RU2790544C1 - Method for remelting structural materials of shells of spent fuel rods and structural materials of spent fuel assemblies - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to the field of metallurgy, namely, to pyrometallurgical processing of metal radioactive waste represented by shells of spent fuel rods. Method includes the stage of pre-mixing fragments of structural materials (SM) of spent shells of fuel rods made of zirconium alloys with SMs of spent fuel assemblies (SFA) made of stainless steels, followed by remelting under a layer of pre-fused three-component flux containing magnesium, calcium, and aluminium oxides. The process is implemented in a chamber with an inert atmosphere using a water-cooled melter equipped with an induction, electric arc, plasma, or combined heating source. The resulting metal ingot does not have surface contamination, is resistant to external effects, therefore being suitable for safe long-term storage. The slag can be reused, whereafter said slag must be immobilised into stable matrices or recycled in order to extract and return fissile materials to the nuclear fuel cycle.

EFFECT: possibility of extracting up to 19% and 70% of the uranium and plutonium contained in the shells, respectively, therefrom.

6 cl, 1 dwg

Description

FIELD OF TECHNOLOGY TO WHICH THE INVENTION RELATES

The invention relates to methods for processing metallic radioactive waste (MRW) generated during the processing of spent nuclear fuel (SNF) of thermal neutron reactors (RTN) of the VVER, RBMK, PWR and other types, using fuel rods with zirconium alloy cladding.

Structural materials (CM) of fuel claddings, formed at the head operations of RTN SNF reprocessing, are fragments (“husks”) made of zirconium alloy containing traces of SNF and neutron activation products. To date, the only industrially mastered method of handling them in Russia involves warehousing in bulk with backfilling with fire-fighting alumina filler in a ratio of 3:1. This approach poses a threat to radiation safety and contributes to the inefficient use of the volumes of specialized storage facilities due to the presence of surface contamination and the low bulk density of the claddings, respectively.

As a solution, a method is proposed for remelting CM of fuel claddings together with CM of spent fuel assemblies (SFA) made of stainless steels under a flux layer, which provides metal compaction, decontamination and the possibility of returning fissile materials (DM) to the nuclear fuel cycle (NFC). The latter circumstance imposes special requirements on the composition of the fluxing composition. It should not include components that hinder the processing of slag by industrially developed hydrometallurgical methods, namely, compounds with silicon, or pollute the final product, such as boron and other neutron poisons.

BACKGROUND OF THE INVENTION

A two-stage method for handling MRAW is known, presented in the document RU 2377675 C2 "Method for the recovery of metals and alloys spent in contact with radioactive materials" (GOU VPO "Ural State Technical University - UPI"). At the initial stage, the oxidation of radionuclides is carried out under a layer of the chloride system, they say. %: 35 NaCl, 31-33 MgCl ₂ and 35 KCl at a temperature of 700°C, and at the second stage - melting of scrap metal waste under a layer of eutectic mixture, mol. %: 50 CaF ₂ - 50 MgF ₂ at a temperature of 1600-1700°C. When remelting stainless steel, the metal is purified by the measured radioactivity by 160-220 times. The disadvantage of this approach is the use of chloride systems characterized by increased vapor pressure, which requires measures to capture them, and hygroscopicity necessitates controlled storage and preparation of reagents before remelting.

To solve the problems of processing metal waste in the method KR 101200780 B1 "Method of treating radioactive metal waste using melt decontamination"), it is proposed to carry out a preliminary classification and decontamination of MRAW before induction slag remelting. The process begins with the selection of metal fragments in accordance with the geometric parameters and the classification of the selected MRAO according to the source of their formation and the quality of the material. Next, the surface decontamination of the classified MRAO is carried out according to their level of activity, after which they are loaded into an induction melting furnace. Surface decontamination of MRAO can be carried out by chemical decontamination, electropolishing, sandblasting, hand polishing, and the like. The melting of the loaded MRAO is carried out with a silicon-containing flux. The resulting slag is downloaded, and the molten metal is poured into the mold by tilting the melting unit. With this approach, the possibility of extracting DM from slag is significantly limited by the presence of silicates in it, which form gels during acid dissolution, hindering the processes of phase separation.

When processing an alloy based on zirconium with the addition of 20 wt. % nickel in the document RU 2172787 C1 "Method of pyrometallurgical processing of waste, spent materials and products" (JSC "VNIIKhT") it is proposed to use borosilicate glass as a flux with a melting point of 1000°C. The composition includes undesirable components: silicon oxide, which complicates the extraction of valuable components by hydrometallurgical methods, as well as compounds of boron isotopes, which are an absorber ( ¹⁰ V) and a reflector of neutrons ( ¹¹ V), the presence of which in the extracted DM is not allowed. The invention RU 2172787 C1 is accepted as a prototype.

