The method of extraction of ceramic nuclear fuel from the fuel elements and fuel assemblies

 

(57) Abstract:

The invention relates to a pyrometallurgical methods of regeneration of the spent nuclear fuel mainly on the basis of uranium and plutonium. The method includes melting of metallic construction materials, namely fuel elements and fuel assemblies, in an inert atmosphere, and separating the melt from the solid fuel. Melting lead in a crucible melting furnace with zinc melt. The process temperature of 600-1200oC. the zinc Content in the melt 50-98 wt.%. After separation of the melt is cooled to 430-1000oC. the Top layer of the cooled melt is drained and returned to the process cycle. The melt after separation from the solid fuel is filtered, the solid fuel is washed zinc melt. Leaching zinc melt after separation from the solid fuel is filtered and sent for removing ceramic nuclear fuel. Extraction of ceramic nuclear fuel and cooling of the molten lead in vacuum induction furnaces, crucibles transparent to electromagnetic fields. Frequency induction currents 50-250000 Hz. The method allows to exclude the operation of disassembling fuel assemblies, to reduce a lot the of asola. 7 C.p. f-crystals.

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

There are ways for mechanical recovery of uranium oxide fuel from the metal shells of the fuel elements (cartridges), manufactured from Zirconia doped with niobium, which consists in the disassembly of the fuel assemblies (FA) to separate the fuel and the subsequent cutting [1], cutting [2] or strain [3] ; or cutting shank and cutting all fuel assemblies into separate pieces without disassembly into individual fuel rods [4]. The disadvantage of these methods are multi-stage and produce large amounts of dust and aerosols oxide fuel, which requires a powerful and highly efficient gas cleaning systems. In the disassembly of FAS and cutting fuel rods produce large quantities of fragments of zirconium membranes (so-called zirconium husk), which are flammable and require the construction of special stores large amounts.

The closest in technical essence and the achieved result is a thermal method of extracting nuclear fuel from the fuel elements [A. S. USSR 357596, op what I shells, according to which the fuel elements are placed in a container made of iron 15-25% by weight of structural parts of the fuel elements and the role of solid additives, and lead the process in a vacuum or inert atmosphere at a temperature of 1000-1300oC.

The disadvantages of this method are:

- applicability only for the reprocessing of fuel rods, with shells made of alloys based on zirconium;

- the need to produce containers of iron, which during the processing of the fuel rods will be part of the radioactive metal waste and cannot be extracted economically acceptable way for reuse in the production cycle.

The technical result of the proposed solution is:

- to develop a universal method of extracting ceramic spent nuclear fuel rods and fuel assemblies of constructional parts (the fuel cladding, the end parts and the spacer grids FA and others) are made of alloys based on zirconium, and alloys based on iron (stainless steel, and other types of steels and their combinations, including reactor fuel elements "Brest", containing lead layer;

- reducing the volume of generated radioactive metal waste, dust and aerosols.

The technical result is achieved in that in the method of extraction of ceramic nuclear fuel from the fuel elements, comprising melting in an atmosphere of inert gas metal structural materials and separation of the resulting melt from the solid fuel according to the invention melt is subjected to fuel elements and fuel assemblies, the process of melting lead in a crucible melting furnace with zinc melt at 600-1200oWith, the zinc content in the melt is maintained within the range of 50-98 wt.%, the melt is separated from the solid fuel, cooled to 430-1000oWith the top layer of the cooled melt is drained and returned to the process cycle.

The melt after separation from the solid fuel filter.

Solid fuel washed zinc melt.

Leaching zinc melt after separation from the solid fuel is sent for removing nuclear fuel from the fuel elements. Leaching zinc melt after separation from the solid fuel filter. The top layer of the cooled melt is drained and vozvratov.

The processes of melting, cooling the melt and lead leaching in vacuum induction furnaces, crucibles transparent to electromagnetic fields, mainly metal split water-cooled crucibles, when the electromagnetic stirring of the melt.

The processes carried out at a frequency induction currents 50-250000 Hz.

