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Home , Decay heat, Isotopes of thorium

FR0104396 LONG-TERM BEHAVIOR OF RADIOTOXICITY AND DECAY HEAT POWER OF SPENT - AND FUEL

A.S.Gerasimov, B.R.Bergelson, T.S.Zaritskaya, G.V.Kiselev, L.A.Myrtsymova, G.V.Tikhomirov State Scientific Center of the Russian Federation Institute of Theoretical and Experimental Physics (SSC RF ITEP), 25, B.Cheremushkinskaya, Moscow, 117259, Russia [email protected]

Changes of a radiotoxicity and decay heat power of from spent U-Pu and Th-U nuclear fuel of PWR-type reactors at long-term storage during 300 are investigated. Extraction of most important for transmutation permits to reduce radiological danger of wastes remaining in storage.

INTRODUCTION

The problem of a radiotoxicity of long-lived radioactive wastes produced in various nuclear fuel cycles is important from the viewpoint of ecological danger of these cycles. Separation of the most important nuclides and extraction them from storage with subsequent transmutation permits to reduce radiological danger of wastes staying in storage. Removal of nuclides with increased decay heat power from storage permits to ease requirements to heat removal systems at long-term storage of wastes. Quantitative comparison of the radiological characteristics of minor actinides produced in various fuel cycles is also of interest. Changes of radiotoxicity and decay heat power of actinides from spent uranium - plutonium and thorium nuclear fuel of VVER-1000 type reactors at storage during 300 years are investigated in the paper. In previous investigations (1, 2), calculation of radiotoxicity and decay heat power of spent uranium fuel were submitted at long-term storage or accumulation in storehouse. Radiation characteristics of fission products from spent uranium-plutonium or thorium fuel are close to those from uranium fuel which are studied in (1, 2). The radiotoxicity RT, of i by water is determined by ratio RT,- = A,- / MPA,-, where A/ - activity of considered amount of a nuclide i, MPA,- - maximum permissible activity of this nuclide by water accepted in Russia in 1999 (3). Total radiotoxicity is a sum of radiotoxicities.

SPENT URANIUM-PLUTONIUM FUEL

Total radiotoxicity of actinides by water and contributions of most important actinides in total radiotoxicity at storage of spent uranium-plutonium MOX-fuel are presented in table 1. Total decay heat power and contributions of most important actinides are given in table 2. T is storage time. The data on half-life times and power in decays are taken from (4). The fresh fuel was a mix of with addition of 3.5 % plutonium-239. The data on contents of actinides in spent uranium - plutonium fuel are obtained by model calculations of fuel burnup in spectrum created by uranium fuel in an active core of VVER type reactor (5). They correspond to burnup of 44 kg of fission products per 1 ton and subsequent cooling during 3 years. of , plutonium, , and without uranium isotopes were taken into account. The radiotoxicity of actinides by water in initial of storage is determined by nuclides Pu-238, Pu-240, Pu-241, Am-241, Cm-244. The contribution of Cm-244 in initial period makes 25%, plutonium isotopes - 55%, Am-241 - 10-15%. After 100 years of a storage, total radiotoxicity of actinides decreases 1.4 times. The main contribution 70 % gives Am-241, plutonium isotopes - P5-07 28%, Cm-244 - 0.8% of total radiotoxicity to the end of 100- storage. The radiotoxicity of actinides of uranium-plutonium fuel appears about 3 times more than for uranium fuel.

Table 1. Radiotoxicity of actinides from 1 ton of spent uranium-plutonium fuel, 10 14 kg water

T, years Nuclide 1 3 10 30 100 300 Pu-238 2.0 1.9 1.8 1.6 0.91 0.19 Pu-239 0.43 0.43 0.43 0.43 0.43 0.42 Pu-240 1.2 1.2 1.2 1.2 1.2 1.1 Pu-241 4.2 3.8 2.7 1.0 3.6-2 6.3-5 Am-241 1.5 2.1 3.6 5.8 6.5 4.8 Am-242m 1.3-2 1.3-2 1.3-2 1.2-2 8.4-3 3.4-3 Am-243 4.5-2 4.5-2 4.5-2 4.5-2 4.5-2 4.4-2 Cm-242 1.3-2 1.2-3 6.4-4 5.8-4 4.2-4 1.7-4 Cm-243 3.5-2 3.4-2 2.8-2 1.7-2 3.2-3 2.5-5 Cm-244 3.5 3.2 2.5 1.2 7.9-2 3.8-5 Total 13 13 12 11 9.2 6.6

Table 2. Decay heat power of actinides from 1 ton of spent uranium-plutonium fuel, Watt

T, years Nuclide 1 3 10 30 1000 300 Pu-238 106 105 99.0 84.6 48.9 10.3 Pu-239 20.1 20.1 20.1 20.1 20.1 20.0 Pu-240 55.0 55.1 55.3 55.5 55.4 54.3 Pu-241 10.6 9.60 6.85 2.62 9.02-2 1.57-4 Pu-242 0.209 0.209 0.209 0.209 0.209 0.209 Am-241 96.5 130 225 363 407 297 Am-243 2.70 2.70 2.70 2.70 2.68 2.63 Cm-242 15.5 1.45 0.763 0.697 0.506 0.203 Cm-243 3.24 3.09 2.60 1.60 0.292 2.25-3 Cm-244 398 369 282 131 9.00 4.26-3 Total 708 696 694 663 544 385

The decay heat power of actinides in beginning of a storage is determined by Cm-244 which gives about 50 % of a power. The plutonium isotopes create 27 %, Am-241 - 15-20 %. After 100 years of a storage, total power of actinides decreases 1.3 times. The main contribution 75 % gives Am-241, plutonium isotopes - 23 %, Cm-244 - 1.6 %. The power of actinides of uranium - plutonium fuel appears about 3 times more than at usual uranium fuel because of greater accumulation of , americium, and curium.

