RU2200766C2 - Method of recovering ceramic nuclear fuel from fuel element packets - Google Patents

Abstract

FIELD: nuclear power engineering. SUBSTANCE: method involves melting, in vacuum or in inert medium, of metallic structural material and separation of melt from solid fuel. Packet of heat-emitting elements is placed into melting crucible to form melt containing zirconium, iron, chromium, and nickel. Additionally, portion of fragments of zirconium elements and portion of used up elements without end details are loaded so that concentrations of said metals are maintained as follows: 72-86, 2-26, 0.2-7, and 0.1-4 wt.%, respectively. Melt is further supplemented by 0.1-15% zinc. Melting is effected at 800-1250 C. EFFECT: facilitated separation of ceramic nuclear fuel from metallic details and reduced amount of radioactive metal waste. 12 cl, 1 dwg, 3 ex

Description

 The invention relates to pyrometallurgical methods for processing spent nuclear fuel (SNF) mainly based on uranium.

There are methods for the mechanical extraction of uranium oxide oxide from the metal shells of fuel elements (fuel elements) made of niobium doped zirconium, which include:
- in the dismantling of fuel assemblies (fuel assemblies) into individual fuel elements and subsequent cutting [1], chopping [2] or deformation [3];
- 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 formation of large quantities of dust and aerosols of oxide fuel, which requires powerful and highly efficient gas cleaning systems. As a result of 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 ceramic uranium fuel from packages of fuel elements [A.S. USSR 357596, publ. 10/31/1972 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 weighing 15-25% by weight of the structural parts of the package of fuel elements and acting as a solid additive, and the opening process is carried out in vacuum or inert atmosphere at 1000-1300 ^o C.

The disadvantages of this method are:
- the need to cut fuel assemblies by cutting the shanks and extracting the fuel rods, which is a very time-consuming operation performed using manipulators, since the work is carried out with irradiated fuel assemblies;
- the need to manufacture containers of iron, which during the processing of fuel rods will be included in the radioactive metal waste and cannot be removed from them in an economically acceptable way.

 The technical result of the proposed solution is to reduce the complexity of the process of separating ceramic nuclear fuel from metal structures of fuel elements and reducing the amount of radioactive metal waste generated. The technical result is achieved by the fact that in the method for extracting ceramic nuclear fuel from packages of fuel elements, comprising melting in a vacuum or inert medium metal structural materials and separating the melt from solid fuel, according to the invention, packages of fuel elements in the composition of fuel assemblies in a melting crucible with a melt are subjected to melting. containing zirconium, iron, chromium and nickel.

 In a crucible with a melt, at the same time, the assembly is additionally charged with a portion of the fragments of zirconium shells of spent fuel elements.

 In a crucible with a melt simultaneously with the fuel assembly additionally load a portion of the spent fuel elements without end parts.

 The content in the melt of zirconium, iron, chromium and nickel is maintained at the level of 72-86; 2-26; 0.2-7.0 and 0.1-4.0 wt.%, Respectively.

 Zinc is added to the melt in an amount of 0.1-15.0 wt.%.

The process is carried out at 800-1250 ^o C.

 Part of the melt after separation from solid fuel is sent to the operation of extracting ceramic nuclear fuel from packages of fuel elements.

 The melt after separation from solid fuel is filtered.

 Solid fuel is washed with zinc melt.

 Wash zinc melt after separation from solid fuel is filtered.

 The process is carried out in vacuum induction furnaces with copper split water-cooled crucibles with electromagnetic stirring of the melt.

 The process is conducted at a frequency of induction currents of 50-250000 Hz.

 The main distinguishing features of the proposed method is that the fuel assemblies are subjected to processing in the configuration in which they were extracted from the nuclear reactor, without any preliminary mechanical disassembly.

 The process is carried out by immersing fuel assemblies in a melting crucible with a zirconium-based melt, which also contains iron, chromium and nickel. This melt is formed during the melting of the metal parts of the fuel assemblies themselves, namely: the claddings of the fuel rods and spacer grids made of zirconium alloyed with niobium, as well as the end parts (shanks) made of steel 12Kh18N10T. This steel contains Fe, Cr, and Ni, which form fusible eutectics with zirconium. As a result of the dissolution of the metal component of the fuel assembly, a two-phase system will be formed, consisting of a metal melt and solid uranium dioxide, with the subsequent separation of phases by specific density directly in the melting crucible and control filtration of the metal melt in order to separate suspended fuel particles when it is drained from the crucible.

A diagram of the fuel assembly of the VVER-1000 reactor is shown in the drawing. It consists of:
- reloading and unloading 1 and landing 4 heads, the so-called end parts or shanks made of stainless steel 12X18H10T, with which the fuel rods are rigidly fixed in a certain position;
- a package of fuel elements 2, which is a set of tubes of zirconium doped with niobium, inside of which there is oxide uranium fuel;
- spacing hexagonal honeycombs 3 of zirconium doped with niobium, through which the fuel rods pass.