The main distinctive features of the proposed process are:

- dip processed fuel rods or fuel assemblies Assembly without prior disassembly into individual fuel rods or bundles of fuel rods in a zinc melt;

maintaining the zinc content in the melt within 50-98 wt.%;

- execution of the process at 600-1200oC;

- cooling of the melt after separation from the solid fuel to 430-1000oC;

- drain and return the top layer of the cooled melt into the production cycle.

The method is based on the fact that the zinc dissolves almost all metals included in the structural materials of the fuel rods and fuel assemblies: Zr, Nb, Fe, Cr, Ni, Pb and others , but practically does not dissolve the components of ceramic nuclear fuel. As a result of dissolution of the metallic structural parts of the fuel rods and fuel assemblies will be formed two-phase system, sooy density directly in the melting crucible. The discharge of the melt from the crucible, if necessary, separation of suspended particles of solid fuel melt is subjected to control filtering.

Before beginning the process of processing in the melting crucible is prepared bath fusible zinc melt. The party spent fuel rods or fuel Assembly evenly immersed in the melt, in which the dissolution of the metallic structural parts under the influence of such factors as high temperatures, corrosive environment zinc metal melt and his intense electromagnetic stirring. It is obvious that the physico-chemical characteristics of the metals included in the materials of construction of the fuel rods and fuel assemblies, after dissolution will be concentrated in the metal melt. The result of this operation in the melting crucible is formed of a two-phase system consisting of a melt on the basis of zinc dissolved metals present in the structural materials of the fuel elements, such as zirconium, niobium, iron, chromium, Nickel, lead and some other, as well as fragments of solid ceramic fuel of various sizes. The latter, as having a greater density (10-13 g/cm3will settle in either the fuel in suspension, is located in the upper part of the crucible, and also fills the open pores of the solid fuel. During processing of the fuel accumulating in the crucible of solid fuels to the maximum allowable values of the melt zinc-based merge in another melting furnace. Present in the poured molten solid fuel particles, if necessary, filtered. The remains of molten metal out of the pores of the solid fuel melt the recycled zinc, which, if necessary, filtered and directed to the removal operation of solid fuel from the fuel rods. Solid fuel is unloaded from the crucible and sent for further processing.

The regeneration of the zinc melt containing, for example, zirconium to niobium or iron with chromium and lead, as well as other dissolved zinc metals, carried out by the method of segregation. The method is based on the loss from the melt when it cools, crystals of intermetallic compounds: Zr3Zn2, ZrZn, ZrZn2, ZrZn3, ZrZn6, ZrZn14, ZrZn22, NbZn, Nb2Zn3, NbZr3, NbZn7, NbZn15, FeZn13, CrZn17, NiZn3, Ni3Zn10, Ni5Zn21, Ni4Zn22and others, as well as to the limited solubility of lead in zinc, sigaram and Nickel (basic construction materials fuel assemblies of WWER-1000), in the segregation receive two layers:

1) the bottom layer is solid intermetallic compounds of the type ZrmZnn, NbmZnn, FeZn13, CrZn17, NimZnn, and others, which are in the form of a spongy mass deposited on the bottom of the crucible;

2) the upper layer of liquid refined (recycled) zinc.

During cooling of the melt on the basis of zinc containing iron, chromium and lead (the main structural materials of the reactor fuel elements "Brest"), get three layers:

1) the bottom layer is a liquid alloy of lead and zinc, containing 5-6 wt.% zinc;

2) middle layer - solid intermetallic compounds of the type FeZn13, CrZn17and others that are in the form of a spongy mass is located above the lead-zinc melt;

3) the top layer is a liquid refined (recycled) zinc.

During processing of the fuel rods and fuel assemblies can be some (since the process is conducted at relatively low temperatures) the restoration of ceramic uranium and plutonium fuel to metals: uranium and plutonium, respectively. However, metallic uranium and plutonium also form intermetallic compounds (U2Zn17, PuZn2Pu2Zn9, PuZn8Pu2Zn17with zinc. Therefore, Brcko will be deleted intermetallic phase together with zirconium, niobium, iron, chromium, Nickel and other metals when not refining. This is a very important positive factor, as the accumulation of uranium-235 and plutonium can cause spontaneous chain reaction (SCR).

