JP6921025B2 - Neutron shielding material and spent fuel assembly storage container - Google Patents

Description

The present invention relates to a neutron shielding material made of epoxy resin and a spent fuel assembly storage container provided with the neutron shielding material.

After nuclear power generation, the spent fuel assembly taken out from the nuclear reactor is cooled in the cooling pool in the nuclear power plant for a certain period of time, and then transported to processing facilities such as fuel reprocessing plants, interim storage facilities, etc. NS. If the amount of spent nuclear fuel generated exceeds the reprocessing capacity, it will be necessary to store the spent nuclear fuel for a long period of time, so dry storage is being considered. When storing spent nuclear fuel inside and outside a nuclear power plant or transporting spent nuclear fuel, a metal dry cask is used as a container for storing spent fuel assemblies.

Spent nuclear fuel continues to emit a large amount of decay heat in addition to high-dose neutron rays and γ-rays even after the fission reaction and radioactive decay are attenuated to some extent by cooling in the cooling pool. Further, when a highly enriched nuclear fuel for high burnup is used, it is expected that the calorific value, neutron intensity, etc. generated by the spent nuclear fuel will further increase. Therefore, the cask that stores or transports the spent fuel assembly is required to have high neutron shielding performance, sealing performance, heat resistance performance, and the like.

Conventionally, a neutron shielding material made of synthetic resin has been used as a lightweight and solid neutron shielding material. Generally, a neutron shielding material made of a synthetic resin is formed by blending a neutron absorber and a flame retardant in a resin matrix. As the synthetic resin, a resin containing a high density of hydrogen atoms having a large elastic scattering cross section of neutrons is used. On the other hand, in applications such as cask where the heat load during storage is a problem, the synthetic resin needs to have high heat resistance which is in a relationship opposite to the hydrogen density.

As a neutron shielding material for cask, epoxy resin, which has a good balance between hydrogen density and heat resistance, can be cured at room temperature, and has excellent space filling property and mechanical strength, is often used. Conventionally, from the viewpoint of improving the hydrogen density of synthetic resins, a technique has been developed in which a resin matrix is formed of a hydrogenated bisphenol type epoxy resin and an amine system having a large number of hydrogens is used as a curing agent. However, since amine-based curing agents do not necessarily have high heat resistance, various studies have been conducted to achieve both hydrogen density and heat resistance.

For example, Patent Document 1 describes a composition for a neutron shielding material containing an epoxy component, a curing agent component, a boron compound as a neutron absorber, and a refractory material. Examples of the epoxy component include _{hydrogenated bisphenol type epoxies having CH 3} , H, F, Cl or Br as a substituent. Examples of the curing agent component include 4,4'-methylenebis (cyclohexylamine), 1,3-bis (aminomethyl) cyclohexane, and an imidazole compound (see claim 3 and the like).

Japanese Patent No. 3634798

As described in Patent Document 1, hydrogenated bisphenol type epoxy resin using 4,4'-methylenebis (cyclohexylamine), 1,3-bis (aminomethyl) cyclohexane, imidazole compound and the like as a curing agent. By introducing a crosslinked structure into the epoxy, heat resistance can be improved while maintaining a high hydrogen density to some extent. However, the epoxy resin neutron shielding material still cannot be said to have sufficient heat resistance, and further heat resistance is desired.

The environment in which the neutron shielding material is installed is 150 ° C to 200 ° C or higher when increasing the number of spent fuel assemblies loaded per cask or storing spent fuel assemblies for high burnup. It is expected that the temperature will be high. Since synthetic resins have a lower thermal conductivity than metals and the like, if the storage period at such a high temperature is prolonged, a huge heat load is applied to the neutron shielding material. Therefore, oxidative deterioration and radiation deterioration are promoted by the heat load, and there is a problem that the epoxy resin is thermally decomposed.

When the epoxy resin in the neutron shielding material is thermally decomposed, hydrogen atoms in the components are lost due to vaporization, and it becomes difficult to maintain the shape of the neutron shielding material, so that the neutron shielding performance is significantly deteriorated. Therefore, there is a demand for a neutron shielding material or cask that has high hydrogen density, high heat resistance, and high resistance to heat aging even when stored in a high temperature environment for applications such as cask. There is.

Therefore, an object of the present invention is to provide a neutron shielding material having high heat resistance and whose neutron shielding performance is unlikely to deteriorate even in a high temperature environment, and a spent fuel assembly storage container provided with the neutron shielding material.

In order to solve the above problems, the neutron shielding material according to the present invention is a neutron shielding material containing an epoxy resin as one of the main components, and the epoxy resin contains at least a compound containing two or more epoxy groups in the molecule. It is composed of a cured product of a resin composition containing a main agent contained as one component and a curing agent for ring-opening polymerization of the epoxy group, and the curing agent is a phenol resin.

Further, the spent fuel assembly storage container according to the present invention is installed in the inner container, the outer container installed outside the inner container, and the inner container, and stores the spent fuel assembly. A basket for the fuel and the neutron shielding material surrounding the inner container.

