A//J-Ai--473 FR0108170

COMPARISON OF RADIOTOXICITY OF URANIUM, PLUTONIUM, AND THORIUM SPENT NUCLEAR FUEL AT LONG-TERM STORAGE

A. GERASIMOV, G. KISELEV, T. ZARITSKAYA, L. MYRTSYMOVA, SSC RFITEP

25, B.Cheremushkinskaya, Moscow 117259, Russia - E-mail: [email protected]

Key words: Spent Fuel, Radwaste, Storage

The radiotoxicity of long-lived actinides and fission products of spent nuclear fuel is a measure of the radiological danger of radwaste during long-term storage. Radiotoxicity of fission products decreases by about five orders of magnitude after decay of Sr-90 and Cs-137. The rest of long-lived nuclides should be directed to final storage or transmutation. Situation with actinides is more serious as they have a high radiotoxicity for a long time. The problem of long-lived actinide management could be solved within the framework of closed nuclear fuel cycle using long-term controllable storage and some kind of nuclear transmutation. The design of long-term storage facility depends on radwaste radiation characteristics. Decay heat power along with radiotoxicity is an important radiation characteristic. Both decay heat power and radiotoxicity of actinides are determined by alpha decays. Thus time dependence of actinide decay and heat power repeats in general that of radiotoxicity.

The development of nuclear power is closely related to new types of nuclear fuel. Plutonium MOX fuel and thorium fuel allow to extend the fuel base of nuclear power. The level of transplutonic nuclides accumulated in MOX fuel is higher than that in uranium fuel. On the contrary, accumulation of plutonium and more heavy nuclides in thorium fuel is much less than in uranium fuel. So, study of the radiotoxicity of these types of fuel is important from the environmental point of view.

Time dependence of radiotoxicity of actinides from spent uranium, MOX-plutonium, and thorium fuel calculated for storage during 1000 years is discussed in the paper. The radiotoxicity RT, of nuclide / by air or by water is determined by the ratio

RT/ = A/ / MPA,, where A is the activity of considered amount of the nuclide /', MPA, - the maximum permissible activity of this nuclide in air or in water according to radiation safety standards. Total radiotoxicity is equal to the sum of radiotoxicities of all nuclides taken in those amounts in which they are contained in considered mixture. For the calculations, current data of MPA accepted in Russia [1] were used.

. 33/04 OGOO

Calculation results

Total radiotoxicity of actinides by water and contribution of most important nuclides in storage of spent uranium and plutonium fuel of light water WER-1000 type power reactor during 1000 years are presented in Tables 1 and 2. The initial amount of actinides corresponds to their contents in 1 ton of spent fuel with a burnup of 40 GW.d/ton and subsequent cooling during 3 years. The fresh uranium fuel is uranium with 4.4 % enrichment. The fresh plutonium fuel is a mixture of depleted uranium with addition of 3.5% Pu-239. Only isotopes of neptunium, plutonium, americium, and curium were considered in the calculations.

T, year Nuclide 1 3 10 30 100 300 1000 Np-237 1.3-4 1.3-4 1.3-4 1.3-4 1.6-4 2.4-4 3.7^ Pu-238 1.4 1.3 1.3 1.1 0.62 0.13 5.4-4 Pu-239 0.23 0.23 0.23 0.23 0.23 0.22 0.22 Pu-240 0.37 0.37 0.37 0.37 0.37 0.36 0.33 Pu-241 1.6 1.5 1.1 0.40 1.4-2 - - Pu-242 1.5-3 1.5-3 1.5-3 1.5-3 1.5-3 1.5-3 1.5-3 Am-241 0.55 0.76 1.3 2.2 2.5 1.8 0.56 Am-242m 1.3-3 1.3-3 1.2-3 1.1-3 8.1-4 3.3-4 - Am-243 1.3-2 1.3-2 1.3-2 1.3-2 1.3-2 1.2-2 1.2-2 Cm-242 2.78-3 1.8-4 - - - - - Cm-243 4.65-3 4.4-3 3.7-3 2.3-3 4.2^ - - Cm-244 1.00 0.93 0.71 0.33 2.3-2 - - Cm-245 2.2-4 2.2^ 2.2^ 2.2-4 2.2^ 2.1-4 2.0-4 Total 5.2 5.1 5.0 4.6 3.8 2.5 1.2 Table 1. Radiotoxicity of actinides from 1 ton of uranium spent fuel, 1014 kg water

T, year Nuclide 1 3 10 30 100 300 1000 Pu-238 2.0 1.9 1.8 1.6 0.91 0.19 1.1-3 Pu-239 0.43 0.43 0.43 0.43 0.43 0.42 0.42 Pu-240 1.2 1.2 1.2 1.2 1.2 1.1 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 1.5 Am-242m 1.3-2 1.3-2 1.3-2 1.2-2 8.4-3 3.4-3 1.4-4 Am-243 4.5-2 4.5-2 4.5-2 4.5-2 4.5-2 4.4-2 4.1-2 Cm-242 1.3-2 1.2-3 6.4^ 5.8^ 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 3.1 Table 2. Radiotoxicity of actinides from 1 ton of spent plutonium fuel, 10 kg water

