RU2361297C2 - Method of isotopic recycling uranium - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention relates to the technology of recycling nuclear energy material. The said method of isotopic recycling uranium involves correction of the composition of burnt uranium isotopic mixtures in a double centrifugal cascade with selection of hexafluoride of uranium isotopic mixtures, purified from dangerous radioactive isotopes U-232 and U-234, through selection of the heavy fraction of second ordinary cascade. Separation of uranium isotopic mixtures in the second ordinary cascade is done in the presence of a carrier gas. The carrier gas has average molecular weight ranging from 346 to 348 amu.

EFFECT: invention reduces the mass of dangerous radioactive wastes and reduces loss of fissile U-235 isotope.

7 cl, 1 dwg, 2 tbl

Description

The invention relates to technology for the recycling of nuclear energy materials and can be used to return uranium extracted from spent nuclear fuel to the fuel cycle of light-water reactors.

The fuel burned out in a nuclear reactor in terms of the isotopic composition of uranium differs significantly from the natural (natural) mixture of uranium isotopes. In the burnt mixture, in addition to the difference in the isotopic concentration of U-235 in significant quantities, there are undesirable even isotopes U-232, U-234 and U-236, which are not separated during the chemical reprocessing of SNF and with an increased content of which are associated with the main difficulties in handling the regenerated uranium.

A method for restoring the suitability of fuel burned out in a nuclear reactor in the form of hexafluoride of a mixture of uranium isotopes for the manufacture of nuclear fuel for reuse in a nuclear reactor [Patent RU No. 2113022 C1, IPC 6 G21C 19/42, 19/50, C01G 43/00. Priority from 05/20/1997. Publ. 06/10/1998. Bull. No. 16] (analogue), consisting in the fact that in the hexafluoride of a burnt mixture of uranium isotopes, the concentration of fissile U-235 isotope is increased compared to the initial concentration to a predetermined value and at the same time the concentration of U-236, U-234 and U-232 isotopes is reduced compared with the initial concentration, mixing among themselves in the liquid and / or gas phases mainly three components. As the first component, i.e. source material, use a hexafluoride burnt mixture of uranium isotopes. As a second, i.e. reducing component, heap uranium hexafluoride of natural origin is used with a concentration of fissile isotope U-235 mainly in the range of 0.15-0.7115 wt.%. As the third, also reducing component, enriched (from 3.6 to 100 wt.%) Uranium hexafluoride of natural origin is used. During the mixing process, the content of both fissile isotope (U-235) and unwanted isotopes (U-232, U-234 and U-236) is monitored and controlled. The sequential addition and mixing of the components is continued until a predetermined concentration of uranium isotopes is obtained within specified limits. Uranium in the chemical form of hexafluoride is mixed.

An analogous method allows the preparation of nuclear fuel for reuse with virtually any reduced composition of undesirable isotopes in a reduced mixture of uranium isotopes. However, the mass of the recovered fuel material exceeds the load mass of the LWR core, from which the SNF is unloaded, and the excess recovered for reuse of the burnt mixture of uranium isotopes is not used.

Also known is a method of isotopic reduction of regenerated uranium [Patent RU 2236053 C2, IPC 7 G21C 19/42, B01D 59/20. Priority from 04/04/2002. Publ. 09/10/2004. Bull. No. 25], which includes direct enrichment of regenerated uranium hexafluoride in an isotope-separation centrifuge cascade. The mixture of uranium isotopes is enriched in the fissile isotope U-235 to 10.0-90.0 wt.%, After which it is diluted with natural uranium to a mass not exceeding the mass of raw uranium regenerate. Dilution is carried out with a mixture of uranium isotopes with a fissile isotope uranium-235 content of from 0.1 to 5.0 wt.%.

The analogue method allows to reduce the absolute content of the undesired isotope U-236 in the regenerated uranium and only the relative concentration of the most radiation-hazardous isotopes U-234 and U-232.