According to the present invention, it is possible to provide a neutron shielding material having high heat resistance and whose neutron shielding performance is unlikely to deteriorate even in a high temperature environment, and a spent fuel assembly storage container provided with the neutron shielding material.

It is a perspective view which shows an example of the spent fuel assembly storage container. It is a figure which shows the result of having measured the weight loss rate with heating of the resin composition of an epoxy resin. It is a figure which shows the result of having measured the glass transition temperature of the resin composition of an epoxy resin.

Hereinafter, the neutron shielding material according to the embodiment of the present invention and the spent fuel assembly storage container provided with the neutron shielding material will be described with reference to the drawings.

<Neutron shielding material>
The neutron shielding material according to the present embodiment is an epoxy resin neutron shielding material containing an epoxy resin as one of the main components. The neutron shielding material according to the present embodiment is used by arbitrarily blending a flame retardant, a neutron absorber, or the like in an epoxy resin forming a resin matrix, and molding the neutron shielding material into an appropriate shape. This neutron shielding material can be used for various purposes of shielding neutrons, and is particularly preferably used for applications in which a high temperature of 150 ° C. to 200 ° C. or higher may continue, for example, as a member of a dry cask.

The epoxy resin, which is the main component of the neutron shielding material, is composed of a cured product obtained by curing the resin composition by a polymerization reaction. The resin composition for forming an epoxy resin contains a main agent containing an epoxy compound containing two or more epoxy groups in the molecule as at least one component, and a curing agent for ring-opening polymerization of the epoxy group of the epoxy compound. Will be done. Further, a curing accelerator that promotes polymerization via an epoxy group is arbitrarily added to the resin composition depending on the type of main agent, curing conditions, and the like.

The neutron shielding material according to this embodiment uses a phenol resin as a curing agent. Acid anhydride, which is generally used as a curing agent, has low moisture resistance and produces free acid that thermally decomposes at about 200 ° C or lower. Therefore, when the neutron shielding material is exposed to a high temperature of 150 ° C to 200 ° C or higher. , Sufficient heat resistance cannot be obtained, causing a decrease in hydrogen density and the like. In addition, since acid anhydride has high reactivity with basic components such as imidazole compounds, which are typical curing accelerators, carbon dioxide gas is generated by the reaction, and the neutron shielding material is expanded to increase the density. Causes a drop. In addition, polyamines and the like, which are generally used as curing agents, often have a flexible skeleton and have a problem in heat resistance. On the other hand, when a phenol resin is used as the curing agent, the glass transition temperature of the epoxy resin becomes high, so that the progress of heat aging is minimized even in a high temperature environment, and a high hydrogen density and a shape of a neutron shielding material can be obtained. Can be maintained.

In addition, in this specification, "phenol resin" means a polymer compound obtained by polymerizing phenol or a phenol derivative with aldehydes. The molecular weight and degree of polymerization of the phenol resin are not particularly limited, and the phenol resin may be modified by a structural unit having a ring structure other than phenol or a phenol derivative. Further, in the present specification, the terms "curing agent" and "curing accelerator" do not mean mutually exclusive concepts, and may include common compounds. For example, the "hardener" may or may not form crosslinks by itself.

Further, in the present specification, the term "heat resistance" means a characteristic evaluated by using a glass transition temperature or a thermal strain temperature as an index. Therefore, the heat resistance of the cured product can be evaluated by measuring the glass transition temperature and the thermal strain temperature. Further, the heat resistance of the neutron shielding material can be indirectly evaluated by using the rate of decrease in hydrogen density or the number of hydrogens with temperature change or the rate of weight decrease as an index.

(Main agent)
As the main agent, an epoxy compound containing two or more epoxy groups in the molecule is used as at least one component. As the main agent, only the epoxy compound may be used, or the epoxy compound and other compounds may be used in combination. Epoxy resin has fluidity near room temperature and can be cured at room temperature, so it has excellent filling property in the space where the neutron shielding material should be placed. Therefore, when the neutron shielding material is applied to applications such as cask, a cured product having a high filling rate and good neutron shielding performance is formed even when the space is narrow or has a complicated shape. can do.

As the main agent, it is preferable to use one or more epoxy compounds of hydrogenated bisphenol A diglycidyl ether and hydrogenated bisphenol F diglycidyl ether. As the main agent, one kind of epoxy compound may be used, or a plurality of kinds of epoxy compounds may be mixed and used. These epoxy compounds have a rigid ring structure, are nuclear hydrogenated, and exhibit a high hydrogen density. Therefore, it is suitable for achieving both high neutron shielding performance and high heat resistance. As the main agent, hydrogenated bisphenol A diglycidyl ether is particularly preferable because a higher hydrogen density can be obtained.

As the main agent, in addition to hydrogenated bisphenol A diglycidyl ether and hydrogenated bisphenol F diglycidyl ether, other epoxy compounds and epoxy resins obtained by polymerizing epoxy compounds can also be used in combination. For example, when a bisphenol type epoxy compound that is not nuclear hydrogenated or a prepolymer obtained by polymerizing such an epoxy compound is used in combination, the viscosity of the resin composition can be adjusted and the strength and toughness of the cured product can be improved. It may be possible to plan.

