RU2497210C1 - Method of isotopic recovery of regenerated uranium - Google Patents

Abstract

FIELD: power engineering.

SUBSTANCE: method of isotopic recovery of regenerated uranium includes increase of content of isotope U-235 in hexafluoride of regenerated uranium to the specified value in the range of 2.0÷5.0 wt %, reduction of relative concentration of isotope U-232 in the mixture of uranium isotopes and direct enrichment of hexafluoride of regenerated uranium with isotope U-235 on the double-cascade plant from separation stages of gas centrifuges. At the same time in the first cascade the regenerated uranium is enriched with isotope U-235 up to 5.0÷10.0 wt % with maintenance of ratio of mass flows of the dump flow and bleed flow of the cascade in the range of (6.9÷18.4):1. Flows of the dump and bleed of the first cascade are sent for supply of the second cascade. The regenerated uranium is bled from the separating stage of the central part of the second cascade.

EFFECT: complete treatment of a burnt mixture of uranium isotopes from the most radiation-hazardous nuclide U-232 and production of commercial low-enriched uranium hexafluoride at minimum retuning of industrial cascades of centrifuges.

2 cl, 1 dwg, 7 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.

Uranium regenerated from spent nuclear fuel (SNF) is a valuable source for reuse in light-water reactors (LWR), since it contains fissile isotope U-235 in an amount not less than natural uranium and allows saving the latter. For this purpose, the burnt mixture of uranium isotopes should be enriched with the U-235 isotope to a content of 2.0 ÷ 5.0 wt.%.

However, in the burnt mixture, in addition to the difference in the concentration of the fissile isotope U-235 in significant quantities, there are undesirable even isotopes U-232, U-234 and U-236, which are not separated during the chemical processing of spent nuclear fuel and which are associated with an increased content of the main difficulties in handling with regenerated uranium.

The economically feasible (acceptable) range of concentrations of the isotopes U-232, U-234, U-236 and U-235 in the restored burnt mixture is justified in [Patent RU 2110855, IPC G21C 3/42, publ. 05/10/1998] and amounts, wt.%: U-235 1 ÷ 10; U-232 4.9 × 10 ^-7 ÷ 3.8 × 10 ^-9 ; U-234 1.7 × 10 ^-1 ÷ 7.3 × 10 ^-3 ; U-236 8.0 × 10 ^-l ÷ 6.4 × 10 ^-3 ; U-238 and other impurities - the rest. The relative deviation of the content from the nominal values from minus 35 to plus 35%.

The term "content" or "concentration" hereinafter refers to the mass content (concentration) of a particular uranium isotope only in a mixture of uranium isotopes. These terms do not refer to the chemical form of uranium.

The enrichment of burned-out nuclear fuel with the U-235 isotope can be carried out by two methods: either by adding to the burned-out mixture of uranium isotopes an isotopic mixture of uranium-reducer with a higher concentration of U-235, i.e. enrichment; or enrichment of the burnt isotope mixture to a higher concentration in accordance with U-235 in isotope-separation gas-centrifuge cascades, i.e. direct enrichment.

A known method of 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 2113 022, IPC G21C 19/42, publ. 05/20/1997], consisting in the fact that the hexafluoride of a burnt mixture of uranium isotopes increases the concentration of the U-235 isotope compared to the initial concentration to a predetermined value and at the same time reduces the concentration of the U-236, U-234 and U-232 isotopes compared to the initial concentration, mixing among themselves in the liquid and / or gas phases three components: hexafluoride of a burnt mixture of uranium isotopes, dump uranium hexafluoride of natural origin with a concentration of U-235 isotope mainly in the range of 0.15 ÷ 0.7115 wt.% and enriched hexafluoride (from 3.6 to 100 ma .%) Of uranium of natural origin. 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.

The mentioned method allows you to prepare nuclear fuel for reuse with almost any reduced composition of undesirable isotopes in the recovered mixture of uranium isotopes. However, mixing components that differ in the isotopic spectrum of uranium isotopes devalues the separation work contained in the component with a higher concentration of fissile isotope U-235.

  A method for the isotopic reduction of regenerated uranium in the fuel cycle is proposed [Patent RU 2236053, IPC G21C 19/42, B01D 59/20, publ. September 10, 2004], which includes the direct enrichment of raw uranium regenerate in an ordinary isotope-separation gas centrifuge cascade. The mixture of uranium isotopes is enriched in U-235 isotope to 10.0 ÷ 90.0 wt.% In a cascade installation, after which it is diluted with natural uranium with a U-235 content from 0.1 to 5.0 wt.%, Thereby devaluing separation work.