The closest in technical essence and the achieved effect is the proposed method for the isotopic reduction of regenerated uranium in the fuel cycle [Patent RU 2242812 C2, IPC 7 G21C 19/42, B01D 59/20. Priority dated 12/17/2002. Publ. 12/20/2004. Bull. No. 35], which consists in the fact that in a burnt mixture of uranium isotopes, the concentration of fissile isotope U-235 is increased to 2.0-7.0 wt.% With a decrease in the absolute and relative concentration of undesirable isotopes U-232, U-234 and U- 236 by direct enrichment of raw uranium regenerate hexafluoride in a centrifuge isotope separation cascade. The raw uranium regenerate is enriched with the fissile isotope U-235 in a double cascade during the selection of the reduced fuel material through the dump stream of the second ordinary cascade. Next, the dump stream of the second ordinary cascade is diluted with natural uranium hexafluoride to a mass not exceeding the mass of raw uranium regenerate hexafluoride. This method is selected as a prototype.

The prototype method allows in the first ordinary cascade to partially clean the raw uranium regenerate from the isotope U-236; in the second ordinary cascade, it is mainly necessary to clean the U-232 isotope (recall that the double cascade is a system of two ordinary cascades in which the selection of the product of the first relative to the external power of the cascade serves as the power of the second cascade). The two-stage correction scheme for the isotopic composition of a burnt mixture of uranium isotopes allows us to achieve the following results:

- the light fraction stream of the second ordinary cascade is 0.014 wt.% by weight of the flow rate of the feed of the initial uranium regenerate to feed the first ordinary cascade.

- up to 85% of the radiation-hazardous isotope U-232 is extracted with a stream of light fraction of the second ordinary cascade sent to the enrichment waste;

- no more than 1.6 wt.% of the fissile isotope U-235 entering the second ordinary cascade goes into the light fraction stream.

The disadvantages of the prototype method include, firstly, insufficiently complete extraction of radiation-hazardous nuclide U-232 from a mixture of uranium isotopes; secondly, significant losses of the fissile isotope U-235 and, as a result, the formation of a relatively large amount of nuclear and radiation hazardous waste; thirdly, a high concentration of γ-emitting nuclide U-232 and its decay products at the end steps of the second ordinary cascade, which create a significant radiation background from centrifuge equipment in this technological area.

The present invention is aimed at solving the following problems:

- reduction in the mass of radiation-hazardous waste during the correction of the composition of the mixture of uranium isotopes in the second ordinary cascade;

- additional extraction from the mixture of isotopes of uranium of the radiation-hazardous isotope U-232 while reducing losses of fissile isotope U-235.

The above objectives are achieved by a technical solution, the essence of which is that in the method of isotopic reduction of regenerated uranium, which includes adjusting the composition of a burnt mixture of uranium isotopes in a double centrifuge cascade during the selection of marketable isotopic products through the selection of the heavy fraction of the second ordinary cascade, separation of the mixture of uranium isotopes into the second ordinary cascade is conducted in the presence of a carrier gas having a molecular weight identical to (close to or equal to) the molecular weight of the isotope hexafluoride oranium.

In addition, the above tasks are also achieved by additional technical solutions, consisting in the fact that they use a carrier gas with an average molecular weight in the range from 346 to 348 amu; that they use a carrier gas chemically inert to uranium hexafluoride, at least up to a temperature of 125 ° C, and having a saturated vapor pressure at a working gas temperature of the second ordinary cascade of at least 133 Pa; that one or more isomers of fluorinated dimethylcyclohexane with the general chemical formula C ₈ H ₃ F ₁₃ are used as a carrier gas; that the flow rate of the carrier gas is not more than 1 wt.% the flow rate of hexafluoride mixture of uranium isotopes for food of the second ordinary cascade; that hexafluoride of a mixture of uranium isotopes and a carrier gas is supplied to various stages of the second ordinary cascade; that the selection of the light fraction of the second ordinary cascade is in contact with the sorbent from granular sodium fluoride, after which the carrier gas is returned to the power of the second ordinary cascade.