Refined (recycled) zinc is poured through the top of the crucible and sent for flushing solid fuel and/or retrieve it (fuel) from the next batch of fuel rods. The bottom layer is an alloy of lead and zinc are removed through the drain device in the pan, and allow for the extraction of zinc by vacuum distillation. Remaining in the crucible spongy mass of chemical compounds (intermetallics) zirconium, niobium, iron, chromium, Nickel and other metals with zinc discharged from the crucible and also directed to the extraction of zinc by vacuum distillation with the goal of returning to the process cycle.

Maintaining the zinc content in the melt at a level not lower than 50 wt.% due to the fact that lower values are formed melts with higher liquidus temperatures, which leads to significant losses of volatile zinc as a result of evaporation and increases the probability of dissolution of the solid fuel in a metal melt and increases Energomash the>

Carrying out the extraction process of ceramic nuclear fuel from the fuel rods at 600-1200oWith due to the fact that in this interval ensures rapid and complete dissolution of the zinc melt all metals included in the structural parts of the fuel rods, but does not dissolve the spent fuel.

Liquation cleaning is carried out at 430-1000oWith depending on the composition of the initial melt. This time interval is determined experimentally and maximum achievement of the required degree of purification of zinc from dissolved metals.

For technically and economically efficient operations extraction of nuclear fuel based on uranium and plutonium from the fuel elements in a hostile environment molten metal requires special melting furnace, which allows to realize the following conditions:

to exclude contamination extractable nuclear fuel material in the melting crucible;

- exclude the absorption of a melting crucible of radioactive elements to avoid the formation of a new type of radioactive waste in the form of waste crucible materials;

to ensure electromagnetic stirring of the melt, stimulating the dissolution of construc atmosphere of inert gas;

to ensure that the movement of the melt and solid fuel inside of the crucible in the desired direction (up or down), and drain the melt of the chute is located in the upper part of the crucible or through the hole in the pan;

- to provide almost instantaneous control of the temperature of the melt;

to provide a radical solution to the problem of durability of the crucible, and it should work for several years and do not require intermediate sweeps and repairs involving staff.

These properties are vacuum induction furnace with metal split water-cooled (cold) crucibles (IPT) with moving the pallet with drain devices, and a drain gutters, located in the upper part of the crucible. As demonstrated industry experience, life IPHT reaches 18 years of age and over, which virtually eliminates the formation of such a class of radioactive waste, spent as crucible materials. The use of induction currents of relatively low frequency range (50-250000 Hz) allows the use of simple and reliable sources of power supply, provides intensive mixing of the molten metal, and C is) spent zinc melt most effectively carried out in IPT of the same type, as for the extraction of ceramic nuclear fuel from the fuel rods.

Experimental validation of the proposed method was carried out in an argon atmosphere in two induction furnaces (current frequency 2400 Hz) transparent to the electromagnetic field copper partitioned water-cooled (cold) crucibles with a diameter of 100 mm with drain troughs in the upper part of the crucible and devices for bottom discharge of the melt in the moving pallets. Under drain gutters and devices for bottom drain installed ceramic filters for particulate solid fuel.

