing tiered louvers, rather than a single tall louver, can include increased structural strength of shorter individual louvers, greater ease of manufacture of shorter individual louvers, and limitation in shorter louvers of the impacts of swelling and buckling.

[0099] The louver reflector 404 can also include three support rings 422, 424, 426. The supporting rings 422, 424, 426 can operate as connecting members to uphold the tiers 416, 418, 420. A fourth ring (not depicted) can overlie the topmost tier (e.g., tier 416) and impart additional structural stability. Each louver 410 can be held in position, or within a range of positions, by one or more support rings.

[0100] In certain embodiments, an outward edge of each louver 410 can be anchored by vertical hinge pins (e.g., one at the top and one at the bottom) set into knuckles in the adjacent support rings. So configured, the motions of the louver 410 can be hinged and limited by pins, ridges, or grooves in the surfaces of the adjacent support plates. Such an arrangement can accommodate louver swelling due to thermal and radiogenic expansion, while concurrently mitigating mechanical stress upon the louvers 410 and support rings 422,424, 426.

[0101] The configuration of the louver reflector 404 can also contribute to a negative reactivity coefficient for the reactor core 406, in contrast to the positive reactivity coefficient of the reactor system 300 of FIG. 3. That is, if radiation from the reactor core 406 increases, radiogenic heating of the louvers 410 increases, causing them to expand. If the position of the outer edge of each of the louvers 410 is constrained by pins or other restraint, and the remainder of each of the louvers 410 can be free to expand into the reactor core 406, expanding louvers 410 can displace fuel from the reactor core 406. This reduction in fuel can decrease reactivity of the reactor core 406 and thus core radiation output. As a result, a desirable negative feedback on reactivity can be achieved. Various other support arrangements are possible and contemplated, additionally or alternatively to the hinged arrangement just described.

[0102] Embodiments of the louver reflector 404 can include louver-support arrangements that provide one or more degrees of freedom to the louvers 410 in combination with driving mechanisms that enable angular and/or radial alteration of the position of the louvers 410. Changing the angles and/or radial positions of one or more of the louvers 410 can alter hydrodynamic, thermal, and neutronic characteristics of the reactor system 400. The relative volumes of downcomer 408 and the reactor core 406 can also be altered in this manner. As an example, the reactor core 406 can decrease in volume while the downcomer 408 increases in volume by the same amount.

[0103] Active reflector reconfiguration can be advantageous during various modes of operation, such as startup. Rapid, precise throttling of reactor power level can be supported by a reconfigurable reflector, which can alter at least one of: volume of the reactor core 406; flow patterns within the downcomer 408, the louver reflector 404, and the reactor core 406; and neutron reflection into the reactor core 406. Active reflector reconfiguration is contemplated for various embodiments that include but are not limited to a reflector such as the louver reflector 404.

[0104] In operation of the reactor system 400 including the louver reflector 404, the downcomer 408 can mix with fuel within the reactor core 406 in a nether region of the reactor system 400. This arrangement can enable substantially continuous upward flow of fuel within the reactor core 406, as indicated by arrow 430 in FIG. 5.

[0105] FIG. 6 is a partial cross-section view of the louver reflector 404. As shown, the louvers 410 can be approximately uniform in overall cross-section. With reference to the horizontal component of fuel flow through channels of the louver reflector 404 (e.g., flow parallel to the plane of the cross-sectional drawing), arrows 502 and 504 indicate that the direction of flow through a channel 414 can be constrained by its local orientation. Thus, upon entry into a channel 414 from the downcomer 408, fuel flows in the direction indicated by arrow 502 (e.g., obliquely to the outer boundary of the louver reflector 404). Upon exit from a channel 414 into the reactor core 406, fuel flows in the direction indicated by arrow 504 (e.g., approximately normally to the inner boundary of the louver reflector 404). The curvature of the channel 414 near the inward surface of the louver reflector 404 can thereby impart a terminal direction to flow exiting the channel 414. Where this flow has a significant tangential component and the terminal outflows of all reflector channels are aligned, as illustrated in FIG. 6, the louver reflector 404 can impart angular momentum to the fuel flowing from the louver reflector 404 into the reactor core 406. [0106] FIG. 6 also shows three illustrative trajectories (e.g., 506, 508, 510) along which neutrons originating in the reactor core 406 can pass through (or partly through) the louver reflector 404. Along a first track 506, a neutron passes through portions of one louver 410 to pass entirely through the louver reflector 404. Along a second track 508, a neutron passes through portions of four louvers 410 to pass entirely through the louver reflector 404. Along a third track 510, a neutron passes through portions of six louvers 410 to pass entirely through the louver reflector 404. Thus, while the number of louvers 410 (and thickness of louver material) that are traversed by a neutron to pass entirely through the louver reflector 404 can exhibit an angular dependence, at nearly all possible angles of incidence such a neutron must pass through more than one louver.