The main distinguishing feature of the method is that in the second ordinary centrifuge cascade, the separation of the mixture of uranium isotopes is carried out in the presence of a carrier gas having a molar mass comparable to the molar mass of the isotopes U-234 and U-232. The introduction of a carrier gas changes the average molecular weight of the separated isotopic mixture and, thus, changes the distribution of the shared molar masses along the length of the cascade. With a molecular weight of the carrier gas from 346 to 348 amu molecules of uranium isotopes of high molecular weight - ²³⁵ UF ₆ (349 amu), ²³⁶ UF ₆ (350 amu) and ²³⁸ UF ₆ (352 amu) - shift to selection of the heavy fraction of the second ordinary cascade, which leads to a decrease in the losses of the fissile U-235 isotope with the light fraction selection stream. In the presence of a carrier gas, molecules of uranium isotope hexafluoride with a lesser or equal molecular weight, i.e. ²³² UF ₆ (346 amu), ²³⁴ UF ₆ (348 amu) are either pushed to the extreme dividing steps for selecting the light fraction of the second ordinary cascade, or mixed with the carrier gas and discharged with it through the selection of the light fraction. Thus, the presence of a carrier gas increases the extraction of the radiation-hazardous isotope U-232 from a burnt mixture of uranium isotopes and reduces the loss of fissile isotope U-235.

A decrease in the fraction of fissile isotope U-235 in the light fraction stream reduces the mass of radioactive waste that is formed upon correction of the isotopic composition of a burnt mixture of uranium isotopes.

Dilution by the carrier gas of uranium hexafluoride in the branch of the light fraction of the second ordinary cascade reduces the molar fraction of the radiation-hazardous isotope U-232 in the working gas stream at the extreme stages of the cascade, which improves the radiation situation in the technological zone of the centrifuge cascade.

To reduce the carrier gas content in the selection of the heavy fraction, its supply to the second ordinary cascade is desirable closer to the end degrees from the side of the selection of the light fraction. The preferred carrier gas flow rate should be up to 1 wt% of the feed stream of the second ordinary cascade, since, on the one hand, a further increase in the mass fraction of the carrier gas no longer leads to an additional positive effect on the correction of the isotopic composition, on the other hand, it only increases the gas content -carrier in a marketable product - an adjusted mixture of uranium isotopes.

To achieve the above technical results, the carrier gas must have certain physical and chemical properties:

- be chemically resistant to uranium hexafluoride, at least to a temperature of 125 ° C, because in certain areas of the rotor of a gas centrifuge, the temperature of the working surfaces reaches the specified value. Otherwise, reduction of uranium hexafluoride to non-volatile uranium tetrafluoride may occur;

- the pressure of saturated vapor of the carrier gas at the temperature of the working gas of the second ordinary cascade should be at least 133 Pa, because otherwise, due to vapor condensation on the surface of the equipment, technical problems with the dosage and transfer of carrier gas through the cascade may occur.

Of the well-known set of volatile fluorine-containing compounds, the isomers of fluorinated dimethylcyclohexane with the general chemical formula C ₈ H ₃ F ₁₃ correspond to the set requirements to the greatest extent. These compounds have a molar mass of 346 to 348 g / mol, taking into account the isotopy of carbon and hydrogen (mainly 346 amu). The saturated vapor pressure of C ₈ H ₃ F ₁₃ at a temperature of 15 ° C is about 2260 Pa. Fluorinated dimethylcyclohexane is chemically inert to uranium hexafluoride to a temperature of 125 ° C.

At the output of the second ordinary cascade, the selection of the light fraction containing almost the entire U-232 isotope, a significant part of the U-234 isotope and the carrier gas are in contact (preferably at a temperature of 80 ÷ 120 ° C) with granular NaF. Due to the formation of the non-volatile adduct UF ₆ · 2NaF, the carrier gas is cleaned of the waste composition correction of the burnt mixture of uranium isotopes with the fixation of the latter on a solid sorbent. The carrier gas is returned again to the separation stage of the second ordinary cascade.

The implementation of the method is illustrated in the drawing, which shows a block diagram of a double centrifugal isotope separation cascade for isotopic reduction of regenerated uranium and carrier gas feed flows to the separation stage of the second ordinary cascade.