One fuel assembly of the VVER-1000 reactor has a total mass of about 787 kg and contains, kg:
Uranium Dioxide - 497
Niobium doped zirconium - 184
Stainless steel 12X18H10T - 106
The calculation shows that when melting the metal component of the fuel assembly, the resulting multicomponent Zr-Fe-Cr-Ni-melt will have a mass of 290 kg and contain, wt.%:
- stainless steel components - 36.3, including Fe - 26, Cr - 6.6 and Ni - 3.7, respectively;
- zirconium doped with niobium (by difference) - 63.7.

It has been experimentally established that for this multicomponent system, the minimum liquidus temperature of 800-1200 ^o With alloys containing a total of 14-28 wt. % of components of stainless steel 12X18H10T: Fe, Cr and Ni. Carrying out the process at such temperatures minimizes the probability of reduction of uranium dioxide by zirconium to metallic uranium, which, in turn, sharply reduces the loss of uranium with zirconium melt.

 To obtain an alloy of the above composition, the zirconium content should be increased from 63.7 to 72-86 wt.%. This is achieved by loading into the melting crucible, together with another FA, up to 200 kg of fragments of irradiated zirconium shells, the so-called zirconium husk, accumulated in large quantities as a result of processing of irradiated fuel elements by the cutting method over several decades. The zirconium content in the melt can also be increased by loading into the crucible simultaneously with the fuel assembly a calculated portion of fuel rods without end parts.

If you need to further reduce the liquidus temperature (up to 750 ^o C and below), it is advisable to add 0.1-15.0% of zinc to the melt, forming low-melting alloys with zirconium and iron. This will further reduce the degree of progress of the uranium reduction reaction to a metallic state.

 The content of iron, chromium and nickel in the melt, equal to: 2-26, 0.2-7 and 0.1-4.0 wt. %, respectively, is determined mainly by the content of these metals in stainless steel 12X18H10T, from which the end parts of the fuel assemblies are made, and the number of zirconium-containing materials added to the melt - zirconium husk or fuel elements without end parts. If necessary, adjust the composition of the melt introduce additional small amounts of iron, chromium and nickel.

 To remove residual metal zirconium-iron-chromium-nickel melt from the pores of solid oxide uranium fuel it (fuel) is washed with low-melting zinc melt. It is advisable to use zinc melt repeatedly, periodically cleaning (regenerating) it from the zirconium, iron, chromium, nickel, and other impurities that accumulate during washing.

For technically and cost-effective operations for the extraction of ceramic nuclear fuel based on uranium from fuel elements in an aggressive environment of a 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;
- provide electromagnetic mixing of the melt, intensifying the dissolution of structural materials of fuel elements in the melt based on zirconium;
- to ensure the implementation of technological operations in the required atmosphere: vacuum, inert gas, air, etc., and it should be possible to change the atmosphere during the process;
- provide the ability to move the melt inside the crucible in the desired direction (up or down), as well as drain or crystallize the melt and stretch in the form of a compact ingot;
- 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 chutes 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, and also greatly simplifies the design of IPHT.

Before starting the processing process in a cold crucible equipped with a drain trough in the upper zone and a water-cooled copper tray with a drain device moving inside, a low-melting bath based on zirconium containing Fe, Cr and Ni is prepared, the proposed composition of which is indicated above. Spent fuel assemblies and a portion of zirconium husk or spent fuel rods are uniformly immersed in the melt, where they dissolve under the influence of factors such as high temperature, aggressive chemical environment of the metal melt and its intense electromagnetic mixing. It is obvious that, due to their physicochemical characteristics, the metal parts of the fuel assembly after dissolution will be concentrated in the metal melt. In a cold crucible, as the fuel assemblies are processed, a two-phase system will be accumulated, consisting of a Zr-Cr-Ni-Fe melt and spent oxide fuel particles of various sizes. The latter, as having a high density (10-11 g / cm ³ ), will settle in the lower part of the crucible. A metallic melt containing small amounts of fine particles of oxide fuel in suspension will be located in the upper part of the crucible. The molten metal will also fill the open pores of the oxide fuel.

 After processing a batch of fuel assemblies, when the amount of solid fuel in the crucible reaches the maximum permissible filling, the zirconium-based melt is drained into the mold by the gutter by moving up the pan. The melt in the mold is crystallized, cooled, the ingot is removed and sent to the storage. It should be noted that compact ingots will require significantly less storage space, and will also be completely fireproof, unlike zirconium husks, currently stored in bulk.

 The metal melt located in the pores of the solid fuel is removed into the heated mold through a bottom discharge device. After washing with zinc melt, solid fuel is discharged from the crucible and sent for further processing. Solid spent fuel particles present in the melted melts are filtered off if necessary.

