RU2702620C1 - Method for isotope recovery of regenerated uranium - Google Patents

Abstract

FIELD: nuclear physics and equipment.

SUBSTANCE: invention relates to closure of nuclear fuel cycle and can be used for return of uranium extracted from spent nuclear fuel (SNF) into fuel cycle of both light-water reactors and other types of reactors operating on enriched uranium. Method for isotopic recovery of regenerated uranium involves high content of isotope ²³⁵U to 3–5 wt% in regenerated uranium with reduction of absolute and / or relative concentration of even uranium isotopes, with separation of raw uranium regenerate raw material isotopes at installation of two successive gas-centrifugal cascades. Moreover, the raw uranium regenerate is enriched in the fissile isotope uranium-235 in the first gas centrifugal cascade to content of 5–15 wt%. ²³⁵U, for production of commercial isotopic mixture, second stage cascade containing 5–14 wt%. ²³⁵U is mixed with low-enriched uranium-diluent of natural origin and concentration ²³⁵U 3–5 wt%, and the second cascade selected flow is enriched by ²³⁵U up to 20 wt% is directed into storage container. After holding and radiochemical cleaning, the next batch at the inlet of the first gas centrifuge cascade is mixed with the recovered uranium.

EFFECT: technical result is elimination of hazardous high concentrations of fissile isotope ²³⁵U is not more than 20 % by isotope ²³⁵U at any stages of the process, providing complete recovery of regenerated uranium in fuel reproduction due to the absence of unused regenerate directed to long-term storage.

1 cl, 1 tbl, 1 dwg

Description

The invention relates to the field of closure of the nuclear fuel cycle and can be used to return uranium extracted from spent nuclear fuel (SNF) to the fuel cycle of both light-water reactors and other types of reactors operating on enriched uranium.

At the present stage of nuclear technology development, uranium accounts for more than 95% of the fissile components of spent fuel. The use of so-called regenerated uranium will reduce the need for natural uranium and significantly reduce the amount of radioactive waste disposed in comparison with the open fuel cycle. This material can be used as raw material for the production of low enriched uranium in cascades of gas centrifuges. The term "cascade" hereinafter means a multi-stage separation unit, which is a group of interconnected gas centrifuges, which serves the processed raw materials (feed cascade) and from which the resulting products (flows of selection and dump).

The use of uranium regenerate for the manufacture of nuclear fuel is difficult due to the presence of ²³² U and ²³⁶ U artificial isotopes in its composition and a higher content of ²³⁴ U relative to the natural mixture. These isotopes are not separated from ²³⁵ U during chemical processing. This factor causes difficulties in the enrichment of regenerated uranium in a standard three-stream (feed, selection, dump) or ordinary cascade of gas centrifuges, since the ²³² U, ²³⁴ U, ²³⁶ U isotopes are enriched together with ²³⁵ U. As a result, their concentration in the product is higher than allowed by standards, specifications and specifications for commodity low enriched uranium (LEU). Hereinafter, by “concentration” we mean the mass concentration (content) of components in a mixture of uranium isotopes without taking into account possible impurities.

The initial content of even isotopes in the regenerated uranium depends on the burnup rate of the nuclear fuel from which it is extracted. In recent decades, there has been a steady tendency to increase the fuel burnup depth of thermal neutron reactors. Starting from some values of the burnup depths, it is possible to enrich a burned-out nuclear fuel in two ways: 1) add to the burned-out mixture of uranium isotopes an isotopic mixture of uranium-reducer with a higher concentration of ²³⁵ U and the absence of even isotopes in it; 2) enrichment of the burnt isotope mixture to a higher concentration of ²³⁵ U using modified cascades of gas centrifuges.

Most of the possible solutions to this problem partially or completely eliminate the difficulties associated with the presence of the ²³² U, ²³⁴ U, and ²³⁶ U isotopes, however, the consumption of regenerated uranium per unit of product is usually such that it does not allow a complete closure of the uranium cycle, namely to achieve equal masses of the regenerate obtained from 1 kg of spent nuclear fuel and spent on the production of 1 kg of commercial grade LEU.