[0107] The total thickness of louver material that is traversed by a neutron to pass entirely through the louver reflector 404, as a function of angle of incidence, can be dependent on degree of curvature of, and spacing between, the louvers 410. In certain embodiments, the thickness of louver material that must be traversed by any neutron originating in the reactor core 406 can be configured to be as independent of angle of incidence as possible. Under circumstances where traversal thickness not independent of angle of incidence, it can be desirable that traversal thickness is maximal for angles of incidence closest to normal. That is, for neutrons exiting radially with respect to the core axis and at angles relatively close to such paths, since the majority of neutrons originating in the core can impinge upon the louver reflector 404 within such a range of angles.

[0108] Other embodiments of the louver reflector 410 can include louvers that taper more dramatically than those of FIG. 6, from a relatively thick outer edge to a relatively thin inner edge. Louver tapering can have at least three advantageous effects. In one aspect, tapering of various degrees can produce channels that narrow coreward, have constant width, or widen coreward, depending on what flow patterns are desired. In another aspect, tapering of louvers can support more rapid heat removal from coreward portions of louvers that tend to be hotter than other portions of the louvers because the inward- flowing fuel in the louver reflector 404 can be heated as it goes, making it less effective at cooling the louvers by the time it reaches their inner portions. In a further aspect, tapering of inward louver edges can force the rapid merging of planar jets or sheets of fuel, as the jets can emerge from the louver reflector 404 into the reactor core 406. [0109] Blunt-edged louvers, in contrast, can tend to cause the development of static eddies (fuel stagnancies) in the sheltered area at the louver edge, bracketed by the planar jets emerging from adjacent channels. Static eddies can be generally undesirable in the reactor core 406. Notably, the narrower an inward edge of a louver, the smaller and more transient any eddies occurring along the edge can tend to be. However, the rate of inward taper of a louver (surface curvature) can be constrained by the need to prevent detachment of inflow from the louver surface, which can give rise to static eddies along the louver itself.

[0110] In other embodiments, louvers reflectors 404 can include louvers whose thickness as a function of position along the louver has one or more inflections. In one example, a louver can be relatively thin along its outward edge, increase in thickness toward the center of the reflector louver, then taper to a thin edge on the coreward side. All such taper\s, as well as more complexly inflected tapers not explicitly described herein, are contemplated and within the scope of the disclosure. Moreover, although the louver reflector 404 is depicted as having an overall circular horizontal cross-section, louver reflectors having non-circular (e.g., elliptical) cross-sections in various planes, including the horizontal plane, are also contemplated.

[0111] FIG. 7 is a cross-sectional view through an embodiment of a tapered louver reflector 704. The tapered louver reflector 704 can include tapered louvers 710 that are curved and having reflector channels 714 positioned therebetween so that fluid exiting each reflector channel 714 can enter the reactor core 406 at an oblique angle. As an example, upon entry into a channel 714 from the downcomer 408, fuel can flow in the direction indicated by arrow 750, approximately normally to the outer boundary of the tapered louver reflector 704. Upon exit from the channels 714 into the reactor core 406, fuel can flow in the direction indicated by arrow 752, obliquely to the inner boundary of the tapered louver reflector 704.

[0112] Considering the inner boundary or surface of the tapered louver reflector 704 as a whole, the configuration of the tapered louvers 710 can be viewed as injection of a rotating, approximately cylindrical sheet of liquid fuel into the reactor core 406. Absent any countervailing force, such an injection can cause the liquid fuel within the reactor core 406 to rotate or swirl. That liquid fuel within the reactor core 406 can also rise, as fuel is extracted at the top of the reactor (e.g., to be passed through the heat exchange unit 106).

Subsequently, the liquid fuel can re-enter the reactor as the downcomer 408 and rejoin the reactor core 406 in a nether region of the reactor vessel 402. [0113] The velocity of each local portion of the rotating cylindrical injection sheet can vary. Even when the tapered louver reflector 704 includes approximately identical tiers, the difference in pressure between the downcomer 408 and the reactor core 406 can tend to be a function of height, with corresponding variations in flow through the channels 714.