For the prototype of the selected method of isotopic reduction of regenerated uranium [Patent RU 2242812, IPC G21C 19/42, B01D 59/20, publ. December 20, 2004], consisting in the fact that in a burnt mixture of uranium isotopes, the concentration of the U-235 isotope 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 gas centrifuge isotope-separation cascade. Raw uranium regenerate is enriched in the U-235 isotope to a content of 21.0 ÷ 90.0 wt.% In a double cascade during the recovery of recovered fuel material through the dump stream of the second ordinary cascade. When the content of the U-235 isotope in the dump stream of the second ordinary cascade is higher than 7.0 wt.%, The enriched regenerated uranium is diluted with natural uranium hexafluoride to a mass not exceeding the mass of the raw uranium regenerate.

A double cascade is a system of two ordinary three-stream cascades, in which the selection stream of the first ordinary cascade with respect to the external supply serves as the supply stream of the second ordinary cascade.

The disadvantages of the prototype method include insufficiently complete extraction of radiation-hazardous nuclide U-232 from a mixture of uranium isotopes.

The objective of the invention is a more complete cleaning of a burnt mixture of uranium isotopes from the most radiation-hazardous U-232 - a nuclide that forms the main dose of external exposure to personnel at all stages of work with regenerated uranium raw materials, as well as the elimination of the dilution of the corrected isotopic composition of regenerated uranium with natural uranium .

The above technical problems are achieved by the fact that in the method of isotopic reduction of regenerated uranium for reuse in a nuclear reactor, which consists in increasing the content of the U-235 isotope in the hexafluoride of regenerated uranium to a value specified in the range 2.0 ÷ 5.0 wt.% values and lower the relative concentration of the U-232 isotope in a mixture of uranium isotopes, including the direct enrichment of regenerated uranium hexafluoride with the U-235 isotope in a two-stage installation from separation stages of gas centrifuges, in the first regenerated uranium is enriched in the U-235 isotope to 5.0 ÷ 10.0 wt.% while maintaining the ratio of the mass flow rates of the dump stream and the cascade selection stream in the range (6.9 ÷ 18.4): 1, dump and selection flows of the first cascade sent to the power of the second cascade, while the isotopically reduced regenerated uranium is taken from the separation stage of gas centrifuges of the central part of the second cascade.

In addition, the above tasks are also achieved by the fact that isotopically reduced regenerated uranium is selected in an amount not exceeding 0.13 mass fraction of the external power flow of a two-stage installation.

The figure shows a block diagram of a two-stage installation for isotopic reduction of regenerated uranium.

The second cascade of the two-stage installation is made in the form of a five-stream (five-pipe) cascade having two power flows in the enrichment and depletion branches of the cascade and three selection flows: a selective stream in the extreme stage of the depletion branch (“dump” stream); a selective stream at the extreme stage of the enrichment branch of the cascade, referred to as the “purge” stream, and a selective stream of isotopically reduced regenerated uranium hexafluoride, organized from the separation stage of the central part of the cascade. In this case, the central part of the cascade means a section of the separation stages of gas centrifuges located between the cascade supply flows.

The supply of the second cascade with the dump and cascade selection flows with external power supply to the cascade unit with a ratio of mass flow rates (6.9 ÷ 18.4): 1 provides such a distribution of the undesired U-232 isotope over the cascade installation, in which U-232 is concentrated mainly in the beneficiation plant branches of the second cascade and has a low content in its central part. An additional selection of regenerated uranium hexafluoride, organized from the central part of the second cascade, will thus contain a reduced concentration of U-232 nuclide.

The content of the U-235 isotope in the selection stream of the first cascade (with external power) of the cascade installation in the range of 5.0 ÷ 10.0 wt.% Allows uranium hexafluoride with a concentration of U-235 2.0 ÷ to be selected from the separation stage of the central part of the second cascade 5.0 wt.%, Which corresponds to the requirements of the patent RU 2110855 for undesired isotopes without additional operations of isotopic adjustment by natural uranium diluent.

The amount of selection of isotopically reduced regenerated uranium from the central part of the second cascade affects the distribution of the nuclide U-232 along the length of the latter. This can lead to an increase in the concentration of U-232 in marketable products with increased selection flows. The restriction of the magnitude of the selection stream of isotopically reduced regenerated uranium by a mass fraction of 0.13 of the magnitude of the external supply stream of the two-stage installation of regenerated uranium hexafluoride allows us not to go beyond the requirements of patent RU 2110855 on undesired isotopes.