There is a method of isotope reduction of regenerated uranium (Patent RU No. 2242812, G21C 19/42, B01D 59/20, publ. 12/20/2004), which consists in increasing the content of the ²³⁵ U isotope in regenerated uranium to (2.0 ÷ 7.0) wt. . % with a decrease in the absolute or relative concentration of ²³² U, ²³⁴ U and ²³⁶ U isotopes, including direct enrichment of raw uranium regenerate hexafluoride in a double isotope-separation cascade of gas centrifuges (analog). In the first cascade, the raw uranium regenerate is enriched in the ²³⁵ U isotope to its content in the selection stream of 3.5 ÷ 10.0 wt. % or 21.0 ÷ 36.0 wt. %, and the second cascade is fed with the selection stream of the first cascade, in which enrichment with ²³⁵ U isotope leads to 21.0 ÷ 90.0 wt. % when selecting recovered fuel material with an isotope content of ²³⁵ U not more than 50.0 wt. % through the dump stream of the second ordinary cascade.

The disadvantage of this method is the use at one stage of high enrichment values of the ²³⁵ U isotope (from 21 to 90%, while in accordance with international restrictions in the technology, enrichment of not more than 20% ²³⁵ U must be used), as well as the inability to provide a given proportion between the feed regenerate and the resulting product.

A known method of isotopic reduction of regenerated uranium (Patent RU No. 2497210, G21C 19/42, B01D 59/20, publ. 05/11/2012), includes increasing the content of isotope ²³⁵ U in hexafluoride of regenerated uranium to a value specified in the range 2.0 ÷ 5.0 wt. % value, a decrease in the relative concentration of the ²³² U isotope in a mixture of uranium isotopes and direct enrichment of regenerated uranium hexafluoride with the ²³⁵ U isotope in a two-stage installation from gas centrifuge separation stages (analogue). Moreover, in the first cascade, regenerated uranium is enriched in the ²³⁵ U isotope to 5.0–10.0 wt. % while maintaining the ratio of the mass flow rates of the blade stream and the cascade selection stream in the interval (6.9 ÷ 18.4): 1. The dump and selection flows of the first cascade are directed to the power of the second cascade. Regenerated uranium is taken from the separation stage of the central part of the second cascade. The invention provides for the complete cleaning of a burnt mixture of uranium isotopes from the most radiation-hazardous nuclide ²³² U and the production of commodity low-enriched uranium hexafluoride with minimal rearrangement of industrial cascades of centrifuges.

The disadvantage of this method is the inability to maintain a predetermined proportion between the raw material regenerate and the resulting product, as well as the loss of separation work when mixing the “waste” in the form of a fraction enriched in ²³² U and ²³⁴ U, which is both a radioactive waste and a product having a relatively high concentration of ²³⁵ U (up to 20 wt.%).

There is a method of isotopic reduction of regenerated uranium (Patent RU No. 2282904, G21C 19/42, B01D 59/20, publ. 08/27/2006), which consists in the separation of the isotopic mixture of the initial regenerated uranium in a cascade of gas centrifuges and subsequent mixing of the separated isotopic mixtures with uranium-diluent, the concentration of the ²³⁵ U isotope in the regenerated uranium is increased to (2.0 ÷ 5.0) wt. % with a decrease in the absolute and (or) relative concentration of even isotopes of uranium in the resulting product (prototype). The separation of the uranium isotope mixture is carried out in a double cascade of gas centrifuges, in the first of which the raw uranium regenerate is enriched in the ²³⁵ U isotope to a content of 90.0 and more wt. %, in the second cascade, ²³² U and ²³⁴ U isotopes are separated into the light fraction, and as a isotopic mixture, the dump stream (heavy fraction) of the second cascade enriched in the ²³⁵ U isotope is sent for mixing with uranium-diluent.

The disadvantages of the method are to obtain at separate stages of the separation of high, more than 90 wt. %, concentrations of ²³⁵ U, loss of separation work to obtain a diluent during mixing, and the inability to maintain a predetermined proportion between the raw material regenerate and the resulting product.

The present invention is directed to solving the following problems and achieving technical results:

достижение требуемого обогащения регенерированного урана изотопом ²³⁵U с одновременным снижением концентраций четных изотопов до заранее заданных пределов;

achieving the required enrichment of regenerated uranium with the ²³⁵ U isotope while reducing even isotope concentrations to predetermined limits;

исключение опасных высоких концентраций делящегося изотопа ²³⁵U (не более 20% по изотопу ²³⁵U) на любых стадиях процесса;

the exclusion of dangerous high concentrations of fissile isotope ²³⁵ U (not more than 20% of the isotope ²³⁵ U) at any stage of the process;

обеспечение условия полного возврата регенерированного урана в воспроизводство топлива (отсутствие неиспользованного регенерата, направляемого на длительное складское хранение);

ensuring conditions for the full return of regenerated uranium to fuel reproduction (absence of unused regenerate sent for long-term storage);

минимизации массы высокоактивного отхода, в котором сконцентрированы нежелательные четные изотопы урана ²³²U, ²³⁴U, ²³⁶U;

minimizing the mass of highly active waste, in which undesirable even isotopes of uranium ²³² U, ²³⁴ U, ²³⁶ U are concentrated;

минимизация массы урана-разбавителя природного происхождения.

minimization of the mass of natural uranium diluent.