The double cascade is organized by ordinary cascades 1 and 2. The flow diagram 3 of the supply of the double cascade with raw uranium regenerate hexafluoride is also shown on the block diagram; stream 4 for selection of the heavy fraction (depleted in fissile isotope U-235) and stream 5 for selection of the light fraction (enriched in fissile isotope U-235) of the isotopic mixture of the first ordinary cascade; streams 6 and 7 of selection, respectively, of heavy and light fractions of the second ordinary cascade; flows 8 and 9, respectively, of the circulation of the carrier gas supply gas carrier gas of the separation stages of the cascade; one of the columns 10 filled with a sorbent of granular sodium fluoride (NaF); a uranium diluent hexafluoride stream 11 and a uranium hexafluoride stream 12 with an energy concentration of fissile isotope U-235, packaged into a container 13 for shipment to the consumer; a container 14 for packing uranium hexafluoride of a light fraction of the second ordinary cascade desorbed from sodium fluoride, as well as a container 15 for packing of uranium hexafluoride of a heavy fraction of the first ordinary cascade.

Raw uranium regenerate enters the uranium plant, usually in the chemical form of oxides. At the uranium plant, the regenerated uranium is additionally purified from fission products by radiochemical technology, after which the uranium is converted into the chemical form of hexafluoride and transported to a centrifuge installation for correcting the composition of the burnt mixture of uranium isotopes in the form of a double cascade 1-2. The double cascade is a cascade association of the separation stages of typical gas centrifuges, which relative to the collectors of the cascade supply flows are conventionally divided into separation stages of the branch of light and heavy fractions.

To feed the separation stages of regenerated uranium hexafluoride to gas centrifuges, they are gasified and, in the form of stream 3, sent to the collector of the feed stream of the first ordinary cascade 1. In streams 5 and 4 of cascade 1, respectively, a light fraction is enriched up to 5–6 wt% by fissile isotope U-235, taking into account the necessary compensation for the presence of a neutron absorber-isotope U-236, and a heavy fraction (dump) containing from 0.1 to 0.3 wt.% Of the isotope U-235. In some cases, enrichment in the first ordinary cascade by fissile isotope U-235 leads to 21 ÷ 90 wt.% To obtain a light fraction purified from the isotope U-236. In this case, the isotope U-236 practically all goes with stream 4 to the dump of cascade 1.

The light fraction of cascade 1 is sent to the collector of the power stream of the second ordinary cascade 2. Stream 7 selection of the light fraction of cascade 2 contains the bulk of the isotope U-232 and a significant part of the isotope U-234. In addition, in the selection of the light fraction, hexafluoride of the fissile isotope U-235 is present.

Stream 6 selection of the heavy fraction of cascade 2 is a hexafluoride mixture of uranium isotopes, purified from radiation-hazardous isotopes U-232 and U-234. Depending on the selected enrichment scheme in the first ordinary cascade, the mixture of uranium isotopes in stream 6 can be either energy uranium hexafluoride or high enrichment uranium hexafluoride. In the latter case, the mixture of uranium isotopes with the adjusted composition is additionally sent to the mixing site with hexafluoride of a natural (natural) mixture of uranium isotopes for the preparation of commercial reactor fuel material.

In order to reduce the losses of the fissile isotope U-235 with the light selection stream 7 and the completeness of separation of the U-232 isotope, a carrier gas 9 with a molecular weight in the range from 346 to 348 a is additionally fed into one or more separation stages of the branches of the light fraction of the centrifuge cascade 2. EM, which allows you to almost completely push the fissile isotope U-235 into stream 6 of the selection of the heavy fraction of the second ordinary cascade.

To separate UF ₆ and carrier gas, stream 7 is directed to one of the sorption columns 10 filled with granular NaF with a sorbent temperature of 80 ÷ 120 ° С. On the sorbent, chemisorption of uranium hexafluoride occurs with the formation of a strong complex compound. The carrier gas under given temperature conditions passes through the NaF layer with virtually no delay. Next, the carrier gas is sent to the return manifold 8, where it is condensed into intermediate containers for temporary storage (not shown in the drawing), and then re-soaked into the separation stages of cascade 2 in the form of stream 9. The carrier gas can also be supplied from the outlet of the column 10 directly into the power stream 9 without intermediate condensation-evaporation operations.

After saturation of the sorbent NaF with uranium hexafluoride, one of the columns 10 is temporarily cut off from stream 7 and transferred to the desorption mode. The desorption mode consists in heating NaF above a temperature of 300 ° C for subsequent desublimation of desorbed UF ₆ into a container 14 for long-term responsible storage.

