RU2194783C1 - Method of ceramic nuclear fuel recovery from fuel elements and fuel assemblies - Google Patents

Abstract

FIELD: pyrometallurgical methods of reprocessing of spent fuel, mainly, fuel based on uranium and plutonium. SUBSTANCE: method includes melting of metal structural materials, particularly, fuel elements and fuel, assemblies in inert atmosphere and separation of melt from solid fuel. Melting is carried out in crucible of melting furnace with zinc melt. Process temperature is 600-1200 C. Zinc content in melt is up to 50-98 mas.%. After separation, melt is cooled to 430-1000 C. Upper layer of cooled melt is discharged and returned to process cycle. Melt after separation from solid fuel is filtered, and solid fuel is washed with zinc melt. Washing zinc melt after separation from solid fuel is filtered and directed for recovery of ceramic nuclear fuel. Process of recovery of ceramic nuclear fuel and melt cooling are carried out in vacuum induction furnaces with crucibles transparent for electromagnetic field. Frequency of induction currents is 50-250000 Hz. EFFECT: exclusion of operations involved in disassembly of fuel assemblies, reduction of multi-stage and labor input in process, reduction of volumes of radioactive metal wastes, dust and aerosols. 8 cl, 2 ex

Description

 The invention relates to pyrometallurgical methods for the regeneration of spent nuclear fuel (SNF) mainly based on uranium and plutonium.

 There are methods for the mechanical extraction of uranium oxide oxide from metal shells of fuel elements (fuel rods) made of niobium doped zirconium, which consists in disassembling fuel assemblies (FA) into individual fuel rods and subsequent cutting [1], cutting [2] or deformation [3] ; or - in cutting the shanks and chopping the entire fuel assembly into separate pieces without disassembling into individual fuel elements [4]. The disadvantage of these methods is the multi-stage and the formation of large quantities of dust and aerosols of oxide fuel, which requires powerful and highly efficient gas cleaning systems. As a result of disassembling fuel assemblies and cutting fuel rods, a large number of fragments of zirconium shells (the so-called zirconium husk) are formed, which are fire hazardous and require the construction of special storage facilities of large volumes.

The closest in technical essence and the achieved result is a thermal method for extracting nuclear fuel from fuel elements [A.S. USSR 357596, publ. 10.31.72 g. Bull. N 33] with the separation of fuel from structural materials by heating to the melting temperature of the shells, according to which the fuel elements are placed in a container made of iron with a mass of 15-25% by weight of the structural parts of the fuel elements and acting as a solid additive, and carry out the process in a vacuum or inert atmosphere at a temperature of 1000-1300 ^o C.

The disadvantages of this method are:
- its applicability only for the processing of fuel elements whose shells are made of zirconium-based alloys;
- the need to manufacture containers of iron, which during the processing of fuel rods will be part of radioactive metal waste and cannot be removed from them in an economically acceptable way for reuse in the production cycle.

The technical result of the proposed solution is:
- development of a universal method for the extraction of ceramic spent nuclear fuel from fuel rods and fuel assemblies, structural parts (fuel cladding, end parts and spacer grids of fuel assemblies, etc.) which are made both from zirconium-based alloys and from iron-based alloys (stainless and others types of steels) and their combinations, including fuel pins of the Brest reactor containing a lead layer;
- the exclusion of operations for the dismantling of fuel assemblies and, as a consequence, the reduction of multi-stage and labor-intensive process;
- reduction of the volume of generated radioactive metal waste, dust and aerosols.

The technical result is achieved in that in a method for extracting ceramic nuclear fuel from fuel elements, including melting in an inert gas atmosphere of metallic structural materials and separating the obtained melt from solid fuel, according to the invention, the fuel elements and fuel assemblies are subjected to melting, the melting process is carried out in a crucible of a melting furnace zinc melt at 600-1200 ^o C., the zinc content of the melt is maintained in the range 50-98 wt.%, the melt is separated from the t erdogo fuel, cooled to 430-1000 ^o C, the top layer of the cooled melt is drained and returned to the process cycle.

 The melt after separation from solid fuel is filtered.

 Solid fuel is washed with zinc melt.

 Washing zinc melt after separation from solid fuel is directed to the extraction of ceramic nuclear fuel from fuel elements. Wash zinc melt after separation from solid fuel is filtered. The upper layer of the cooled melt is drained and returned to the washing of the solid fuel and / or to the extraction of ceramic nuclear fuel from the fuel elements.

 The processes of melting, melt cooling and washing are carried out in vacuum induction furnaces with crucibles transparent to the electromagnetic field, mainly metal split water-cooled crucibles, with electromagnetic stirring of the melt.

 The processes are conducted at a frequency of induction currents of 50-250000 Hz.