To solve these problems, a method of isotope reduction of regenerated uranium is proposed, which consists in increasing the content of the ²³⁵ U isotope to 3-5 wt. % In the recovered uranium by lowering the absolute and / or relative concentration of the even isotopes of uranium, comprising the isotope separation feed uranium regenerate on installation of two consecutive gas centrifuge cascades mixing the selected commodity isotopic mixture with uranium diluent, thus enriching the raw uranium reclaimed for fissile isotope ²³⁵ U in the first gas centrifuge ordinary cascade to a content of 5-15 wt. % ²³⁵ U, to obtain a marketable isotopic mixture, the dump of the second cascade containing 5-14 wt. % ²³⁵ U is mixed with low enriched uranium diluent of natural origin and a concentration of ²³⁵ U 3-5 wt. %, and the selection stream of the second cascade enriched in ²³⁵ U to 20 wt. % is sent to a storage tank, and after aging, they are mixed with regenerated uranium of the next batch at the entrance to the first cascade of gas centrifuges.

In addition, after exposure, radiochemical purification of the isotope mixture is carried out.

By “double gas centrifuge cascade” we mean a circuit that is a series connection of two ordinary cascades in which the selection of the first cascade serves as the external power supply to the second cascade.

The main distinguishing features of the method is that the enrichment in the first cascade is carried out to concentrations of not more than 15% wt.,

- hexafluoride restored by the isotopic composition of uranium is obtained by mixing the dump of the second cascade containing 5-14 wt. % ²³⁵ U, with low enriched uranium-diluent of natural origin with a selected concentration in the range of 3-5 wt. %, in order to ensure a predetermined proportion between the flow of raw uranium regenerate and commercial LEU.

In addition, the isotopic uranium mixture in the selection of the second cascade, enriched in ²³⁵ U to 20 wt. % and contaminated with ²³² U, ²³⁴ U and ²³⁶ U isotopes, are mixed with regenerated uranium hexafluoride from the next batch of enrichment and again sent to the recovery cycle of regenerated uranium in the proposed double cascade scheme.

Mixing of all streams is carried out in the chemical form of uranium hexafluoride.

The implementation of the method is illustrated in the drawing, which shows a block diagram of a double gas centrifuge separation cascade for cleaning raw uranium regenerate from even isotopes of uranium, a scheme for mixing the selection of the heavy fraction of the second cascade (purified from ²³² U, ²³⁴ U ²³⁶ U isotopic mixture) with uranium diluent to obtain a commodity LEU and mixing scheme for the selection of the light fraction of the second cascade (contaminated with ²³² U, ²³⁴ U ²³⁶ U of the isotopic mixture) with the regenerate of the next batch of enrichment.

The double cascade consists of ordinary cascades 1 and 2. The stream of raw regenerated uranium 3 hexafluoride enters cascade 1, where it is enriched to intermediate enrichments (the choice of the ratio of enrichments of the first and second cascades can be made taking into account the isotopic composition of the raw uranium regenerate).

The dump of the first cascade 4 is sent for storage of depleted uranium hexafluoride with the possibility of further conversion to nitrous oxide and storage or disposal in this form. Stream 5 enriched in ²³⁵ U mixture (5-15 wt.%) Enters the input of the second ordinary cascade, where it is divided into two parts. Uranium hexafluoride with a reduced content of ²³² U, ²³⁴ U and ²³⁶ U isotopes is collected in the dump of the second cascade 6 in comparison with stream 5. Uranium hexafluoride with a high content of ²³² U, ²³⁴ U ²³⁶ U and enriched in isotope ^{235 is} accumulated in the selection stream of the second cascade 7 U up to 20 wt. %

At the same time, in a factory for natural uranium enrichment, a diluent hexafluoride is supplied according to the standard scheme. It is supplied for the mixing section with stream 6 uranium hexafluoride. Production of commercial know-how hexafluoride (stream 8 in the flowchart) is carried out by mixing flows 6 and 9 in a given proportion in gas phase after desublimation of the uranium-diluent received at the mixing site. In this case, the concentration of ²³⁵ U in stream 8 must be selected in such a way as to provide a predetermined proportion between the raw uranium regenerate and commercial LEU.