The cascade installation (see drawing) is formed of two cascades of gas centrifuges - the first three-stream cascade 1 and the second five-stream cascade 2. The flowchart 3 shows the external power supply of the cascade with raw uranium regenerate hexafluoride from container 12; an intermediate dump stream 4 of uranium hexafluoride of the first cascade 1 depleted in the U-235 isotope relative to the external feed stream 3; selective stream 5 of the first ordinary cascade enriched in the U-235 isotope relative to stream 3 of the external power supply. Streams 4 and 5 serve as power flows of the second cascade, respectively, to the depletion and enrichment branches of the latter. The second cascade in the depleted branch has a stream 6, which is a dump stream of the cascade installation, and in the enrichment branch, a selection stream 7, which is a selection ("cleaning") stream of the cascade installation. Selective stream 7 "cleaning" of the cascade installation can be enriched in the fissile isotope U-235 up to 90 wt.% (Preferably up to 20 wt.%) And contains the radiation-hazardous isotope U-232 with increased concentration, and the dump stream 6 is depleted in the isotope U -235 to 0.1-0.3 wt.%. Stream 8 of an additional selection of isotopically reduced regenerated uranium (target commercial product) is organized in the central part of the cascade from the separation stage, where the content of the U-235 isotope corresponds to the ordered value of the commercial product in the range of 2.0-5.0 wt.%. The target product is packaged by desublimation in container 9 for shipment to the reactor fuel fabrication.

Positions 10 and 11 are containers intended for packing uranium hexafluoride of the dump stream 6 and uranium hexafluoride of the cleanup stream 7, respectively.

The filled containers 10 and 11 are sent to storage or to the mixing section, where the contents of the container 11 are poured into the container 10, as a result of which the uranium hexafluoride enriched in the U-235 isotope to 20 wt.% Or more is diluted to the category of depleted uranium. At the same time, the isotopic composition of depleted uranium decreases in the concentration of U-232 stream 7 "cleaning". Moreover, in the container 10, the gamma radiation of the U-232 isotope is additionally shielded by depleted uranium hexafluoride, as a high-density substance. This decision minimizes the risks of long-term storage of unclaimed products of isotopic adjustment of regenerated uranium.

Specific examples of isotopic reduction of regenerated uranium with an initial concentration of fissile isotope U-235 of 0.8 wt.% Are shown in tables 1-7.

Table 1 shows the results of the isotopic reduction of regenerated uranium to a U-235 content of 4.0 wt.% By direct enrichment in an ordinary cascade, Table 2-7 shows the results of the isotopic reduction of regenerated uranium to a content of U-235 4.0 wt.% direct enrichment in a two-stage installation of gas centrifuges according to the proposed method.

A batch of 148 tons of hexafluoride (or 100 tons of uranium) of raw uranium regenerate was subjected to isotopic reduction, the isotopic composition of which is given in Tables 1-7, column 2.

As can be seen from table 1, the direct enrichment of raw uranium regenerate in an ordinary cascade does not ensure the achievement of the requirements of the economically feasible concentration range of the nuclide U-232 given in patent RU 2110855.

The results of the isotopic reduction of the uranium regenerate according to the proposed technical solution with intermediate enrichment in the first ordinary cascade of the cascade installation using the U-235 isotope 5.0 wt.% With a ratio of the mass flows of the dump and selection equal to 91.304: 8.696≈10.5: 1, and intermediate enrichment in the first ordinary cascade of a cascade installation using the U-235 isotope of 10 wt.% with a ratio of the mass flows of the dump and selection equal to 94.84: 5.155≈18.4: 1, are given in Table 2-5. The U-235 isotope enrichment in the second cascade in Tables 2 and 3 (column 6) was 20 wt.%, In Tables 4 and 5 (column 6) - 40 and 60 wt.%, Respectively.

Table 6 shows the results of isotopic reduction of the uranium regenerate with enrichment in the first ordinary cascade of the cascade unit using the U-235 isotope of 5.0 wt.% With a ratio of the mass flows of the dump and selection equal to 6.9: 1, and in Table 7 - the results of the isotopic reduction of the uranium regenerate with enrichment in the first ordinary cascade of the cascade installation using the U-235 isotope 10.0 wt.% with the ratio of the mass flows of the dump and selection equal to 15.4: 1.