As the other epoxy compound, a bifunctional epoxy compound containing two epoxy groups in the molecule may be used, or a polyfunctional epoxy compound containing three or more epoxy groups in the molecule may be used. Examples of other epoxy compounds include glycidyl ether type epoxy compounds, glycidyl ester type epoxy compounds, glycidyl amine type epoxy compounds, and alicyclic epoxy compounds. Further, as other epoxy resins, glycidyl ether type epoxy resin obtained by polymerizing these epoxy compounds, glycidyl ester type epoxy resin, glycidyl amine type epoxy resin, alicyclic epoxy resin and the like, and these epoxy resins Examples thereof include an epoxy resin which is a modified product and corresponds to a prepolymer.

Examples of the glycidyl ether type epoxy compound include tetrabromobisphenol A diglycidyl ether, bisphenol A diglycidyl ether, bisphenol F diglycidyl ether, 4,4'-biphenyldiyl bis (glycidyl ether), and naphthalenediyl bis (glycidyl ether). , Dicyclopentadiene diglycidyl ether, hydroquinone diglycidyl ether, resorcinol diglycidyl ether, cyclohexanediol diglycidyl ether, biphenol diglycidyl ether, diglycidylphenyl glycidyl ether, 4,4'-dihydroxybenzophenone diglycidyl ether, triglycidyl cyanurate Ether, tris (4-hydroxyphenyl) methanetriglycidyl ether, 1,1,1-tris (4-hydroxyphenyl) ethanetriglycidyl ether, 1,1,1-tris (4-hydroxyphenyl) propanthriglicidyl ether, Examples thereof include 1,1,2,2-tetra (p-hydroxyphenyl) ethanetetraglycidyl ether and the like, and derivatives thereof.

Examples of the glycidyl ether type epoxy resin include brominated epoxy resin obtained by polymerizing these epoxy compounds, bisphenol A type epoxy resin, bisphenol F type epoxy resin, biphenyl type epoxy resin, naphthalene type epoxy resin, and dicyclopentadiene. Examples thereof include type epoxy resin, trisphenol methane type epoxy resin, and phenol resin type epoxy resin.

Examples of the phenol resin type epoxy resin include phenol novolac obtained by polymerizing phenols and aldehydes, cresol novolac obtained by polymerizing cresol and aldehydes, 1,2-xylylene group, 1, Xylylene novolak with a 3-xylylene group or a 1,4-xylylene group introduced, bisphenol A novolak obtained by polymerizing bisphenol A and aldehydes, or polymerizing phenols, salicylaldehyde, and hydroxybenzaldehyde. Phenol resins such as the obtained triphenylmethane novolac, biphenyl novolac modified with biphenyls, dicyclopentadienephenol novolac modified with dicyclopentadiene, and terpenphenol novolac modified with terpenes, epichlorohydrin and the like. Examples thereof include an epoxy resin that has been epoxidized by reacting with.

Examples of the glycidyl ester type epoxy compound include phthalic acid diglycidyl ester, isophthalic acid diglycidyl ester, terephthalic acid diglycidyl ester, tetrahydrophthalic acid diglycidyl ester, hexahydrophthalic acid diglycidyl ester, 1,2,4-benzene. Tricarboxylic acid triglycidyl ester, 1,3,5-benzenetricarboxylic acid triglycidyl ester, trimellitic acid glycidyl ester, pyromellitic acid glycidyl ester, cyclohexanedicarboxylic acid diglycidyl ester, naphthalenedicarboxylic acid diglycidyl ester and derivatives thereof. And so on. Examples of the glycidyl ester type epoxy resin include cyclic glycidyl ester type epoxy resins obtained by polymerizing these epoxy compounds.

Examples of the glycidylamine type epoxy compound include N, N-diglycidylaniline, N, N-diglycidyltoluidine, 1,3-phenylenebis (N, N-diglycidylmethaneamine), and 4,4'-methylenebis [ N, N-bis (oxylanylmethyl) aniline], N, N, N'-triglycidyl-4,4'-diaminodiphenylmethane, N, N, N', N'-tetraglycidyl-4,4'- Diaminodiphenylmethane, N, N, N', N'-tetraglycidyl-1,3-benzenedi (methaneamine), N, N, N', N'-tetraglycidyl-4,4'-diaminodiphenyl ether, diglycidyl aminophenol , Triglycidylaminophenol, diglycidylaminocresol, triglycidylaminocresol, triglycidyl isocyanurate, N, N-diglycidyl-4-glycidylaniline, N, N-diglycidyl-4-oxylanyloxyaniline, N, N- Examples thereof include diglycidyl-4-phenoxyaniline, diglycidyl piperazine, N, N-bis (2,3-epoxypropyl) -4- (2,3-epoxypropoxy) aniline and derivatives thereof. Examples of the glycidyl amine type epoxy resin include aromatic amine type epoxy resins, aminophenol type epoxy resins, and aliphatic amine type epoxy resins obtained by polymerizing these epoxy compounds.