DISCLOSURE OF THE INVENTION

The objectives of the invention are the conditioning of MRAW, represented by SM SFAs made of stainless steels and zirconium SM fuel claddings, formed as a result of the implementation of the head operations of processing SNF RTN, while extracting DM from them for the purpose of subsequent return to the NFC. This is achieved by the fact that, in contrast to the known technical solutions, fuel rod cladding CMs made of zirconium alloys are remelted together with SFA CMs (heads, tails, spacer gratings, etc., made of 12Kh18N10T and/or 08Kh18N10T stainless steels) under a layer of oxide fused refining flux containing, wt. %: 19.5-26.6 CaO, 2.5-16.6 MgO and 57.2-78 Al ₂ O ₃ in an inert gas atmosphere. At the same time, the content of CM SFA in the metal charge ranges from 0 to 90 wt. %. The process can be carried out in water-cooled melters with various types of heating: induction, electric arc, plasma or combined. The flux and metal are loaded at the same time; additional loading of fragments of CM fuel rods and CM SFAs and, if necessary, flux can also be carried out. Upon completion of the process, the block of melting products is completely removed from the crucible, the metallic and non-metallic phases (slag) are separated mechanically or by dissolving/etching the slag with an acid solution. The resulting metal ingot is characterized by a high density corresponding to the theoretically possible for a compact metal; there are no oxide films with radioactive contamination on its surface. Radionuclides that have not passed into slag are reduced to their elemental state and introduced into a metal matrix that is resistant to external thermal, chemical and radiation effects, which makes it possible to consider it as a final form suitable for long-term safe storage and/or burial. The resulting slag is an acid-soluble oxide composition, which can be opened by hydrometallurgical methods commercially mastered in radiochemical production in order to extract DM to be returned to the nuclear fuel cycle. In this case, the spent slag can be reused until the assimilative capacity decreases. If the goal of extracting DM is not pursued, the slag can be transferred for conditioning by vitrification or inclusion in a mineral-like matrix. When using acid etching of the slag at the stage of separation from the ingot, the resulting product solution is transferred to the hydrometallurgical extraction of DM.

DESCRIPTION OF THE DRAWINGS

In FIG. Figure 1 shows the sequence of operations for handling fuel element cladding CMs and RTN SFA CMs.

IMPLEMENTATION OF THE INVENTION

Fuel rod cladding CMs, which are +20-40 mm fragments made of E110, E125, E635 alloy or zircaloy, are mixed with SFA CMs, represented by heads fragmented up to 30 mm, shanks, spacer grids, caps and springs made of stainless steels 12Kh18N10T and/or or 08X18H10T are placed in a water-cooled melting unit with an induction, electric arc, plasma or combined heating source. The content of CM SFA in the metal cage is from 0 to 90 wt. %. Also in the melter is loaded pre-alloyed flux containing, wt. %: 19.5-26.6 CaO, 2.5-6.6 MgO and 57.2-78 Al ₂ O ₃ , in an amount corresponding to 5-10% by weight of the metal charge. During the melting process, additional loading of the metal fraction and/or flux can be carried out in specified quantities. Next, heating is applied, the metal charge and the flux are melted. The temperature of the process carried out in an atmosphere of high purity argon is maintained at a level not exceeding 1800-1900°C. During the melting process, part of the radionuclides that were present on the surfaces of the CM of fuel rod cladding is assimilated in the slag phase, and part is reduced to the elemental state and dissolved in the metal. Upon completion of the process, the reaction mass is cooled, forming a two-phase block of smelting products containing a compact metal ingot and a layer of slag.

Remelting with a given flux composition ensures the extraction into slag of up to 18% U and up to 70% Pu, which were present in the surface radioactive contamination on CM fuel claddings. After that, the block of smelting products is removed, the slag is separated from the metal phase mechanically or by chemical etching of the slag in an acid solution. The resulting metal ingot does not have surface radioactive contamination, is characterized by compactness, high resistance to thermal, chemical and radiation effects. This ensures the possibility of its long-term safe effective storage without additional engineering barriers, in contrast to the original shells, which require significant storage volumes due to low bulk weight (1-2 kg/dm ³ ) and pouring with fire-fighting filler. The separated slag is processed according to the hydrometallurgical technology existing in the production or transferred for conditioning.

The effectiveness of the proposed method is confirmed by the results of experiments on the remelting of the charge containing:

- metal charge obtained by the method of electric arc remelting 82.8 wt. % alloy E110 and 17.2 wt. % steel 12X18H10T;

- three-component flux containing Al ₂ O ₃ , MgO and CaO obtained by fusion in an electric arc furnace;

- simulator of radioactive surface contamination in the form of a powder consisting of UO ₂ and CeO ₂ (Ce - plutonium simulant), taken in quantities of 0.02 and 0.0007 wt. % of metal charge;

The production of fluxes and metal charges was carried out in a 5SA laboratory electric arc furnace (Centorr Vacuum Industries, USA).