Table 1 Direct isotopic reduction of regenerated uranium in an ordinary cascade Options Raw uranium regenerate Isotope Reduced Uranium Regenerate one 2 3 Mass flow rate, t U one hundred one hundred Product weight, t U - 14,890 U-235, wt.% 0.80 4.0 U-232, wt.% 1.5 × 10 ^-7 9.8 × 10 ^-7 U-234, wt.% 0.016 0.1 U-236, wt.% 0.35 1.27 U-238, wt.% rest rest

table 2 Isotopic reduction of regenerated uranium with the ratio of the mass flows of the dump and selection in the first cascade ~ 10.5: 1 Options Raw material regenerate The first ordinary cascade Second Ordinary Cascade Selection Blade Cleaning Product Blade one 2 3 four 5 6 7 Mass flow rate, t U one hundred - - - - - Mass flow, t U - 8,696 91,304 0.735 11,030 88,253 U-235, wt.% 0.80 5,0 0.4 20,0 4.0 0.24 U-232, wt.% 1.5 × 10 ^-7 1.51 × 10 ^-6 2.0 × 10 ^-8 1.1 × 10 ^-5 5.91 × 10 ^-7 4,5 × 10 ^-9 U-234, wt.% 0.016 0.122 0.0053 0.671 0.0817 0.00232 U-236, wt.% 0.35 1,449 0.245 3,965 1,377 0.191 U-238, wt.% rest rest rest rest rest rest

In tables 2 and 6, the concentration of the nuclide U-232 in the target product, respectively equal to 5.91 × 10 ^-7 wt.% And 5.37 × 10 ^-7 wt.% (Columns 6), is reduced compared with the concentration of the nuclide U -232 in the raw uranium regenerate, and although it remains very high, it falls within the interval specified in patent RU 2110855 (4.9 × 10 ^-7 wt.% + 35%).

In tables 3, 4, 5, 7, the concentration of U-232 nuclide in the selection stream 8 is significantly reduced, and amounted to 1.69 × 10 ^-7 wt.% (Table 3, column 6), 1.50 × 10 ^-7 wt.% (table 4, column 6), 1.44 × 10 ^-7 wt.% (table 5, column 6), and 1,194 × 10 ^-7 wt.%, (table 7, column 6), which comply with the standards declared in RU 2110855.

Table 3 Isotopic reduction of uranium regenerate with the ratio of the mass flows of the dump and selection in the first cascade ~ 18.4: 1 Options Raw material regenerate The first ordinary cascade Second Ordinary Cascade Selection Blade Cleaning Product Blade one 2 3 four 5 6 7 The mass of the feed stream, t U one hundred - - - - - Mass flow, t U * - 5,155 94,845 2,152 3,586 94,262 U-235, wt.% 0.80 10.0 0.3 20,0 4.0 0.24 U-232, wt.% 1.5 × 10 ^-7 2.76 × 10 ^-6 8.2 × 10 ^-9 6.5 × 10 ^-6 1.69 × 10 ^-7 4.22 × 10 ^-9 U-234, wt.% 0.016 0.251 0,00323 0.552 0,0564 0.00224 U-236, wt.% 0.35 2,718 0.220 4,586 1,750 0.20 U-235, wt.% rest rest rest rest rest rest

Table 4 Isotopic reduction of uranium regenerate with the ratio of the mass flows of the dump and selection in the first cascade ~ 18.4: 1 Options Raw material regenerate The first ordinary cascade Second Ordinary Cascade Selection Blade Cleaning Product Blade one 2 3 four 5 6 7 Mass flow rate, t U one hundred - - - - - Mass flow, t U - 5,155 94,845 0.956 4,782 94,262 U-235, wt.% 0.80 10.0 0.3 40,0 4.0 0.24 U-232, wt.% 1.5 × 10 ^-7 2.76 × 10 ^-6 8.2 × 10 ^-9 1.5 × 10 ^-5 1.50 × 10 ^-7 4.22 × 10 ^-9 U-234, wt.% 0.016 0.251 0,00323 1,186 0,0534 0.00224 U-236, wt.% 0.35 2,718 0.220 7,745 1,824 0.20 U-238, wt.% rest rest rest rest rest rest

Table 5 Isotopic reduction of uranium regenerate with the ratio of the mass flows of the dump and selection in the first cascade ~ 18.4: 1 Options Raw material regenerate The first ordinary cascade Second Ordinary Cascade Selection Blade Cleaning Product Blade one 2 3 four 5 6 7 Mass flow rate, t U one hundred - - - - - Mass flow, t U - 5,755 94,845 0.615 5,123 94,262 U-235, wt.% 0.80 10.0 0.3 60.0 4.0 0.24 U-232, wt.% 1.5 × 10 ^-7 2.76 × 10 ^-6 8.2 × 10 ^-9 2.3 × 10 ^-5 7.44 × 10 ^-7 4.22 × 10 ^-9 U-234, wt.% 0.016 0.251 0,00323 1,827 0,0519 0.00224 U-236, wt.% 0.35 2,718 0.220 10.46 1,890 0.20 U-238, wt.% rest rest rest rest rest rest