Examples of the alicyclic epoxy compound include 3,4-epoxycyclohexylmethyl-3', 4'-epoxycyclohexanecarboxylate, and 3,4-epoxy-6methyl-cyclohexylmethyl-3', 4'-epoxy-6. Methyl-cyclohexanecarboxylate, bis (3,4-epoxycyclohexylmethyl) adipate, bis (3,4-epoxy-6methyl-cyclohexylmethyl) adipate, ethyleneoxy-3,4-epoxycyclohexane, biscarboxylic acid (3, 4-epoxycyclohexane-1-ylmethyl) ester, 1,1'-bi (2,3-epoxycyclohexane), 4,4'-bi (1,2-epoxycyclohexane), limonendiepoxide, dicyclopentadienediepoxide , 1,4-Cyclohexadiene epoxide, 4-vinylcyclohexanediepoxide, 1,5-cyclooctadiene epoxide and the like, derivatives thereof and the like. Examples of the alicyclic epoxy resin include an oligomer-type alicyclic epoxy resin obtained by polymerizing these epoxy compounds.

(Curing agent / curing accelerator)
As the curing agent or curing accelerator, a compound that opens and polymerizes the epoxy group of the epoxy compound is used. As the curing agent, only the phenol resin may be used, or the phenol resin and other compounds may be used in combination. As the other curing agent or curing accelerator, a compound that itself forms a crosslink by an addition reaction, a condensation reaction, or the like may be used, or a compound that acts as a basic catalyst or an acidic catalyst may be used.

Examples of the phenolic resin include phenol novolac, cresol novolac, naphthol novolac, xylylene novolac, bisphenol A novolac, bisphenol F novolac, triphenylmethane novolac, biphenyl novolac, dicyclopentadiene novolac, terpenphenol novolac and the like. As the phenol resin, only one type may be used, or a plurality of types may be used in combination. The phenol resin is obtained by polymerizing a phenol or a nuclear-substituted phenol derivative, a compound having a ring structure serving as a cross-linking group, and aldehydes such as formaldehyde, acetaldehyde, benzaldehyde, and salicylaldehyde in the presence of an acid catalyst. be able to. The phenol resin is preferably used in combination with another curing accelerator that promotes the polymerization reaction. When a phenol resin is used, a rigid ring structure is introduced, so that the heat resistance and mechanical strength of the neutron shielding material can be improved. As the phenol resin, a resin having high hydrogen density and rigidity is preferable, and phenol novolac is particularly preferable.

As the curing accelerator, it is preferable to use one or more of imidazoles, imidazole salts, phosphines, phosphonium salts, tertiary amines, and tertiary amine salts, and phosphines, phosphonium salts, tertiary amines, and , It is more preferable to use one or more of the tertiary amine salts. Since phosphines and tertiary amines do not have an imidazole structure, unlike imidazole compounds having a nitrogen atom at the 1st and 3rd positions of the 5-membered ring, active sites are unlikely to remain after curing. Therefore, even if the neutron shielding material is exposed to a high temperature of 150 ° C. to 200 ° C. or higher, the reaction activity does not increase remarkably, and the thermal decomposition of the epoxy resin or the like present in the cured product is reduced. Therefore, it is possible to obtain a neutron shielding material having high heat resistance, which shows higher resistance to heat aging.

As the curing agent or curing accelerator, a combination of the phenol resin and one or more of the tertiary amine and the tertiary amine salt, a combination of the phenol resin and one or more of the imidazoles and the imidazole salt, and the like are preferable. According to such a combination with a general curing accelerator, a cured product having high heat resistance can be obtained by surely advancing the polymerization reaction in a short time even in normal temperature curing or low temperature curing. Further, according to such a combination, the dilution of the hydrogen density by the curing agent or the curing accelerator can be minimized by adjusting the compounding ratio of the phenol resin or selecting the type having a large number of hydrogens. Can be done.

The main agent and the curing agent are preferably blended in the range of 0.7 or more and 1.3 or less in terms of the equivalent ratio of the epoxy equivalent and the active hydrogen equivalent. The equivalent ratio is more preferably 0.8 or more and 1.2 or less, and further preferably 0.9 or more and 1.1 or less. Within such a range, the amount of unreacted epoxy compound and curing agent remaining after the polymerization reaction is reduced. Therefore, it is possible to prevent the hydrogen density from decreasing due to the vaporization of the unreacted epoxy compound or the curing agent. The amount of the curing accelerator that mainly acts as a catalyst can be adjusted appropriately according to the type, curing conditions, and the like.

(Flame retardants)
As the flame retardant, an appropriate type can be used as long as it is chemically stable without impairing the mechanical properties of the epoxy resin and the like. However, when heat curing is performed, it is preferable to use a compound that is stable at the heating temperature. Specifically, a compound having an action at 150 ° C. or higher is preferable, and a compound having an action at 200 ° C. or higher is more preferable. As the flame retardant, one type may be used, or a plurality of types may be used in combination. The flame retardant may not be blended depending on the use of the neutron shielding material, usage conditions, usage environment, and the like.