 The melt from the heated mold is poured into an empty melting crucible, the next fuel assembly and the calculated portion of zirconium husk or spent fuel rods are immersed in it, i.e. the processing cycle of the next batch of fuel assemblies is repeated.

 An experimental verification of the proposed method was carried out in an induction furnace with a transparent cut-off water-cooled (cold) crucible with a diameter of 100 mm transparent to the electromagnetic field, with a device for bottom discharge of the melt. A ceramic filter was installed under the drain device. The current frequency of the power source 2400 Hz provided intensive mixing of the metal melt.

 Example 1. The initial charge weighing 4268 kg, simulating the fuel assemblies of the VVER-1000 reactor, consisted of eight zirconium tubes filled with sintered uranium dioxide tablets, as well as pieces of 12X18H10T stainless steel. The mass of uranium dioxide was 2485 g, zirconium - 920 g, stainless steel 12X18H10T - 503 g.

The mixture of this composition was loaded into a cold crucible with a melt weighing 3423 g containing zirconium, iron, chromium and nickel and kept for 27 min at 1050 ^° C in an argon atmosphere until the metal fraction of the mixture was completely dissolved in the melt and the density was separated by melt and solid dioxide uranium. The total mass of the metal melt in the crucible was 4846 g, which had the following chemical composition, wt.%: Zr - 79.73, Fe - 14.48, Cr - 3.72, Ni - 2.07. After that, the drain plug was opened and the melt, passing through the filter, fell into the mold, where it crystallized into an ingot.

 The analysis showed that the uranium content in the obtained ingot weighing 4770 g was 0.36 wt.%. The residual total content of the metal phase in the uranium dioxide was 1.9 wt.%.

Example 2. The experiment was carried out as described in example 1. The difference was that in a melt weighing 3423 g containing zirconium, iron, chromium and nickel, 250 g of zinc was also added. A mixture with a mass of 4268 kg, the composition of which is given in Example 1, was loaded into the obtained melt weighing 3673 g. The process was carried out for 19 min at 950 ^° C in argon atmosphere until the metal fraction of the charge was completely dissolved in the melt and the density separation of the melt and solid uranium dioxide was completed. The total mass of the metal melt in the crucible was 5096 g, which had the following chemical composition, wt.%: Zr - 76.12; Fe - 13.79; Cr 3.25; Ni - 1.93; Zn - 4.91. After that, the drain plug was opened and the melt, passing through the filter, fell into the mold, where it crystallized into an ingot.

 The analysis showed that the uranium content in the resulting ingot weighing 5035 g was 0.23 wt.%. The residual total content of the metal phase in the uranium dioxide was 1.2 wt.%.

Example 3. The experiment was carried out as described in example 1. The difference was that to wash the solid uranium dioxide remaining in the crucible, 3 kg of zinc was loaded, melted and the melt was held at 800 ^° C for 10 minutes. Next, the zinc melt was poured through a bottom discharge device, passed through a filter and crystallized. The analysis showed that the uranium content in the zinc ingot weighing 3040 g was less than 0.02 wt.%. The residual content of the metal phase in the uranium dioxide was 0.6 wt.%.

 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. 511.

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

Claims (12)

 1. A method of extracting ceramic nuclear fuel from packages of fuel elements, comprising melting in a vacuum or inert medium metal structural materials and separating the melt from solid fuel, characterized in that the packages of fuel elements in the fuel assemblies in a melting crucible with a zirconium melt are subjected to melting. , iron, chrome and nickel.  2. The method according to p. 1, characterized in that in the crucible with the melt simultaneously with the fuel assembly load an additional portion of fragments of zirconium shells of spent fuel elements.  3. The method according to p. 1, characterized in that in the crucible with the melt simultaneously with the fuel Assembly load an additional portion of the spent fuel elements without end parts.  4. The method according to any one of paragraphs. 1-3, characterized in that the content in the melt of zirconium, iron, chromium and nickel is maintained at the level of 72-86; 2-26; 0.2-7.0 and 0.1-4.0 wt. % respectively.  5. The method according to p. 1, characterized in that zinc is added to the melt in an amount of 0.1-15.0 wt. % 6. The method according to p. 1 or 5, characterized in that the melting is carried out at 800-1250 ^o C.  7. The method according to p. 1, characterized in that part of the melt after separation from solid fuel is directed to the operation of extracting ceramic nuclear fuel from packages of fuel elements.  8. The method according to p. 1 or 7, characterized in that the melt after separation from solid fuel is filtered.  9. The method according to p. 1, characterized in that the solid fuel is washed with zinc melt.  10. The method according to p. 9, characterized in that the washing zinc melt after separation from solid fuel, is filtered.  11. The method according to p. 1 or 9, characterized in that the melting is carried out in vacuum induction furnaces with copper split water-cooled crucibles with electromagnetic stirring of the melt.  12. The method according to p. 11, characterized in that the process is conducted at a frequency of induction currents of 50-250000 Hz.