The main distinguishing features of the proposed process are:
- immersion of the processed fuel rods or fuel assemblies without preliminary disassembly into individual fuel rods or bundles of fuel rods in a zinc melt;
- maintaining the zinc content in the melt in the range of 50-98 wt.%;
- carrying out the process at 600-1200 ^o C;
- cooling the melt after separation from solid fuel to 430-1000 ^o C;
- draining and returning the upper layer of the cooled melt to the production cycle.

 The method is based on the fact that almost all metals that are part of the structural materials of fuel elements and fuel assemblies: Zr, Nb, Fe, Cr, Ni, Pb, etc. are dissolved in zinc, but the components of ceramic nuclear fuel are practically not dissolved. As a result of the dissolution of the metal structural parts of fuel elements and fuel assemblies, a two-phase system will be formed, consisting of a metal melt and solid ceramic fuel, followed by phase separation by specific density directly in the melting crucible. When draining the melt from the crucible, if necessary, separation of suspended particles of solid fuel, the melt is subjected to control filtration.

Before starting the processing process in a melting crucible, a bath of fusible zinc melt is prepared. A batch of spent fuel rods or a fuel assembly is uniformly immersed in the melt, in which the dissolution of the metal structural parts occurs under the influence of factors such as high temperature, the aggressive environment of the metal zinc melt and its intense electromagnetic mixing. It is obvious that, due to their physicochemical characteristics, the metals included in the structural materials of fuel elements and fuel assemblies, after dissolution, will be concentrated in the metal melt. As a result of this operation, a two-phase system is formed in the melting crucible, consisting of a zinc-based melt with metals dissolved in it, which are part of the structural materials of fuel elements, such as zirconium, niobium, iron, chromium, nickel, lead, and some others, as well as fragments of solid ceramic fuels of various sizes. The latter, as having a high density (10-13 g / cm ³ ), will settle in the lower part of the crucible. A metal melt containing small amounts of finely divided particles of nuclear fuel in suspension is located in the upper part of the crucible, and also fills the open pores of solid fuel. During the processing of fuel rods, as solid fuel accumulates in the crucible to the maximum permissible value, the zinc-based melt is poured into another melting furnace. The solid fuel particles present in the melted melts are filtered off, if necessary. The residual metal melt is washed from the pores of the solid fuel with a regenerated zinc melt, which, if necessary, is filtered and sent to the operation for extracting solid fuel from the fuel elements. Solid fuel is discharged from the crucible and sent for further processing.

The regeneration of a zinc melt containing, for example, zirconium with niobium or iron with chromium and lead, as well as other metals dissolved in zinc, is carried out by segregation. The method is based on the precipitation of intermetallic crystals from the melt upon cooling: Zr ₃ Zn ₂ , ZrZn, ZrZn ₂ , ZrZn ₃ , ZrZn ₆ , ZrZn ₁₄ , ZrZn ₂₂ , NbZn, Nb ₂ Zn ₃ , NbZr ₃ , NbZn ₇ , NbZn ₁₅ , FeZn ₁₃ , CrZn ₁₇ , NiZn ₃ , Ni ₃ Zn ₁₀ , Ni ₅ Zn ₂₁ , Ni ₄ Zn ₂₂ and others, as well as the limited solubility of lead in zinc, which decreases upon cooling. In the process of cooling a melt based on zinc containing zirconium niobium, iron, chromium and nickel (the main structural materials of the fuel assemblies of the WWER-1000 reactor), two layers are obtained as a result of segregation:
1) the lower layer - solid intermetallic compounds of the type Zr _m Zn _n , Nb _m Zn _n , FeZn ₁₃ , CrZn ₁₇ , Ni _m Zn _n , etc., which in the form of a spongy mass settle to the bottom of the crucible;
2) the top layer is liquid refined (regenerated) zinc.

In the process of cooling a melt based on zinc containing iron, chromium and lead (the main structural materials of the fuel elements of the Brest reactor), three layers are obtained:
1) the lower layer is a liquid alloy of lead with zinc containing 5-6 wt.% Zinc;
2) the middle layer - solid intermetallic compounds of the type FeZn ₁₃ , CrZn ₁₇ and others, which are located in the form of a spongy mass above the lead-zinc melt;
3) the top layer is liquid refined (regenerated) zinc.

In the process of processing fuel elements and fuel assemblies, insignificant (as the process is conducted at relatively low temperatures) reduction of ceramic uranium and plutonium fuel to metals: uranium and plutonium, respectively, can occur. However, metallic uranium and plutonium also form intermetallic compounds (U ₂ Zn ₁₇ , PuZn ₂ , Pu ₂ Zn ₉ , PuZn ₈ Pu ₂ Zn ₁₇ ) with zinc. Consequently, during the circulation of the zinc melt in the technological cycle, uranium and plutonium will not accumulate in it, since they will be removed into the intermetallic phase together with zirconium, niobium, iron, chromium, nickel and other metals during segregation refining. This is a very important positive factor, since the accumulation of uranium-235 and plutonium can lead to a spontaneous chain reaction (SCR).