[0114] Moreover, various embodiments of the tapered louver reflector 704 can include reflector tiers of non-identical design. As an example, the channels of a lowest, first tier can be wider than those of a higher tier. In general, the more tiers within the tapered louver reflector 704 for a given reactor height, the finer the control of flow that can be enabled in the vertical dimension.

[0115] The introduction of controlled swirl to the liquid fuel (e.g., in the reactor core 406) can be beneficial. The swirl in the flow can includes a rotational component of overall fuel flow within a reactor component. Maintaining swirl in bodies of fuel can produce inherently more stable and predictable patterns of flow and can accelerate mixing of fuel. Advantages of swirled flow and enhanced mixing can include, but are not limited to, one or more of the following. In one aspect, more uniform power production, with delayed neutrons tending to stay in the middle of the reactor. In another aspect, lessened material fatigue from localized hot regions (e.g., those associated with stagnancies). In a further aspect, more easily predicted relationships between temperature, power, and neutronics throughout the fuel.

[0116] FIG. 8 is a cross section of the tapered louver reflector 704. As shown, the arrows indicate an overall pattern of flow of the cylindrical injection sheet in the immediate vicinity of an inner surface of the tapered louver reflector 704.

[0117] FIG. 9 is a cross-sectional view through a portion of an embodiment of a contoured louver reflector 904. The contoured louver reflector 904 can include contoured louvers 910 that are doubly curved such that fuel entering a reflector channel (e.g., channel 914) from the downcomer 408 can do so at an oblique angle (direction indicated by arrow 906). Fuel exiting each of the channels 914 can enter the reactor core 406 at an oblique angle (direction indicated by arrow 908). The contoured louver reflector 904 can impart angular momentum to fuel entering the reactor core 406. The double curvature of the contoured louver reflector 904 can also ensure that neutrons originating anywhere in the reactor core 406 cannot pass out through the contoured louver reflector 904 via a fuel-only (or mostly-fuel) path. The channels 914 can be approximately uniform in cross-section and can therefore present less resistance to inward fuel flow than channels that narrow inward.

[0118] FIG. 9 also illustrates various trajectories along which neutrons originating in the reactor core 406 could encounter the contoured louver reflector 904 (e.g., 952, 954, 958). Along a first trajectory 952, a neutron can pass through portions of twelve of the contoured louvers 910 to pass entirely through the contoured louver reflector 904. Along a second trajectory, 954, a neutron can pass through portions of ten of the contoured louvers 910 to pass entirely through the contoured louver reflector 904. Along a third trajectory, 958, a neutron can pass through portions of six of the contoured louvers 910 to pass entirely through the contoured louver reflector 904. A neutron encountering the contoured louver reflector 904 could pass through as few as two contoured louvers 910, but in doing so would necessarily align with the contoured louvers 910, and thus encounter a large thickness of material of the contoured louver reflector 904. Thus, the contoured louver reflector 904 can force neutrons originating in various portions of the reactor core 406 to encounter a large number of contoured louvers 910 and/or a great thickness of louver material before passing entirely through the contoured louver reflector 904. In general, for a given reflector design, the more louver material must be traversed by an average neutron originating in the reactor core 406 if that neutron were to exit the reflector, the more neutrons can be reflected back into the reactor core by the contoured louver reflector 904 and the more effective it can be vis-a-vis its reflective function.

[0119] FIG. 10 is an angled cross-sectional view of portions of the contoured louver reflector 904. The contoured louver reflector 904 can include six vertically stacked louver tiers 916. In some embodiments, each of the louver tiers 916 can be approximately the same. As an example, each louver tier 916 can be approximately 0.75 meter high and approximately 30 cm thick. Support rings 918 can provide connecting members to support and position the contoured louvers 910 of the contoured louver reflector 904.

[0120] FIG. 11 is a top-down cross-sectional view of the contoured louver reflector 904.

[0121] A variety of embodiments of neutron reflectors have been shown (e.g., 404, 704, 904). Each of these reflectors and variations thereof can be used in a liquid- fuel reactor system such as the reactor system 400. [0122] FIG. 12 illustrates an embodiment of a reactor system 400 that includes a reactor vessel 402 and a neutron reflector 1204 disposed within the reactor vessel 402. The neutron reflector 1204 can be any of those shown herein that include a plurality of louvers or any variant thereof that includes a plurality of louvers. The neutron reflector 1204 can include a plurality of louvers defining a shell with passages therethrough. The neutron reflector 1204 can surround a reactor core 406 of the reactor vessel 402. A liquid fuel, such as a molten salt, can be disposed within the reactor vessel 402. The liquid fuel can surround and fill at least a portion of neutron reflector 1204 and the passages.