Experiments on slag fusion were carried out using a modified laboratory induction furnace UVIP-25-50-0.001 (OOO NPP Elterm-S, Russia). Analysis of the melt products was carried out using an Elan 9000 inductively coupled plasma mass spectrometer (Perkin Elmer Inc., CUIA).

Example 1

A metal ingot obtained in an electric arc furnace from four mass parts of the E110 alloy and one mass part of stainless steel 12X18H10T together with a pollution simulator and a flux containing 19.5 wt. % CaO, 2.5 wt. % MgO 78.0 wt. % Al ₂ O ₃ , remelted for 60 minutes at an average temperature of 1807°C. The mass ratio of metal : flux was 10:1.

To study the chemical composition, the metal ingot was dissolved in a mixture of 8M HNO ₃ + 1M HF, and the slag was dissolved in 8M HNO ₃ . When opening the slag, a complete dissolution of the product was observed without the formation of sludge. The measurement results showed that about 45.9% Ce and 18.1% U are extracted into the slag, the rest was assimilated by the metal phase.

Example 2

A metal ingot obtained in an electric arc furnace from four mass parts of the E110 alloy and one mass part of stainless steel 12X18H10T, together with a pollution simulator and a flux containing 26.6 wt. % CaO, 16.6 wt. % MgO and 57.2 wt. % Al ₂ O ₃ , remelted for 60 minutes. The mass ratio of the metal and slag phases was 10:1, and the average process temperature was 1803°C.

To study the chemical composition, the metal ingot was dissolved in a mixture of 8M HNO ₃ + 1M HF, and the slag was dissolved in 8M HNO ₃ . The slag dissolved without residue. The measurement results showed that about 69.1% Ce and 14.2% U are concentrated in the non-metallic phase.

LIST OF DIFFERENCES FROM THE PROTOTYPE

The proposed method differs from the invention RU 2172787 C1 (prototype), namely:

1. Remelting is carried out under a layer of flux composition, wt. %: 19.5-26.6 CaO, 2.5-16.6 MgO and 57.2-78 Al ₂ O ₃ .

2. The amount of flux introduced is 5-10% by weight of the metal charge.

3. The metal charge consists of CM SFAs made of 12Kh18N10T and/or 08Kh18N10T stainless steels and CM fuel claddings made of zirconium alloys E110, E125, E635 or zircaloy, while the content of CM SFA fragments can vary from 0 to 90 wt. %.

4. Remelting can be carried out in a melter with an induction, arc, plasma or combined heating source.

5. The resulting slag is subject to processing in order to extract DM in order to return them to the NFC.

CONCLUSION ON THE RESULTS OF THE PRELIMINARY SEARCH

Industrially implemented and described in security documents methods of refining remelting do not provide for the possibility of returning fissile materials to the nuclear fuel cycle, which is due to the use of fluxes that form slags that are difficult to process (containing silicates and boron compounds). Thus, the proposed method, involving the use of a flux containing only metal oxides, is patentable.

Claims (6)

1. The method of remelting structural materials (CM) of fuel rod cladding and CM of spent fuel assemblies (SFA) includes mixing fragments of CM of fuel rod cladding and CM of SFA formed in the head operations of processing spent nuclear fuel of thermal neutron reactors, and subsequent remelting under a layer of oxide silicon-free flux composition, wt.%: 19.5-26.6 CaO, 2.5-16.6 MgO and 57.2-78 Al ₂ O ₃ in the amount of 5-10% by weight of the metal charge at temperatures up to 1800-1900 ° C in an inert environment using a water-cooled melter with induction, electric arc, plasma or combined heating, resulting in a block of melt products consisting of a compact ingot suitable for long-term safe storage without additional engineering barriers and a slag phase that can be processed by the hydrometallurgical method to extract fissile materials or conditioned by incorporation into a stable matrix. 2. The method of remelting fuel rod cladding CM and SFA CM according to claim 1, characterized in that the slag can be separated from the metal ingot mechanically or by pickling in acid for subsequent transfer of the product solution to hydrometallurgical extraction. 3. A method for remelting fuel rod cladding CM and SFA CM according to claim 1, characterized in that the metal charge contains from 0 to 90 wt.% SFA CM. 4. A method for remelting fuel rod cladding CMs and SFA CMs according to claim 1, characterized in that SFA CMs are made of stainless steel, and fuel cladding CMs are made of zirconium alloys. 5. The method of remelting CM of fuel rod cladding and CM of SFA according to claim 1, characterized in that additional loading of the metal phase and/or flux can be carried out in the process of remelting. 6. Method for remelting fuel rod cladding CMs and SFA CMs according to claim 1, characterized in that the flux can be reused until its assimilating ability decreases.

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