 Refined (regenerated) zinc is poured through the top of the crucible and sent to the operation of washing solid fuel and / or removing it (fuel) from the next batch of fuel rods. The bottom layer is an alloy of lead with zinc is removed through a drain device in the pan and sent to extract zinc by vacuum distillation. The spongy mass of chemical compounds (intermetallic compounds) of zirconium, niobium, chromium iron, nickel, and other metals with zinc remaining in the crucible is discharged from the crucible and also sent to the zinc extraction by vacuum distillation in order to return to the technological cycle.

 Maintaining the zinc content in the melt at a level of not less than 50 wt.% Is due to the fact that at lower values, melts are formed with higher liquidus temperatures, which leads to significant losses of volatile zinc as a result of evaporation, and also increases the likelihood of solid SNF dissolving in the metal melt and increases the energy intensity of the process. When the zinc content in the melt is more than 98 wt.%, The process productivity sharply decreases.

The process of extraction of ceramic nuclear fuel from fuel rods at 600-1200 ^o C is due to the fact that in this interval provides a quick and complete dissolution in the zinc melt of all metals that are part of the structural parts of the fuel rods, but does not dissolve SNF.

Liquidation cleaning is carried out at 430-1000 ^o C depending on the composition of the initial melt. This interval is established experimentally and is due to the maximum achievement of the necessary degree of purification of zinc from metals dissolved in it.

For technically and cost-effective operations for the extraction of nuclear fuel based on uranium and plutonium from fuel elements in an aggressive environment of metal melt, a special melting furnace is needed that allows the following conditions to be realized:
- eliminate contamination of recoverable nuclear fuel with the material of the melting crucible;
- to eliminate the absorption by the melting crucible of radioactive elements in order to avoid the formation of a new type of radioactive waste in the form of spent crucible materials;
- to provide electromagnetic mixing of the melt, intensifying the dissolution of structural materials of fuel elements and fuel assemblies in zinc melt;
- ensure the implementation of technological operations in an inert gas atmosphere;
- to provide the ability to move the melt and solid fuel inside the crucible in the desired direction (up or down), as well as drain the melt through a trench located in the upper part of the crucible or through an opening in the pallet;
- provide almost inertialess control of the temperature of the melt;
- to provide a radical solution to the problem of longevity of the crucible, namely it should work for several years and not require intermediate cleanings and repairs with the participation of maintenance personnel.

 Such properties are possessed by vacuum induction furnaces with metal split water-cooled (cold) crucibles (IPCT), equipped with moving pallets with drain devices, as well as drain troughs located in the upper part of the crucible. As industrial experience has shown, the life cycle of IPCP reaches 18 years or more, which virtually eliminates the formation of a class of radioactive waste such as spent crucible materials. The use of induction currents in a relatively low frequency range (50-250000 Hz) allows the use of simple and reliable sources of power supply, provides intensive mixing of the metal melt, and also greatly simplifies the design of IPCT.

 It is obvious that liquation refining (regeneration) of spent zinc melt is most efficiently carried out in IPCP of the same type as for the extraction of ceramic nuclear fuel from fuel elements.

 An experimental verification of the proposed method was carried out in an argon atmosphere in two induction furnaces (current frequency 2400 Hz) with transparent sectioned water-cooled (cold) crucibles with a diameter of 100 mm, transparent for the electromagnetic field, with drain chutes in the upper part of the crucibles and devices for bottom discharge of the melt in moving pallets. Ceramic filters were installed under the drain troughs and bottom discharge devices to capture solid fuel particles.

Example 1. In a cold crucible of the first furnace, 4 kg of pure zinc were charged and melted. A VVER-1000 reactor fuel assembly simulator was loaded into the melt: 340 g of steel 12Kh18N10T and 600 g of tubes of alloys 110-E and 125-E (zirconium base + 1 and 2.5 wt.% Niobium, respectively) containing 1.6 kg of dioxide inside uranium. The process was conducted until complete dissolution of stainless steel and shells of alloys 110-E and 125-E for 25 min at 750 ^o C and electromagnetic stirring of the melt. After that, the plug of the drain device was opened, the melt containing 87 wt. % of zinc was poured onto the filter, and after filtration, into a portable heated mold, from which the melt was poured into the cold crucible of the second furnace. This melt was cooled to 550 ^° C for 50 minutes to allow the precipitation of crystals of intermetallic compounds of the type Zr _m Zn _n , Nb _m Zn _n , Fe _m Zn _n , Cr _m Zn _n , Ni _m Zn _n , etc. Next, the drain plug was opened and the molten refined (regenerated) zinc was poured into the heated mold, and from it into the crucible of the first furnace to wash the uranium dioxide located there. The washing process was carried out at 700 ^o C for 15 min with electromagnetic stirring of the melt. After that, the drain plug was opened and the washing melt was poured onto the filter, after which it fell into the mold. The washed uranium dioxide was discharged from the crucible, weighed, and the difference was determined by the loss of uranium dioxide during technological operations, which amounted to 0.08 wt. % Also analyzed the composition of the zinc melt after refining (regeneration) by segregation: the content of zirconium, niobium, iron, chromium, nickel and uranium in it was 0.15; 0, 008; 0.08; 0.007; 0.012 and 0.0015 wt.%, Respectively. The content of zirconium, niobium, iron, chromium, nickel and uranium in the wash melt was 0.23; 0,012; 0.13; 0.011; 0.019 and 0.0017 wt.%, Respectively.