Table 6 Isotopic reduction of uranium regenerate with the ratio of the mass flows of the dump and selection in the first cascade ~ 6.9: 1 Options Raw material regenerate The first ordinary cascade Second Ordinary Cascade Selection Blade Cleaning Product Blade one 2 3 four 5 6 7 The mass of the feed stream, t U one hundred - - - - - Mass flow, t U - 72.60 87.50 0.860 12,898 86,242 U-235, wt.% 0.80 5,0 0.2 20,0 4.0 0.13 U-232, wt.% 1.5 × 10 ^-7 2,182 × 10 ^-6 2.62 × 10 ^-9 9.30 × 10 ^-6 5,370 × 10 ^-7 7.30 × 10 ^-10 U-234, wt.% 0.016 0.166 0,00172 0.624 0,077 0,00085 U-236, wt.% 0.35 1,567 0.176 4,210 1,466 0.145 U-238, wt.% rest rest rest rest rest rest

As can be seen from the results of Table 3-5, the increase in enrichment for the U-235 isotope in the second cascade from 20 to 60 wt.% With a ratio of the mass flows of the dump and selection equal to 94.84: 5.155≈18.4: 1 is small affects the concentration of U-232 nuclide in a mixture of uranium isotopes of reduced uranium regenerate (see column 6). An increase in the ratio of the mass flows of the dump and the selection in the first cascade of more than 18.4: 1 is also not advisable, since the concentration of the U-232 nuclide in the mixture of uranium isotopes of reduced uranium regenerate can be reduced only slightly.

Table 7 Isotopic reduction of uranium regenerate with the ratio of the mass flows of the dump and selection in the first cascade ~ 15.4: 1 Options Raw material regenerate The first ordinary cascade Second Ordinary Cascade Selection Blade Cleaning Product Blade one 2 3 four 5 6 7 Mass flow rate, t U one hundred - - - - - Mass flow, t U - 6,122 93,878 2,546 4,242 93,212 U-235, wt.% 0.80 10.0 0.2 20,0 4.0 0.13 U-232, wt.% 1.5 × 10 ^-7 2,412 × 10 ^-6 2.45 × 10 ^-9 5.67 × 10 ^-6 1,194 × 10 ^-7 6.77 × 10 ^-10 U-234, wt.% 0.016 0.936 0,00165 0.517 0,0494 0,00081 U-236, wt.% 0.35 2,877 0.155 4,882 1,950 0.153 U-238, wt.% rest rest rest rest rest rest

You can influence the extraction of the nuclide U-232 from a mixture of isotopes of uranium regenerate by varying the flow rate of the selection of isotopically reduced uranium regenerate (product). Isotopically reduced regenerated uranium is selected in an amount not exceeding 0.13 mass fractions of the external feed stream of a two-stage installation.

In Table 2, the amount of selection of isotopically reduced regenerated uranium from the central part of the second cascade was 0.11 mass fraction of the external power supply stream of the cascade installation (11.030: 100 = 0.11, where “11.030” is the mass of the product flow, column 6, “100 "Is the mass of the power flow, column 2). In tables 3-7, the selection values of isotopically reduced regenerated uranium from the central part of the second cascade were 0.036, respectively; 0.048; 0.051; 0.129 and 0.042.

The proposed method for correcting the isotopic composition of regenerated uranium provides for obtaining commodity low enriched uranium hexafluoride with minimal restructuring of industrial cascades of gas centrifuges of the plant.

Claims (2)

1. The method of isotope reduction of regenerated uranium for reuse in a nuclear reactor, which consists in increasing the content of the U-235 isotope in the regenerated uranium hexafluoride to a value specified in the range 2.0 ÷ 5.0 wt.% And lowering the relative concentration of the isotope U -232 in a mixture of uranium isotopes, including direct enrichment of regenerated uranium hexafluoride with U-235 isotope in a two-stage installation from gas centrifuge separation stages, characterized in that the regenerated uranium in the first stage quench with the U-235 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 range (6.9 ÷ 18.4): 1, the dump and selection flows of the first cascade are sent to power the second cascade, while the isotopically reduced regenerated uranium is taken from the separation stage of the gas centrifuges of the central part of the second cascade. 2. The method according to claim 1, characterized in that the isotopically reduced regenerated uranium is selected in an amount not exceeding 0.13 of the mass fraction of the external feed stream of a two-stage installation.

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