Examples of the flame retardant include metal hydroxides such as magnesium hydroxide, aluminum hydroxide and calcium hydroxide, hydrates of metal oxides such as trihydrate alumina, and inorganic phosphoric acid compounds such as ammonium polyphosphate. , Phosphate compounds such as organic phosphorus compounds such as phosphoric acid esters, halogen compounds such as hexabromobenzene and tetrabromobisphenol A, and the like.

As the flame retardant, magnesium hydroxide or aluminum hydroxide is particularly preferable. These flame retardants are stable up to about 200 ° C., and cause an endothermic reaction at higher temperatures. For example, magnesium hydroxide starts dehydration at about 290 ° C to 310 ° C. Therefore, these flame retardants are less likely to lose their action even when they are cured at a high temperature. Therefore, even when the neutron shielding material is exposed to a high temperature of 150 ° C. to 200 ° C. or higher, thermal deterioration and combustion of the epoxy resin can be more reliably suppressed.

The flame retardant is preferably blended in an amount of 20% by mass or more and 70% by mass or less, and preferably 30% by mass or more and 60% by mass or less per neutron shielding material. When a flame retardant is added, the hydrogen density of the neutron shielding material may decrease or the viscosity of the resin composition may increase. However, with such a blending amount, it is possible to suppress thermal deterioration and combustion of the epoxy resin while maintaining high hydrogen density of the neutron shielding material and fluidity of the resin composition.

(Neutron absorber)
As the neutron absorber, an appropriate substance having a large neutron absorption cross section can be used as long as it is chemically stable without impairing the mechanical properties of the epoxy resin and the like. As the neutron absorber, one type may be used, or a plurality of types may be used in combination. The neutron absorber may not be blended depending on the use, usage conditions, usage environment, etc. of the neutron shielding material.

Examples of the neutron absorber include boron compounds such as boron carbide, boron nitride, boric anhydride, metaboric acid, and ash boric acid, single metals such as cadmium, gadolinium, and samarium, and oxides containing these elements. Examples include compounds.

As the neutron absorber, boron carbide or boron nitride is particularly preferable. These neutron absorbers are unlikely to react with a curing agent such as an epoxy compound or an acid anhydride, or a flame retardant such as magnesium hydroxide even when cured at a high temperature. Therefore, the neutron shielding performance can be further improved without significantly impairing the mechanical properties, hydrogen density, heat resistance, and the like.

The neutron absorber is preferably blended in an amount of 0.1% by mass or more and 10% by mass or less per neutron shielding material. With such a blending amount, neutrons can be efficiently absorbed without significantly impairing the deceleration performance of the neutron shielding material, the kneadability of the resin composition, and the like.

<Manufacturing method of neutron shielding material>
The neutron shielding material is a resin composition composed of a main agent containing an epoxy compound as at least one component, a curing agent for ring-opening polymerization of the epoxy group of the epoxy compound, and an optionally added curing accelerator. Can be obtained by curing by a polymerization reaction via an epoxy group. By using a phenol resin as the curing agent, a cured product having a high glass transition temperature can be obtained.

The polymerization reaction can be carried out at an appropriate temperature depending on the use of the neutron shielding material, the conditions of use, the environment of use and the like. As the curing temperature, for example, a low temperature range of 5 ° C. or higher and lower than 50 ° C., a medium temperature range of 50 ° C. or higher and lower than 80 ° C., a high temperature range of 80 ° C. or higher and 200 ° C. or lower, or the like can be used. When the curing temperature is room temperature, the heating cost and the dimensional change of the cured product can be reduced, but the higher the curing temperature, the higher the heat resistance of the epoxy resin tends to be.

When heat curing is performed at a temperature higher than room temperature, for example, heating is performed at 50 ° C. or higher and 200 ° C. or lower, preferably 130 ° C. or higher and 170 ° C. or lower, for 1 hour or longer and 12 hours or shorter, preferably 1 hour or longer and 3 hours or lower. conduct. Further, the heat curing is more preferably performed stepwise from the viewpoint of reducing thermal strain and uniformly curing. For example, two-stage heating of 80 ° C. or higher and 130 ° C. or lower for 2 hours or more and 4 hours or less and 140 ° C. or higher and 170 ° C. or lower for 4 hours or more and 12 hours or less, or one day at room temperature. It is preferable to perform three-stage heating of primary heating of about 7 days or more, secondary heating of 120 ° C. or higher and 150 ° C. or lower, and tertiary heating of 180 ° C. or higher and 200 ° C. or lower, or heating equivalent thereto.

According to the above neutron shielding material, since phenol resin is used as the curing agent, the glass transition temperature of the epoxy resin becomes high, and even when the neutron shielding material is exposed to a high temperature of 150 ° C. to 200 ° C. or higher. , Epoxy resin and other components are less likely to be thermally decomposed. Therefore, even in a high-temperature environment such as dry storage where spent nuclear fuel continues to generate heat, it is possible to prevent a decrease in hydrogen density due to vaporization and the generation of non-shielding portions due to deformation of the neutron shielding material. Therefore, it is possible to obtain a neutron shielding material having high heat resistance and whose neutron shielding performance is unlikely to deteriorate even in a high temperature environment.