[0123] In operation, the reactor vessel 402 can include the downcomer 408 and the reactor core 406. Various components and structural details are omitted for clarity. Different flows are illustrated in FIG. 12 and discussed below. The sizes of arrows do not necessarily correspond to the relative magnitudes of the flows.

[0124] As shown, an overall downward flow of fuel through the downcomer 408 is indicated by downward-pointing arrows 1250. Predominantly radial inward flow of fuel from the downcomer 408 to the reactor core 406 through channels of the neutron reflector 1204 is indicated by angled arrows 1252.

[0125] The channels of the neutron reflector 1204 can inject a rotating cylindrical sheet of fuel into the reactor core 406. The direction of flow of this sheet in the immediate vicinity of the neutron reflector 1204 is indicated by circled-dot and circled-X symbols.

[0126] Mixing of the flow of the downcomer 408 with the reactor core 406 in the nether region of the reactor vessel 402 is indicated by hooked arrows 1254. In an example, approximately 90% of fuel entering the top of the downcomer 408 can exits at the bottom of the downcomer 408. The remainder can reach the reactor core 406 through the channels of the neutron reflector 1204. Upward pointing arrows 1256 and helical symbol 1280 indicate that the overall motion of fuel in the reactor core 406 is rotating and upward.

[0127] FIG. 13 shows the reactor system 400 with additional aspects indicated for clarity. In particular, a downcomer pressure (PD) is shown to vary with height in a typical mode of operation of the reactor system 400. Within the downcomer 408, there can be an average pressure PDI across the height of an uppermost reflector tier 1220, an average pressure PD2 across the height of a middle tier 1222, and an average pressure PD3 across the height of a bottom tier 1224. Similarly, within the reactor core 406 and in the immediate vicinity of the neutron reflector 1204, there can be an average core pressure Pci across the height of the uppermost reflector tier 1220, an average pressure Pc2 across the height of the middle tier 1222, and an average pressure Pc₃ across the height of the bottom tier 1224. Across the uppermost tier 1220, there can be an average pressure difference ΔΡι = P_DI - Pci, across the middle tier 1222 there can be an average pressure difference ΔΡ2 = P_D2 - Pc2, and across the bottom tier 1224 there can be an average pressure difference ΔΡ₃ = P_D3 - Pc₃- In general, the following relationships can be preferable:

Pci < PDI

Pc2 < PD2

Pc3 < PD3

PDI > PD2 > PD3

[0128] In a typical state of operation of the reactor system 400, a bulk of fissions can occur within a roughly spindle-shaped volume 1226 of the reactor core 406, indicated by a dashed outline. The distance from the centroid of the volume 1226 to a typical point 1228 within the uppermost tier 1220 of the neutron reflector 1204 is indicated by a vector having length ΐ\. Corresponding distances ¾ and ¾ are indicated for typical points 1232 and 1234 of the middle tier 1222 and the lower tier 1224 of the neutron reflector 1204, where η = ¾ > ¾. Assuming a uniform origination of radiation within the volume 1226, the middle tier 1222 can be exposed to more radiation, overall, than either the uppermost tier 1220 or the lower tier 1224. This can result in uneven radiative aging of the three tiers 1220, 1222, 1224 and uneven thermal expansion or contraction in response to changing core radiation output. Both these effects can be undesirable.

[0129] FIG. 14 is a vertical cross-sectional view through a fluid-fuel spheroid reactor system 1400 according to an embodiment of the disclosure. The spheroid reactor system 1400 can includes a spheroid reactor vessel 1402 and a spheroid neutron reflector 1404. The spheroid neutron reflector 1404 can includes a plurality of louvers 1410 that form a shell with passages therethrough. The spheroid neutron reflector 1404 can surrounds a reactor core 1406 of the spheroid reactor vessel 1402. A liquid fuel such as a molten salt can be disposed within the spheroid reactor vessel 1402. In certain embodiments, the liquid fuel can surround and fills at least a portion of the spheroid neutron reflector 1404 and the passages. [0130] The spheroid reactor system 1400 does not result in significantly uneven irradiation of different portions of the spheroid neutron reflector 1404. The spheroid reactor system 1400 can be approximately spherical in overall form and include a three-tier spheroid neutron reflector 1404 whose louvers 1410 can be of substantially uniform cross section in planes of constant r and Θ with reference to a standard spherical coordinate system (r, θ, φ) whose origin is at about the center of the spheroid reactor system 1400. That is, the louvers 1410 of the spheroid neutron reflector 1404 can be spherical-coordinate analogues of, for example, the vertical louvers 1410 and the spheroid neutron reflector 1404 can be a substantially spherical coordinate analogue of, for example, the cylindrical louver reflector 404.