Example 1. In the cold crucible first furnace was loaded with 4 kg of pure zinc and melted. In the melt downloaded the simulator fuel assemblies of VVER-1000 reactor: 340 g steel 12X18H10T and 600 g tubes of alloy 110-e and 125-e (base Zirconia + 1 and 2.5 wt. % niobium, respectively), containing within 1.6 kg of uranium dioxide. The process led to the complete dissolution of stainless steel and shells from alloys 110-e and 125-e for 25 min at 750oWith and electromagnetic stirring of the melt. After that he opened the tube of the discharge device, the melt containing 87 wt. % zinc, poured on a filter and after filter - portable heated mold, from which the melt of the Bay is and deposition of crystals of intermetallic compounds of the type ZrmZnn, NbmZnn, FemZnn, CrmZnn, NimZnnetc. Then opened the drain device and poured the melt is refined (regenerierung) zinc in a heated mold, and from there in the first crucible furnace for washing there are uranium dioxide. The washing process was conducted at 700oC for 15 min with electromagnetic stirring of the melt. After this, the drain of the device was opened and poured the wash melt on the filter, after which it fell into the mold. Washed uranium dioxide was discharged from the crucible, weighed and the difference was determined by loss of uranium dioxide during manufacturing operations, which amounted to 0.08 wt. %. Also analyzed the composition of the zinc melt after refining (regeneration) by the method of 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 leaching melt was 0,23; 0, 012; 0,13; 0,011; 0,019 and 0,0017 wt.% respectively.

Example 2. In the cold crucible first furnace was downloaded 10 kg of pure zinc and melted. In the melt downloaded the simulator of a fuel rod in the reactor "Brest": 600 g tubes and the e 260 g of lead. The process led to the complete dissolution of the shells of alloy EP-823 and lead for 20 min at 700oWith and electromagnetic stirring of the melt. After that he opened the tube of the discharge device, the melt containing 92 wt.% zinc was poured on a filter and after filter - portable heated mold. From the mold, the melt was poured into a cold crucible second furnace. This melt for 40 min, cooled to a temperature of 450oTo complete the process of deposition of crystals of intermetallic compounds FeZn13and CrZn17and to be formed in the bottom area of the crucible layer of molten lead containing zinc. Next, the pallet of the crucible was moved up to the complete removal through the drain trench of the upper layer of the melt containing refined (recycled) zinc in a heated mold. After that he opened the tube of the discharge device in the pan and poured the molten lead containing 5.7 wt.%. zinc in the mold, where it crystallized. The resulting melt is refined (recycled) zinc was poured into the first crucible furnace for washing there are a mixture of mononitrides uranium and plutonium. The washing process was conducted at 650oC for 15 min with electromagnetic stirring of the melt. Procanico. The washed mixture mononitrides unloaded from the crucible, weighed and the difference was determined losses during manufacturing operations, which amounted to 0.06 wt.%. Also analyzed the composition of the zinc melt after refining (regeneration) by the method of segregation: zderzenie of iron, chromium, uranium and plutonium was 0.1; 0,019; 0,0012 and 0.0002 wt.% respectively. The content of iron, chromium, uranium and plutonium in the washing melt was 0.18; 0,024; 0,0013 and less of 0.0001 wt.% respectively.

Examples confirm the effectiveness of the proposed method.

Literature

1. Patent UK 1096745, 1967.

2. Patent UK 1171257, 1969.

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

4. Kondrat'ev A. N. and others, in Proc. of the Symposium Proceedings CMEA "research in the field of reprocessing of irradiated fuel, so 1, Prague, ed. The AEC, USSR, 1972, S. 174.

1. The method of extraction of ceramic nuclear fuel from the fuel elements, comprising melting in an atmosphere of inert gas metal structural materials and separation of the resulting melt from solid fuel, characterized in that the melt is subjected to fuel elements and fuel assemblies, priprava maintained within the range of 50-98 wt. %, the melt is separated from the solid fuel, cooled to 430-1000oWith the top layer of the cooled melt is drained and returned to the process cycle.

2. The method according to p. 1, characterized in that the melt after separation from the solid fuel filter.

3. The method according to p. 1, characterized in that the solid fuel is washed zinc melt.

4. The method according to p. 3, characterized in that the leaching of the zinc melt, after separation from the 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 leaching of the zinc melt, after separation from the solid fuel filter.

6. The method according to p. 1, characterized in that the upper layer of the cooled zinc melt is drained and returned to the leaching of solid fuel and/or removing 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 lead leaching in vacuum induction furnaces, crucibles transparent to electromagnetic fields, mainly metal split water-cooled crucibles, when the electromagnetic stirring of the melt.


 

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