The above neutron shielding material shall be applied to the radiation shielding part of containers for storing or transporting radioactive materials, such as used fuel assembly storage containers, radioactive material transport containers, radioactive material storage containers, and reactor vessels. Can be done. In addition, it can be applied to radiation shielding parts provided in radioactive material handling facilities such as nuclear fuel reprocessing facilities, spent nuclear fuel storage facilities, and accelerator facilities.

Next, a cask (spent fuel assembly storage container) provided with the above-mentioned neutron shielding material will be described.

FIG. 1 is a perspective view showing an example of a spent fuel assembly storage container.
As shown in FIG. 1, the cask 1 has an inner cylinder (inner container) 2, an outer cylinder (outer container) 3 installed outside the inner cylinder (inner container) 2, and an inner cylinder (inner container) 2. A basket 5 for storing the spent fuel assembly and a neutron shielding material 6 surrounding the inner cylinder (inner container) 2 are provided. In FIG. 1, a part of the cask 1 having a substantially cylindrical shape is cut out to show the internal structure and the cross-sectional structure thereof.

The cask 1 is a storage container used for storing or transporting a spent fuel assembly, and the container body is composed of a bottomed inner cylinder 2 and an outer cylinder 3. The inner cylinder 2 is made of carbon steel or the like in order to shield gamma rays. A heat transfer fin 4 that dissipates heat from the spent fuel assembly is provided on the side surface of the inner cylinder 2. The heat transfer fin 4 is formed of, for example, a metal plate such as copper, aluminum, or an alloy thereof. The side surface of the inner cylinder 2 is covered with an outer cylinder 3 installed outside the heat transfer fins 4.

The basket 5 is installed inside the inner cylinder 2 and has a grid-like partition. The basket 5 contains spent fuel assemblies (not shown) stored in the cask 1 so as to have a load of, for example, about 30 to 70. Spent fuel assemblies are stored separated from each other by metal partition plates provided in a grid pattern.

The inner cylinder 2 constituting the container body of the cask 1 has a primary lid 7 and a secondary lid 8 attached to the upper opening. The spent fuel assembly housed in the cask 1 is sealed in the inner cylinder 2 by a double structure of an inner primary lid 7 and an outer secondary lid 8. A large number of trunnions 9 are provided on the side surface of the outer cylinder 3 as a portion for supporting the cask 1 on a support stand or the like and fixing the cask 1 to a transport hanger or the like.

In the cask 1 shown in FIG. 1, a plurality of heat transfer fins 4 extend along the circumferential direction of the container body between the inner cylinder 2 and the outer cylinder 3. The heat transfer fins 4 are arranged at intervals from each other, and a space is provided between the inner cylinder 2 and the outer cylinder 3 so as to surround the inner cylinder 2. Further, the lid body such as the primary lid 7 can be provided in a hollow structure while ensuring the shielding performance against radiation from the spent fuel assembly, secondary gamma rays and the like. The neutron shielding material containing the epoxy resin as a main component can be arranged in one or more of these spaces so as to surround the inner cylinder 2.

The neutron shielding material 6 preferably has a hydrogen density of 5 × 10 ²² / cm ³ or more. Further, when exposed to a high temperature of 150 ° C. to 170 ° C. for 10,000 hours, the weight loss rate under the condition of the maximum reaching temperature of 150 ° C. is 0.02% or less, and the weight loss rate under the condition of the maximum reaching temperature of 170 ° C. is 0. It is preferably 1% or less. Generally, the maximum operating temperature of a dry cask is assumed to be about 170 ° C to 200 ° C. If the neutron shielding material 6 has such a weight reduction rate, it is possible to maintain a high neutron shielding ability while sufficiently suppressing heat aging in the operating temperature range of the cask 1.

The neutron shielding material 6 may be formed by filling a space surrounding the inner cylinder 2 with a resin composition and curing at room temperature or heating. Alternatively, the molding die may be filled with the resin composition, molded into a predetermined shape by normal temperature curing or heat curing, and the molded cured product may be arranged in a space surrounding the inner cylinder 2. The flame retardant and the neutron absorber may or may not be blended in the resin composition depending on the usage conditions, the usage environment, and the like of the neutron shielding material 6.

According to the cask (spent fuel assembly storage container) described above, a neutron shielding material having high heat resistance and whose neutron shielding performance is unlikely to deteriorate even in a high temperature environment is provided at a position surrounding the inner cylinder 2. Therefore, even if the spent fuel assembly, which is generally expected to have a storage period of about 50 years, continues to generate heat during storage, the thermal aging of the neutron shielding material is suppressed, and the neutron shielding performance of the cask deteriorates. It becomes difficult and the storage of the spent fuel assembly can be safely continued for a long period of time.

For example, according to the above cask, the neutron shielding performance is unlikely to deteriorate even when the number of used nuclear fuel assemblies loaded on the cask is increased or when the spent nuclear fuel aggregates with high burnup are stored. A storage container can be provided. That is, even when the cooling period in the cooling pool in the nuclear power plant is short or when the spent nuclear fuel aggregate with high burnup is stored, the number of used nuclear fuel aggregates loaded on the cask can be increased. It is possible to reduce the weight of the cask, reduce the occupied volume for storage, and the like.