Example 2. In a cold crucible of the first furnace, 10 kg of pure zinc were charged and melted. The Brest reactor fuel element simulator was loaded into the melt: 600 g of tubes made of EP-823 alloy (iron base + 12 wt.% Chromium) containing 1.7 kg of a mixture of uranium and plutonium mononitrides and 260 g of lead inside. The process was carried out until the dissolution of the shells of alloy EP-823 and lead for 20 min at 700 ^o C and electromagnetic stirring of the melt. After that, the drain plug was opened, the melt containing 92 wt.% Zinc was poured onto the filter, and after filtration, into a portable heated mold. From the mold, the melt was poured into the cold crucible of the second furnace. This melt was cooled to a temperature of 450 ^o C for 40 minutes until the precipitation of crystals of FeZn ₁₃ and CrZn ₁₇ intermetallic crystals proceeded completely, as well as to form a lead melt layer containing zinc in the lower zone of the crucible. Next, the crucible pan was moved upward until the upper melt layer containing refined (regenerated) zinc in the heated mold was completely removed through the drain trough. After that, the plug of the drain device in the pan was opened and the lead melt containing 5.7 wt.% Was drained. zinc into the mold, where it crystallized. The obtained melt of refined (regenerated) zinc was poured into the crucible of the first furnace for washing the mixture of uranium and plutonium mononitrides located there. The washing process was carried out at 650 ^o C for 15 min with electromagnetic stirring of the melt. The drain plug in the pan was opened and the washing melt was poured onto the filter, after which it fell into the mold. The washed mixture of mononitrides was unloaded from the crucible, weighed, and losses were determined by the difference during technological operations, which amounted to 0.06 wt.%. The composition of the zinc melt after refining (regeneration) by segregation was also analyzed by the segregation method: the containment of chromium, uranium, and plutonium iron was 0.1; 0.019; 0.0012 and 0.0002 wt.%, Respectively. The content of iron, chromium, uranium and plutonium in the wash melt was 0.18; 0.024; 0.0013 and less than 0.0001 wt.%, Respectively.

 The above examples confirm the effectiveness of the proposed method.

Literature
1. British patent 1096745, 1967.

 2. British patent 1171257, 1969.

 3. Abdel-Rassoni A. et al. J. Nuclear Energy, 1969, 23, p. 551.

 4. Kondratiev A.N. et al., in Sat Proceedings of the CMEA Symposium "Research in the field of processing of irradiated fuel", vol. 1, Prague, ed. KAE, USSR, 1972, p. 174.

Claims (8)

1. A method of extracting ceramic nuclear fuel from fuel elements, including melting in an inert gas atmosphere of metallic structural materials and separating the obtained melt from solid fuel, characterized in that the fuel elements and fuel assemblies are melted, the melting process is carried out in a crucible of a zinc-melting furnace at 600-1200 ^o C, the zinc content in the melt is maintained in the range of 50-98 wt. %, the melt is separated from solid fuel, cooled to 430-1000 ^o C, the upper layer of the cooled melt is drained and returned to the technological cycle.  2. The method according to p. 1, characterized in that the melt after separation from solid fuel is filtered.  3. The method according to p. 1, characterized in that the solid fuel is washed with zinc melt.  4. The method according to p. 3, characterized in that the washing zinc melt, after separation from solid fuel, is directed to the extraction of ceramic nuclear fuel from the fuel elements.  5. The method according to p. 3, characterized in that the washing zinc melt, after separation from solid fuel is filtered.  6. The method according to p. 1, characterized in that the upper layer of chilled zinc melt is drained and returned to the washing of solid fuel and / or to extract fuel from the fuel elements.  7. The method according to p. 1 or 3, characterized in that the processes of melting, cooling the melt and washing are carried out in vacuum induction furnaces with crucibles transparent to the electromagnetic field, mainly metal split water-cooled crucibles, with electromagnetic stirring of the melt.  8. The method according to any one of paragraphs. 1, 3 and 7, characterized in that the process is carried out at a frequency of induction currents of 50-250000 Hz.