The heavy fraction 4 of cascade 1 is packed into a container 15, and then sent for long-term storage, since the comparable content of the fissile isotope U-235 and neutron absorber U-236 (1: 1) in this isotopic mixture makes this depleted uranium unpromising for future use.

Specific examples of the implementation of the method are shown in tables 1-2. Three batches of 10,000 kg of raw uranium regenerate hexafluoride (RepU) with a mass fraction of fissile isotope U-235 of 0.9 wt% (see Table 1, column 2) were subjected to experimental adjustment of the isotopic composition (isotopic reduction) in order to obtain ~ 1000 kg (metal) of energy uranium hexafluoride. Uranium enrichment was carried out in cascades of constant width (the same number of gas centrifuges in separation stages). The main goal of the research was to achieve the maximum return of the fissile isotope U-235 to the fuel cycle.

A two-stage correction scheme for the isotopic composition of regenerated uranium without the use of a carrier gas made it possible to obtain (see: Table 1, column 6) uranium energy hexafluoride in which the content of the U-232 isotope is at the level of ~ 4.9 × 10 ^-8 wt.%. The selection of the light fraction of the second ordinary cascade (see table 1, column 5) amounted to 0.15% of the initial amount of raw materials and consisted of 82.22 wt.% From the fissile isotope U-235 and 17.37 wt.% From the isotope U-234.

Table 1
Correction of RepU isotopic composition in a double stage without the use of carrier gas (basic version) Options Raw RepU RepU ₁ ^*) (selection) RepU ₁ (blade) RepU ₂ (selection) RepU ₂ (blade) Cascade 1 Cascade 2 one 2 3 four 5 6 Nutrition, kg UF ₆ 10,000 10,000 10,000 ~ 1500 ~ 1500 Mass flow, kg UF ₆ - ~ 1500 ~ 8500 ~ 15 ~ 1485 U-235, wt.% 0.9 5,510 ~ 0.1 82.22 4,735 U-232, wt.% 2.0 × 10 ^-7 1.32 × 10 ^-6 1.27 × 10 ^-9 1.26 × 10 ^-4 4.9 × 10 ^-8 U-234, wt.% 0,03 0.193 0.001 17.37 0.019 U-236, wt.% 0.5 2,702 0,111 0.395 2,735 U-238, wt.% 98.57 91.59 99.98 0.001 92.52 ^*) - the index in the designation RepU corresponds to the number of the ordinary cascade.

A study of the influence of an extraneous chemical component on the process of purification of a raw uranium regenerate from the U-232 isotope was carried out using Freon ¹² C ₈ H ₃ F ₁₃ as an example, having a molecular weight close to ²³² UF ₆ (346 amu). At a temperature of 15 ° C, the saturated vapor pressure of ¹² C ₈ H ₃ F ₁₃ is 2287.6 Pa; it is chemically stable in the environment of uranium hexafluoride up to 300 ° C. Since the separation characteristics of gas centrifuges are mainly determined by the molecular weight of the working substance, the effect of Freon ¹² C ₈ H ₃ F ₁₃ on the hydraulic and separation characteristics of serial separation equipment has not been noted. For a fixed value of the flow F 5 supply the working gas (UF ₆₎ of the second stage in the ordinary separation stage cascade stream 9 is further fed Freon ¹² C ₈ H ₁₃ F ₃ F value under the condition P / F≈const = 0,01. Carrier gas was supplied to the separation stage of the working gas supply (table 2, option 1) or to the separation stage of the light fraction selection branch (table 2, option 2).

table 2
Purification of RepU ₁ (selection) from the U-232 isotope in the second ordinary cascade Options RepU ₁ (selection) RepU ₂ (selection) RepU ₂ (blade) RepU ₂ (selection) RepU ₂ (blade) Option 1 Option 2 one 2 3 four 5 6 Flow% one hundred one 99 one 99 U-235, wt.% 5,510 0.629 5,504 0,057 5,509 U-234, wt.% 0.193 2,512 0.167 0.270 0.190 U-236, wt.% 2,702 0.002 2,702 <0.001 2,702 U-238, wt.% 91.59 <0.001 91.60 <0.001 91.59 Freon, wt.% one 96.85 0,021 99.67 0.003 U-232, wt.% ^**) 1.32 × 10 ^-6 1.28 × 10 ^-4 2.7 × 10 ^-8 1.32 × 10 ^-4 4.4 × 10 ^-9 **) - content of the U-232 isotope in a mixture of uranium hexafluoride and freon.