Although the embodiments of the neutron shielding material and the spent fuel assembly storage container according to the present invention have been described above, the present invention is not limited to the above-described embodiments and can be various as long as it does not deviate from the technical scope. Modifications are included. For example, the above embodiment is not necessarily limited to those having all the configurations described. Further, it is possible to replace a part of the configuration of the embodiment with another configuration or add another configuration to the configuration of the embodiment. Further, it is also possible to add another configuration, delete the configuration, or replace the configuration with respect to a part of the configuration of the embodiment.

Hereinafter, the present invention will be specifically described with reference to examples, but the technical scope of the present invention is not limited thereto.

Neutron shielding materials were prepared by changing the types of curing agents and curing accelerators, and the heat resistance was evaluated based on the weight reduction rate by thermogravimetric analysis.

As the main agent, hydrogenated bisphenol A type epoxy resin (EP-4080E manufactured by ADEKA CORPORATION) was used.

As the curing agent, acid anhydride or phenol resin was used. As the acid anhydride, methyl-3,6-endomethylene-1,2,3,6-tetrahydrophthalic anhydride (MNA) was used. As the phenol resin, the phenol resin shown in Table 1 was used. The amount of the curing agent blended was 65 parts by weight with respect to 100 parts by weight of the main agent for the acid anhydride. For the phenol resin, the epoxy equivalent and the active hydrogen equivalent were set to equal 1: 1.

As the curing accelerator, 2-ethyl-4-methylimidazole (2E4MZ) (manufactured by Shikoku Chemicals Corporation) was used. The blending amount of the curing accelerator was 5 parts by weight with respect to 100 parts by weight of the main agent.

Aluminum hydroxide "CL-310" (manufactured by Sumitomo Chemical Co., Ltd.) was used as the flame retardant. The blending amount of the flame retardant was 170 parts by weight with respect to 100 parts by weight of the resin composition.

Boron carbide powder "F-400" (manufactured by Denka Co., Ltd.) was used as the neutron absorber. The blending amount of the neutron absorber was 4 parts by weight with respect to 100 parts by weight of the resin composition.

First, the glass transition temperature of a resin composition containing a main agent, a curing agent, and a curing accelerator was measured using a differential thermogravimetric analyzer. As a measurement sample, 10 to 15 mg of fine pieces cut out from a cured product that had been heat-cured was used. The heat curing was carried out by drying in a dryer adjusted to 45 ° C. for 2 days, heating at 80 ° C. for 2 hours, and further heating at 150 ° C. for 2 hours. The glass transition temperature was determined by measuring the endothermic change in the temperature rise process, with the measurement conditions being a temperature rise of 10 ° C./min from the initial temperature to 250 ° C. The results are shown in Table 2.

Next, the resin composition containing the main agent, the curing agent, and the curing accelerator was subjected to thermogravimetric measurement using a differential thermogravimetric analyzer. As the measurement sample, 10 to 15 mg of fine pieces cut out from either a cured product cured at room temperature using phenol novolac as a curing agent or a cured product cured by heating was used. The room temperature curing was performed by leaving the resin composition at room temperature of 20 to 25 ° C. Further, the heat curing was carried out by drying in a dryer whose temperature was adjusted to 45 ° C. for 2 days, heating at 80 ° C. for 2 hours, and further heating at 150 ° C. for 2 hours.

The thermogravimetric analysis was performed with the maximum temperature reached as one of 150 ° C, 160 ° C and 170 ° C. For each of the prepared measurement samples, raise the temperature from the initial temperature to 40 ° C. at 5 ° C./min, hold at 40 ° C. for 10 minutes, raise the temperature from 40 ° C. to each maximum temperature at 5 ° C./min, and at each maximum temperature. After holding for 600 minutes, the weight of the measurement sample was weighed.

Then, based on the result of thermogravimetric analysis, the weight loss rate after heating for 600 minutes was determined. The weight loss rate was calculated according to the following mathematical formula (1), where the weight before the heating test was X ₀ and the weight after heating for 600 minutes was X _t.
Weight loss rate (%) = (X _0- X _t ) / X ₀ x 100 ... (1)

Further, using the same measurement sample, the glass transition temperature was measured for each of the cured product cured at room temperature and the cured product cured by heating. The heating conditions were the same as those of the measurement shown in Table 2.

FIG. 2 is a diagram showing the results of measuring the weight loss rate of the epoxy resin composition with heating. Further, FIG. 3 is a diagram showing the results of measuring the glass transition temperature of the resin composition of the epoxy resin.
As shown in FIGS. 2 and 3, when an acid anhydride was used as the curing agent (see the graph on the right), when phenol novolac was used as the curing agent (see the graph on the left), the temperature was 150 ° C. and 160. The weight loss rate was greatly reduced and the glass transition temperature was remarkably increased by both heating at ° C. and 170 ° C. In addition, the weight loss rate decreased and the glass transition temperature increased when the epoxy resin was heat-cured (see the left side of the graph) as compared with the case where the epoxy resin was cured at room temperature (see the center of the graph). As shown in Table 2, all epoxy resins containing various phenol resins show a high glass transition temperature of around 150 ° C. When phenol resin is used as a curing agent, the heat resistance of the epoxy resin is improved. It was confirmed that the higher the value and the more the heat was cured, the greater the heat resistance was improved.