The sampling stream 7 of the second cascade, which is a contaminated ²³² U, ²³⁴ U, ²³⁶ U fraction, containing ²³⁵ U enriched up to 20%, is sent to buffer tank 10, where it is stored for ~ one and a half years, and then, after radiochemical cleaning, in the form stream 11 is directed to the inlet of the first gas centrifuge cascade, where it is mixed with the next batch of 3 that came to be enriched in raw uranium regenerate.

Further, the process can be repeated repeatedly in the process of implementing the proposed method.

Specific examples of the implementation of the method are given in the table.

The fuel cycle of a light-water reactor with full use of regenerated uranium is considered on the example of a VVER-1200 reactor operating in the steady state overload conditions. As an example, consider the enrichment of regenerated uranium, which undergoes a second irradiation cycle. That is, at the “zero” stage, fuel based on LEU from natural raw materials survived irradiation as part of the regular campaign of the reactor, then, after the stages of processing and separation of the uranium fraction, the obtained regenerate was enriched using the simplest modification of the ordinary cascade due to the low contents of ²³² U and ²³⁴ U. After that, the obtained LEU was subjected to the next irradiation cycle. However, the regenerate isolated from it, due to the higher content of even isotopes, can no longer be enriched in the simplest modifications of the ordinary cascade with the simultaneous fulfillment of all necessary conditions. It is to restore the mixture of raw material regenerate under such conditions and the proposed method is directed.

As values of nominal enrichment, i.e. enrichment in the case of using natural uranium as raw materials selected values of 4.4 and 4.95 wt. % The fraction obtained after the restoration of the first batch of the regenerate, enriched in ²³² U isotope and having a 20% content of the ²³⁵ U isotope, is sent for temporary storage to buffer tank 10, where this mixture is stored until a new batch of regenerate arrives (in the considered example, this is a year and a half). After that, it is subjected to radiochemical purification from decay products of ²³² U. ²³⁴ U, ²³⁶ U, and then mixed with the regenerate obtained after processing the second batch of SFAs and sent for further enrichment. The results of calculating the parameters of double cascades for this case are given in the table. When calculating the isotopic composition of mixed uranium hexafluoride directed to the input of the cascade during fuel production of the second (or third) overload from the table, the change in the isotopic composition of the returned fraction during storage in the buffer tank was taken into account. In the case when there is no return of the selection stream, the fuel parameters for all transshipments of the second recycling coincide with the parameters of the first transshipment from the table, and the isotopic composition of uranium falling into the waste at each transshipment, and its amount, coincides with the composition of the returned fraction of the first transshipment from the table .

As can be seen from the table, the organization of such a recycling scheme really allows you to completely use up both the initial regenerated uranium and the fraction formed in the second cascade with a 20% content of the ²³⁵ U isotope and the highest content of even isotopes relative to all other flows of the scheme.

Moreover, the resulting product meets the compensation condition of ²³⁶ U, the concentration of ²³² U does not exceed the limitation of 5⋅10 ^-7 %, and the relative concentration of ²³⁴ U and ²³⁵ U does not exceed the value of 0.02.

The table shows the isotopic compositions and costs of natural uranium for the manufacture of fuel for overloading a VVER-1200 reactor (excluding twigs): fuel of the second recycling

Claims (2)

1. The method of isotopic reduction of regenerated uranium, which consists in increasing the content of the isotope ²³⁵ U to 3-5 wt. % in regenerated uranium with a decrease in the absolute and / or relative concentration of even isotopes of uranium, including the separation of isotopes of raw uranium regenerate in an installation of two successive gas centrifuge cascades, mixing the separated commodity isotopic mixture with uranium-diluent, characterized in that the raw uranium regenerate is enriched in the ²³⁵ U isotope in the first gas centrifuge ordinary cascade to a content of 5-15 wt. % ²³⁵ U, to obtain a commodity isotope mixture dump of the second cascade containing 5-14 wt. % ²³⁵ U, mixed with low enriched uranium diluent of natural origin and a concentration of ²³⁵ U 3-5 wt. %, and the selection stream of the second cascade enriched in ²³⁵ U to 20 wt. %, sent to a storage tank, and after exposure, mixed with regenerated uranium of the next batch at the entrance to the first cascade of gas centrifuges. 2. The method according to p. 1, characterized in that after exposure to conduct radiochemical purification of the isotopic mixture.

Images