[0131] In the spheroid reactor system 1400, the spheroid reactor vessel 1402 can encloses a downcomer 1408 and a reactor core 1406. Various components and structural details are omitted for clarity. The overall downward flow of fuel through the downcomer 1408 is indicated by downward-pointing arrows 1412. The primarily radial inward flow of fuel from the downcomer 1408 to the reactor core 1406 through the channels 1414 of the spheroid reactor vessel 1402 is indicated by angled arrows 1451. The channels 1414 can inject a rotating, approximately spherical sheet of fuel into the reactor core 1406. The direction of flow of this sheet in the immediate vicinity of the spheroid neutron reflector 1404 is indicated by circled-dot and circled- X symbols. Mixing of downcomer 1408 with the reactor core 1406 in the nether region of the spheroid reactor vessel 1402 is indicated by hooked arrows (e.g., arrow 1454). Preferably, approximately 90% of fuel entering the top of the downcomer 1408 can exit at the bottom of the downcomer 1408. The remainder can reach the reactor core 1406 through the channels 1414. An upward pointing arrow 1456 and a helical symbol 1458 indicate that the overall motion of fuel in the reactor core 1406 is rotating and upward. The indicated motions are overall and schematic only.

[0132] In a typical state of operation, a bulk of fissions occurring in the spheroid reactor system 1400 can occur within a roughly spherical volume 1435 of the reactor core 1406, indicated by a dashed outline. The various tiers of the spheroid neutron reflector 1404 can be irradiated in an approximately uniform manner in the spheroid reactor system 1400.

Moreover, as will be clear to persons familiar with the art of nuclear reactor design, the mass of fuel in the reactor core 1406 needed for criticality can be at a minimum for a spherical core. In contrast, cylindrical cores can require a larger amount of fuel in the core for criticality. In general, a spherical liquid-fuel reactor core can produce equal power output with less fuel and less massive ancillary components (e.g., reflector and vessel) than a cylindrical core.

[0133] FIG. 15 is a cutaway view through a neutron reflector 1504 that includes a plurality of annular louvers 1510 in stacked configurations. As shown, the neutron reflector 1504 includes eight radially arranged louver stacks 1524. In certain embodiments, each of the louver stacks 1524 can be approximately the same. As an example, each louver stack 1524 can be approximately 5 meters high and approximately 30 cm thick. Vertical connecting members 1526 can support and position the annular louvers 1520 of the neutron reflector 1504.

[0134] FIG. 16 illustrates a cross-sectional view through a portion of the neutron reflector 1504. The annular louvers 1510 of the neutron reflector 1504 can be of approximately uniform radial cross section, arranged in planes of constant r and Θ with reference to a standard cylindrical coordinate system (r, Θ, z) whose origin is at the center of the neutron reflector 1504. The annular louvers 1510 can have a substantially constant vertical cross section. The annular louvers 1510 can be formed as rings or segments of rings centered on an approximately cylindrical reactor core 1506 and they can be stacked vertically to form the neutron reflector 1504. In various other embodiments, a spherical analogue of the neutron reflector 1504 can surround a spherical core (see, e.g., the spheroid neutron reflector 1404 in the spheroid reactor system 1400 surrounding a spherical core). Also, other basic geometries in which a reflector surrounds or partly surrounds a reactor core are contemplated and within the scope of the disclosure.

[0135] When the neutron reflector 1504 is used in an operating liquid fuel reactor system, a liquid fuel of the system can flow down as a downcomer 1508, upwards within the reactor core 1506 of a reactor system, and through the channels 1514. Arrows 1554 and 1556 indicate that direction of flow through a channel 1514 is constrained by local channel orientation. Thus, upon entry into the channel 1514 from the downcomer 1508, fuel in a channel 1514 can flow in the direction indicated by arrow 1554 (e.g., obliquely and downward with respect to the outer boundary of the neutron reflector 1504). Upon exit from the channel 1514 into the reactor core 1506, fuel flows in the direction indicated by arrow 1556 (e.g., obliquely and upward with respect to the inner boundary of the neutron reflector 1504). [0136] The curvature of the channel 1514 near the inward surface of the neutron reflector 1504 can imparts a terminal direction to flow exiting the channel 1514. Although the motion of fuel flowing in a channel 1514 can have a circumferential component, there is nothing in the structure of the neutron reflector 1504 itself to impart such a component. Accordingly, the primary component of the motion of fuel exiting the channel 1514 can be upward and inward. Considering the inner boundary or surface of the neutron reflector 1504 as a whole, the configuration of the annular louvers 1510 can be viewed as injecting a non-rotating, upwardly moving and approximately cylindrical sheet of liquid fuel into the reactor core 1506.