Next, thermogravimetric analysis was performed on a neutron shielding material in which a flame retardant and a neutron absorber were mixed with a resin composition of an epoxy resin. The neutron shielding material was prepared by the following procedure. First, the main agent, the curing agent, and the curing accelerator were weighed to a predetermined blending amount, and these components were kneaded using a planetary mixer. Then, the flame retardant and the neutron shielding material were added so as to have a predetermined blending amount, and further kneaded to obtain a resin composition in which the flame retardant and the neutron shielding material were mixed. This resin composition was filled in a SUS pipe having a diameter of 20 mm and a length of 100 mm and cured by heating to obtain a neutron shielding material composed of a cured product of an epoxy resin.

The thermogravimetric analysis of the produced neutron shielding material was performed using a differential thermogravimetric analyzer. The curing conditions, measurement samples, and measurement conditions of the neutron shielding material were set to 150 ° C. in the same manner as for the resin composition described above. Table 3 shows the composition of the neutron shielding material and the results of the weight loss rate after heating for 600 minutes.

As shown in Table 3, No. 1 using an acid anhydride as a curing agent. It was confirmed that the neutron shielding material using the phenol resin had a significantly reduced weight loss rate at 150 ° C. and improved heat resistance as compared with No. 3. In addition, No. 1 containing a flame retardant. In No. 2, no flame retardant was added. Compared with 1, the weight loss rate was reduced and the heat resistance was higher.

1 cask (spent fuel assembly storage container)
2 Inner cylinder (inner container)
3 Outer cylinder (outer container)
4 Heat transfer fins 5 Basket 6 Neutron shielding material 7 Primary lid 8 Secondary lid 9 Trunnion

Claims (9)

A neutron shielding material containing an epoxy resin as one of its main components, wherein the epoxy resin is ring-open polymerized with a main agent containing a compound containing two or more epoxy groups in the molecule as at least one component. A neutron shielding material composed of a curing agent and a cured product of a resin composition containing the curing agent, wherein the curing agent is a phenol resin. The neutron shielding material according to claim 1, wherein the main agent contains one or more of hydrogenated bisphenol A diglycidyl ether and hydrogenated bisphenol F diglycidyl ether. The neutron shielding material according to claim 1, wherein the phenol resin is one or more of phenol novolac, xylylene novolac, bisphenol A novolak, triphenylmethane novolac, biphenyl novolac, dicyclopentadiene novolac, and terpenphenol novolac. .. The epoxy resin contains a curing accelerator that promotes polymerization via the epoxy group.
The neutron shielding material according to claim 1, wherein the curing accelerator is one or more of imidazoles, imidazoles salts, phosphines, phosphonium salts, tertiary amines, and tertiary amine salts. The main agent contains one or more of hydrogenated bisphenol A diglycidyl ether and hydrogenated bisphenol F diglycidyl ether.
Furthermore, bisphenol A type epoxy compound, bisphenol A type epoxy resin, bisphenol F type epoxy compound, bisphenol F type epoxy resin, phenol resin type epoxy compound, phenol resin type epoxy resin, glycidyl ether type epoxy compound, glycidyl ether type epoxy resin, The neutron shielding material according to claim 1, which comprises one or more of a glycidyl ester type epoxy compound, a glycidyl ester type epoxy resin, a glycidyl amine type epoxy compound, and a glycidyl amine type epoxy resin. The main agent contains one or more of hydrogenated bisphenol A diglycidyl ether and hydrogenated bisphenol F diglycidyl ether.
The neutron shielding material according to claim 1, which comprises a polyfunctional epoxy compound containing three or more epoxy groups in the molecule and one or more of the polyfunctional epoxy resins obtained by polymerizing the epoxy compound. The resin composition contains a flame retardant and contains
Claim 1 in which the flame retardant is one or more of magnesium hydroxide, aluminum hydroxide, calcium hydroxide, ammonium polyphosphate, an inorganic phosphoric acid compound, a phosphoric acid ester, hexabromobenzene, and tetrabromobisphenol A. The neutron shielding material described in. The resin composition contains a neutron absorber and contains
The neutron shielding material according to claim 1, wherein the neutron absorber is one or more of boron carbide and boron nitride. An inner container, an outer container installed outside the inner container, a basket installed inside the inner container for storing spent fuel assemblies, and a neutron shielding material surrounding the inner container. With
The neutron shielding material is a neutron shielding material containing an epoxy resin as one of the main components, and the epoxy resin contains a main agent containing a compound containing two or more epoxy groups in the molecule as at least one component, and the epoxy. A used fuel assembly storage container composed of a cured product of a resin composition containing a curing agent for ring-opening polymerization of a group, wherein the curing agent is a phenol resin.

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