SPENT THORIUM-URANIUM FUEL

Total radiotoxicity of actinides by water and contributions of most important actinides in total radiotoxicity at storage of spent thorium-uranium fuel are presented in table 3. Total decay heat power and contributions of most important actinides are given in table 4. The data on contents P5-07 of actinides in spent thorium-uranium fuel are calculated for neutron spectrum created by basic uranium fuel of VVER type reactor. They correspond to burnup of basic uranium fuel 44 kg of fission products per 1 ton and subsequent cooling during 3 years. The fresh fuel was a mix of thorium with addition of 3.3 % uranium-233. , uranium, and more heavier were taken into account.

Table 3. Radiotoxicity of actinides from 1 ton of spent thorium-uranium fuel, 10 kg water

T, years Nuclide 1 3 10 30 100 300 Th-228 8.3-2 0.18 0.25 0.21 0.10 1.2-2 Th-232 6.3-5 6.3-5 6.3-5 6.3-5 6.3-5 6.3-5 U-232 1.2 1.2 1.1 0.92 0.45 6.0-2 U-233 2.7-4 2.7-4 2.7-4 2.7-4 2.7-4 2.7-4 U-234 4.4-3 4.4-3 4.4-3 4.4-3 4.4-3 4.4-3 Pu-238 7.0-2 6.9-2 6.5-2 5.6-2 3.2-2 6.6-3 Pu-239 3.5-5 3.5-5 3.5-5 3.5-5 3.5-5 3.5-5 Pu-240 2.6-5 2.6-5 2.6-5 2.6-5 2.6-5 2.5-5 Pu-241 8.0-5 7.2-5 5.2-5 2.0-5 6.8-7 - Am-241 2.5-5 3.5-5 6.4-5 1.1-4 1.2-4 8.7-5 Total 1.4 1.5 1.4 1.2 0.59 8.6-2

Table 4. Decay heat power of actinides from 1 ton of spent thorium-uranium fuel, Watt

T, years Nuclide 1 3 10 30 1000 300 Th-228 88.1 190 263 222 110 14.7 Th-232 2.46-3 2.46-3 2.46-3 2.46-3 2.46-3 2.46-3 U-232 44.9 44.0 41.0 33.5 16.6 2.22 U-234 1.00 1.00 1.00 1.00 1.00 1.00 Pu-238 3.76 3.70 3.51 2.99 1.72 0.355 Pu-239 1.67-3 1.67-3 1.67-3 1.67-3 1.67-3 1.66-3 Pu-240 1.24-3 1.23-3 1.23-3 1.23-3 1.22-3 1.20-3 Am-241 1.58-3 2.21-3 3.99-3 6.62-3 7.45-3 5.45-3 Total 138 238 309 259 129 18.3

The radiotoxicity of actinides during whole storage time is determined by U-232 and its daughter nuclides first of which is Th-228. The half-life period of U-232 makes 68.9 years, Th228 - 1.9 years. The subsequent daughter nuclides in of U-232 after Th-228 are short-lived. Among other actinides, the most important are Pu-238 and U-234. Their contribution is 1-2 order lower. The contribution of Pu-239, Pu-240, Pu-241, Am-241, Th-232 is 4 order lower than that of U-232. At 100 years storage, the total radiotoxicity decreases 2.4 times. The radiotoxicity of actinides of thorium-uranium fuel with account of U-232 appears 3 times less than for uranium fuel. The decay heat power of actinides during whole time of a storage is determined by a nuclides U-232 and Th-228 together with short-lived daughter nuclides. Among other actinides, the most important are Pu-238 and U-234. Their contribution is 1-2 order lower. The power of actinides of thorium-uranium fuel appear about 5 times less than for uranium fuel. P5-07

CONCLUSION

The data presented permit to make conclusions about the question: what actinides should be extracted first of all from stored spent fuel for transmutation. Unfortunately, we can consider only chemical separation of different chemical elements instead of isotopic separation of high-active nuclides. In case of uranium and uranium - plutonium spent fuel for which partial contributions of different nuclides in total radiotoxicity or decay heat power is identical, it is expedient to perform chemical separation of plutonium, americium, curium before long-term storage. It is expedient to separate americium after 50-70 year period of storage sufficient for conversion Pu-241 in Am"241. Curium can be separated in beginning of a storage. This will allow to reduce a radiotoxicity of staying actinides by 20-30 %. If we abandon a separation of curium then it decays in 100 years almost fully. Extracted americium (possibly, with long-lived curium isotopes) should be directed to transmutation and plutonium - to repeated use. The separation of actinides is expedient also from the view point of reduction of decay heat power. So, extraction of americium after Pu-241 decay and decay of greater part of Pu-238 permits to reduce essentially decay heat power of plutonium fraction. In case of thorium-uranium fuel when the overwhelming share of radiotoxicity is determined by U-232 which is of the same as main fuel U-233. It is obvious that the repeated use of thorium-uranium fuel connected with various variants of new U-233 addition will be accompanied by accumulation of a radiotoxicity. At unitary use of thorium-uranium fuel with deep U-233 burnup, it is necessary to perform additional deep burn-out (transmutation) of uranium fraction containing both U-233 and U-232. The further reduction of radiotoxicity by several orders can be related with extraction and transmutation of plutonium faction (Pu-238). The transmutation of Th-228 - daughter nuclide of U- 232 - is not necessary because Th-228 decays practically completely in 10 years together with its short-lived daughter nuclides.

REFERENCES

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