[0137] As will be clear to persons familiar with hydraulics, channels formed by louvers of varying curvature (e.g., multiply curved) can inject liquid fuel primarily in an upward direction, horizontally, or in a downward direction. All such variations are contemplated and within the scope of the disclosure. Variations discussed hereinabove with reference to the vertical-louver reflectors, including variously tapered louvers, moveable louvers, and the like, can apply also to the neutron reflector 1504 and to other embodiments including ring-shaped louvers. Further ring-shaped louvers can be constructed in various embodiments as entire rings, or assembled from segments, and various structures can be employed to hold them in position (e.g., weight-bearing vertical members set at intervals around the reflector, to which rings or ring segments are attached).

[0138] FIG. 17 is a vertical cross-sectional view of portions of an embodiment of a liquid- fuel reactor 1700. The reactor 1700 can be approximately cylindrical in overall form and it can include a reflector 1702 having stacked horizontal vanes. A vessel 1704 can enclose a downcomer 1706 and a reactor core 1708. Various components and structural details are omitted for clarity. The overall downward flow of fuel through the downcomer 1706 is indicated by downward-pointing arrows 1710). The predominantly radial inward flow of fuel from the downcomer 1706 to the reactor core 1708 through the channels of the reflector 1702 is indicated by small down-angled arrows 1712. The channels of the reflector 1702 can inject a substantially non-rotating, rising, cylindrical sheet of fuel into the reactor core 1708; where the direction of flow of this sheet in the immediate vicinity of the reflector 1702 is indicated by up-angled arrows 1714. Mixing of downcomer flow with the reactor core 1708 in the nether region of the vessel 1704 is indicated by hooked arrows 1718. In an example, approximately 90% of fuel entering the top of the downcomer 1706 can exits at the bottom of the downcomer 1706. The remainder can reach the reactor core 1708 through the channels of the reflector 1702. Upward pointing arrows 1720 indicate that the overall motion of fuel in the core is upward. Since no structures in the reactor 1700 can tend to impart circumferential motion to fuel, rotatory motion of fuel in the reactor core 1708 can be relatively slight and is not indicated.

[0139] Various embodiments of the disclosed reflectors (e.g., 404, 704, 904, 1504, 1702) can include both tiers of vertical louvers and tiers of horizontal louvers. In an example, the two upper tiers of a three-tier reflector can include vertical louvers, while the bottom-most tier can include horizontal louvers. All such hybrid permutations are contemplated and within the scope of the disclosure.

[0140] Notably, vertical louvers can have an advantage that their louvers tend to be shorter in various feasible reflector geometries and short louvers can be less subject to undesirable flow-induced vibration than long louvers.

[0141] Vertical louver arrangements (e.g., those of FIGS. 5 and 10, which have uniform horizontal cross-section) can impart primarily circumferential motion (angular momentum) to fuel entering the reactor core. In contrast, horizontal or annular louver arrangements (e.g., those of FIGS. 15 and 16, which have uniform radial cross-section) can be used to impart primarily vertical motion to fuel entering the reactor core.

[0142] Circumferential and vertical motion can be simultaneously imparted to fuel entering the reactor core by some doubly non-uniform louvers. That is, louvers that have uniform cross-section in neither horizontal nor radial planes. Twisted vertical louvers, for example, can be doubly non-uniform in this sense. In another example, the same can hold for a spiraling trough-shaped louver. Various embodiments can include doubly non-uniform louvers additionally or alternatively to louvers of uniform horizontal and/or radial cross section.

[0143] FIG. 18 shows a portion of a singly-curved louver 1810.

[0144] FIG. 19 shows a portion of a double non-uniform louver 1910. By comparison of the double non-uniform louver 1910 to the singly-curved louver 1810, it can be observed that when liquid fuel is flowed by the double non-uniform louver 1910, the geometry of the double non-uniform louver 1910 can impart both vertical and rotational flow to the liquid fuel.

[0145] Individual louvers can be non- uniform and asymmetric in every plane and their channels can even be interrupted (e.g., louvers can touch or merge at various points). The behaviors of such non-uniformly (and possibly reconfigurable) channeled reflectors, and the reactor core and downcomer flows formed in the presence of such reflectors in various states of operation, can be predicted and designed using computational tools.