In each of the studied options, an increase was obtained both in the absolute value of the concentration of fissile isotope U-235 in the stream of the heavy fraction of the cascade, and in the total extraction of the isotope U-235 (see Table 2, columns 4 and 6). This indicates that the U-235 isotope, when Freon was added to the cascade nutrition, was almost completely pushed into the heavy fraction selection branch. In the light fraction selection stream, the content of the U-235 isotope became lower than the concentration of the U-234 isotope (see Table 2, columns 3 and 5).

The supply of carrier gas to the separation stage of the light fraction selection branch made it possible to reduce the freon content in the heavy cascade selection stream by an order of magnitude (see Table 2, lines 4 and 6) and increase the extraction of the U-232 isotope in the light cascade selection stream.

The data in Table 2 also show that an increase in the mass fraction of the carrier gas in the supply stream of the second ordinary cascade above 1% does not make sense, since it only leads to an increase in the freon content in the selection stream of the heavy fraction.

A mixture of carrier gas and UF ₆ selection of the light fraction of the cascade was passed through a column with a granular NaF sorbent heated to 80 ° C. All UF ₆ of the selection stream and about 0.01 wt.% Of the carrier gas were adsorbed on NaF. The non-adsorbed carrier gas was re-directed to the second ordinary cascade. As uranium hexafluoride accumulated, the columns with the sorbent were transferred to the desorption mode by heating to 350 ° C. Desorbed UF _{6 by} desublimation was packaged in containers and sent for storage.

It is understood that the invention is not limited to the examples given. There are other possible examples of examples within the scope of the proposed claims. The optimal correction option for the isotopic composition is selected based on the isotopic composition of the raw uranium regenerate, customer requirements for the permissible limits of harmful isotopes, as well as the ratio of the prices of raw natural uranium and separation work.

Thus, the given examples of practical options for the implementation of the proposed method for correcting the isotopic composition of regenerated uranium have clearly shown that the addition of a carrier gas makes it possible to extract at least 99 wt.% Of the radiation-hazardous isotope U-232 from the raw uranium regenerate. Losses of the fissile isotope U-235 in the second ordinary cascade in this case are reduced 100–1000 times and do not exceed 0.01–0.1 mass%. The proposed method is suitable for use in multiple SNF recycling.

Claims (7)

1. The method of isotopic reduction of regenerated uranium, including the correction of the composition of a burnt mixture of uranium isotopes in a double centrifuge cascade during the selection of hexafluoride of a mixture of uranium isotopes purified from radiation-hazardous isotopes U-232 and U-234, through the selection of the heavy fraction of the second ordinary cascade, characterized in that the separation of the mixture of uranium isotopes in the second ordinary cascade is carried out in the presence of a carrier gas having an average molecular weight of 346 to 348 amu 2. The method according to claim 1, characterized in that a carrier gas is used that is chemically inert to the hexafluoride of a mixture of uranium isotopes, at least to a temperature of 125 ° C and having a saturated vapor pressure at a working gas temperature of the second ordinary cascade of at least 133 Pa . 3. The method according to claim 1 or 2, characterized in that as the carrier gas using one or more isomers of fluorinated dimethylcyclohexane with the General chemical formula C ₈ H ₃ F ₁₃ . 4. The method according to claim 1, characterized in that the consumption of carrier gas is not more than 1 wt.% Of the consumption of hexafluoride of a mixture of uranium isotopes for food of the second ordinary cascade. 5. The method according to claim 1, characterized in that the carrier gas in the second ordinary cascade is brought into the separation stage of the hexafluoride supply of a mixture of uranium isotopes. 6. The method according to claim 1, characterized in that the carrier gas in the second ordinary cascade is brought into the separation stage of the selection branch of the light fraction. 7. The method according to claim 1, characterized in that the selection of the light fraction of the second ordinary cascade is contacted with a sorbent from granular sodium fluoride, after which the carrier gas is returned to the power of the second ordinary cascade.