[0146] By suitable configuration of non-uniform louvers and channels throughout the structure of a reflector, substantially every sub-area of the injection sheet produced by a reflector can be locally engineered (e.g., its direction and speed specified). Hydrodynamics of reactor cores and downcomers can be strongly influenced by the specified character of the injection sheet. Such fine control over reactor hydrodynamics have not been previously developed. Also, non-uniform (and possibly reconfigurable) neutron reflectivity can be a property of various embodiments and can be combined with non-uniform flow forming. Since hydraulic, nuclear, and thermal processes can be interdependent in a fluid-fuel reactor, allowing for fine specification of both neutron reflectivity and flow can be advantageous, e.g., for the purpose of preventing persistent vortices.

[0147] Of note, the embodiments illustrated hereinabove do not address the question of neutron reflection at the top or bottom of a reactor core, although such reflection can be desirable for the same reasons that it can be desirable around other portions of the core (e.g., enhancement of reactor core reactivity and reduction of radiation injury to structural materials). Various embodiments can address the need for neutron flux management over the whole surface of the reactor core in either or both of two ways.

[0148] In one aspect, shielding by fuel or other materials can reduce or eliminate the need for neutron flux management around portions of a reactor core not proximate to a reflector. As an example, various embodiments can include one or more columns of subcritical fuel, above the reactor core, below the reactor core, or both. Fuel in an ascending column can feed the reactor core from below, mix with the reactor core and be heated therein, and participate in an ascending column of hot fuel exiting the top of the reactor core on its way to a heat exchanger. If fuel columns above and below openings in the reactor are of sufficient length, they can tend to act as shields, blocking a majority of the radiation from the reactor core that can otherwise escape through the cross-section of the column. Typically, in such

embodiments, a column of fuel can be approximately the same shape and diameter as a gap in the reflector structure with which the column is aligned.

[0149] In another aspect, a neutron reflector can be extended to the nether region of a reactor. In general, it can be challenging to extend fuel-cooled reflector structures over the top of a reactor, since hot (e.g., pre-heat-exchanger) fuel can exit the top of a reactor. Thus, various embodiments can include a fuel-cooled neutron reflector that surrounds the sides and bottom of a reactor core, as well as an ascending column of hot (but subcritical) fuel at the top of the reactor that is tall enough to acceptably reduce neutron flux out of the core through the fuel exit opening.

[0150] FIG. 20 illustrates a cutaway view through a reactor system 2000. The reactor system 2000 can include a fuel-cooled neutron reflector 2004 with structures located both laterally and at the bottom of a reactor vessel 2002. In particular, a reactor core 2006 can be surrounded laterally by the neutron reflector 2004 including horizontally-oriented vanes 2010 and channels. While not shown, reflectors including vertically-oriented structures and doubly non-uniform structures are also contemplated. Relatively cool downcomer fuel (downward- pointing arrows 2056) can flow between the lateral side of the neutron reflector 2004 and the reactor vessel 2002. The majority of the downcomer fuel can enter the reactor core 2006 below the bottom of the lateral portion of the neutron reflector 2004, as indicated by upward- curved arrows 2050. A lesser portion of the downcomer can flow into the reactor core 2006 through channels in the neutron reflector 2004, as indicated by slanted arrows 2052.

[0151] Another portion of the downcomer can flow into a nether space 2034 between a bottom portion of the neutron reflector 2004 and the reactor vessel 2002. Fuel from the nether space 2034 can pass through and cool the bottom portion of the neutron reflector 2004 via a number of channels 2038 into the reactor core 2006, as indicated by upward-pointing arrows 2060. The channels 2038 can be straight, tubular openings, which can allow some neutrons to escape from the reactor core 2006 through a fuel-only route. In other

embodiments (not shown), curved or devious channels can be employed that can allow through flow of downcomer fuel into the core while placing reflector material in the path of any neutron originating in the reactor core. [0152] A number of the reflectors discussed herein can include channels of a sheet-like or curviplanar character. However, reflector perforations forming channels of a tube-like character as in FIG. 20 (e.g., radially oriented spiraling tubes) are also contemplated and within the scope of the disclosure. Also, various embodiments include not only the later and nether reflector structures depicted and discussed hereinabove, but additional reflector structures (e.g., vanes or louvers stationed in the downcomer).

[0153] FIG. 21 illustrates an embodiment of a method 2101 of operating a reactor system. The method 2101 can include operating 2113 the reactor by steps that include the step of pumping 2119 a fuel salt from the reactor core of a reactor vessel and into a heat exchanger. In certain embodiments, the method 2101 can include heating 2125 a secondary fluid (2d) by the heat exchanger and sending 2127 the secondary fluid (2d) to a downstream process to transfer the added heat to the downstream process. The method 2101 can also include the step of flowing 2139 the fuel salt from the heat exchanger into an area between the outside of a neutron reflector and an inner wall of the reactor vessel (e.g., into the downcomer). The method 2101 can also include the step of allowing 2145 the fuel salt to return from this area to the core via passages through the neutron reflector and through a lower portion of the reactor. A beneficial feature of the method 2101 is that the method 2101 can include removing heat from the neutron reflector or shield in step 2157 by flow of the fuel salt through the passages. In preferred embodiments, the neutron reflector can include a plurality of louvers defining a shell with the passages therethrough, and the neutron reflector surrounds the core of the reactor vessel.

[0154] Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics can be combined in any suitable manner in one or more embodiments.

[0155] The terms and expressions which have been employed herein are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described (or portions thereof), and it is recognized that various modifications are possible within the scope of the claims. Accordingly, the claims are intended to cover all such equivalents.

[0156] A number of implementations have been described above, but it will be understood that various modifications can be made. Accordingly, other implementations are within the scope of the following claims.

Claims

CLAIMS What is claimed is:

1. A reactor system comprising:

a reactor vessel; and

a neutron reflector comprising a plurality of louvers defining a shell with passages therethrough, wherein the neutron reflector is disposed within, and surrounds a reactor core of, the reactor vessel.

2. The reactor system of claim 1, wherein the plurality of louvres consists essentially of stainless steel.

3. The reactor system of claim 1, wherein the plurality of louvers are connected by at least one connecting member.

4. The reactor system of claim 3, wherein the at least one connecting member defines a substantially circular rail.

5. The reactor system of claim 4, wherein the shell substantially defines a cylindrical shape.

6. The reactor system of claim 1, further comprising a molten fuel salt in the reactor vessel wherein the fuel salt surrounds and fills at least a portion of the neutron reflector and the passages.

7. The reactor system of claim 6, wherein the fuel salt comprises a fast-spectrum molten chloride salt.

8. The reactor system of claim 6, further comprising a pumping system configured to move the molten salt upwards from the core.

9. The reactor system of claim 8, wherein when the pumping system is operating, the fuel salt flows: upwards through the core, downwards on the outside of the neutron reflector, and through the passages.

10. The reactor system of claim 9, wherein the flowing of fuel salt through the passages removes heat from the neutron reflector.

11. A method of operating a reactor system, the method comprising:

pumping a fuel salt from the core of a reactor vessel and into a heat exchanger;

flowing the fuel salt from the heat exchanger into an area between the outside of a neutron reflector and an inner wall of the reactor vessel;

allowing the fuel salt to return from the area to the core via passages through the neutron reflector and through a lower portion of the reactor; and

removing heat from the neutron reflector by means of the fuel salt flowing through the passages.

12. The method of claim 11, wherein the neutron reflector comprises a plurality of louvers defining a shell with the passages therethrough, wherein the neutron reflector surrounds the core of the reactor vessel.

13. The method of claim 12, further comprising adding heat to a secondary fluid by the heat exchanger and flowing the secondary fluid to a downstream process to transfer the added heat to the downstream process.

14. The method of claim 13, wherein the downstream process comprises the operation of a turbine to generate electricity.

15. The method of claim 13, wherein the shell substantially defines a cylindrical shape.

16. The method of claim 13, wherein the fuel salt comprises a fast-spectrum molten chloride salt.

17. An apparatus for use as a neutron reflector, the apparatus comprising a plurality of louvers connected together by at least one connecting member to define a shell surrounding a core space, wherein the louvers define openings through the shell.

18. The apparatus of claim 17, wherein the at least one connecting member comprises a substantially circular rail from which the plurality of louvers extend.

19. The apparatus of claim 17, wherein each of the louvers is movably connected to the at least one connecting member.

20. The apparatus of claim 17, wherein the plurality of louvers comprise steel.

21. The apparatus of claim 17, wherein the plurality of louvers and the at least one connecting member consist essentially of stainless steel.

22. The apparatus of claim 17, wherein any straight line from the core space, through the shell, to a space outside of the shell extends through at least two of the louvers.

23. The apparatus of claim 17, wherein the shell is substantially cylindrical in shape.

24. The apparatus of claim 17, wherein surfaces of the louvers are curved and have portions that are not parallel to, or normal to, an idealized cylinder concentric with the shell.