Analogous data for actinides from spent thorium fuel of WER-1000 type reactor are presented in Table 3. The fresh fuel is a mix of thorium with addition of 3.3 % U-233. Actinide content in spent thorium fuel was calculated for neutron spectrum created by oooo basic uranium fuel of WER type reactor, burnup of 42 GW.d/ton, and subsequent cooling during 3 years. Isotopes of thorium, uranium, and heavier nuclides were taken into account. Data of Table 3 as well as those of Tables 1 and 2 are normalized to the amount of actinides extracted from 1 ton of spent fuel.

T, year Nuclide 1 3 10 30 100 300 1000 Th-228 0.25 0.25 0.25 0.21 0.10 1.2-2 1.5-5 Th-232 6.3-5 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 6.5-5 U-233 2.7-4 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 4.4-3 Pu-238 7.0-2 6.9-2 6.5-2 5.6-2 3.2-2 6.6-3 2.6-5 Pu-239 3.5-5 3.5-5 3.5-5 3.5-5 3.5-5 3.5-5 3.4-5 Pu-240 2.6-5 2.6-5 2.6-5 2.6-5 2.6-5 2.5-5 2.4-5 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 2.8-5 Total 1.5 1.5 1.4 1.2 0.59 8.6-2 5.4-3 Table 3. Radiotoxicity of actinides from 1 ton of spent thorium fuel, 10 kg water

The data presented demonstrate that the radiotoxicity of actinides of spent uranium fuel by water in the initial period of storage is determined by nuclides Pu-241, Cm- 244, and Pu-238. Their contribution at the beginning of storage is about 80%. All isotopes of plutonium give 70%, Cm-244 - 20%. In addition, Am-241 creates 10% of radiotoxicity. During storage Pu-241 decays into Am-241. After 100 years of storage, total radiotoxicity of actinides decreases 1.4 times. The main contribution, 68% is given by Am-241. The contribution of plutonium isotopes makes 33%. The amount of Cm-244 decreases essentially because of decay. Its radiotoxicity falls 43 times and makes 0.6% of total radiotoxicity at the end of 100-year storage. In the period of 100-1000 years, the main contribution is made by Am-241. After 1000 years, the radiotoxicity decreases 4.3 times. The main nuclides are Am-241, Pu-240, Pu-239.

The radiotoxicity of actinides of spent plutonium fuel by water in the initial period of storage is determined by the same nuclides Pu-241, Cm-244, and Pu-238 as in the case of uranium fuel. All plutonium isotopes give a contribution of 60% of the total radiotoxicity, Cm-244 gives 27%, Am-241 gives 12%. After 100 years of storage, total radiotoxicity of actinides decreases 1.4 times. The main contribution, 70 % is given by Am-241. The contribution of plutonium isotopes makes 26%, Cm-244 - 0.9% of the total radiotoxicity. After 1000 years, the radiotoxicity decreases 4.2 times. The main nuclides are Am-241, Pu-240, Pu-239. It should be noted that time dependence of radiotoxicity of actinides of plutonium spent fuel roughly follows that of uranium spent fuel. This is explained by the fact that main nuclides Pu-239, Pu-240, Pu-241, and Cm-244 are contained in similar proportions in both uranium and plutonium fuel. However, the absolute amount of Pu-239, Pu-240, Pu-241, and Cm-244 in spent plutonium fuel is 2-3 times higher than in spent uranium fuel.

The radiotoxicity of actinides of spent thorium fuel during first 100-300 years is determined by nuclide U-232 and its daughter nuclides first of which is Th-228. The half-life period of U-232 is 68.9 years, Th-228 - 1.9 years. The subsequent daughter oooe

nuclides of Th-228 in decay chain of U-232 are short-lived nuclides. Among other actinides, the most important are Pu-238 and U-234. Their contribution is 1-2 order of magnitude lower. The contribution of Pu-239, Pu-240, Pu-241, Am-241, Th-232 is 4 orders lower than that of U-232. After 100 years storage, the total radiotoxicity by water decreases 2.6 times, at 1000 years it decreases 280 times. After 1000 years storage, the U-232 with daughter nuclides gives no contribution to radiotoxicity. This one is determined by U-234, Th-230.

The total radiotoxicity by water at the beginning of the storage for actinides of uranium, plutonium, and thorium fuel makes 5.2-1014, 1.3-1015, and 1.5 -1014 kg of water per 1 ton of fuel, respectively. The radiotoxicity of actinides of plutonium fuel appears about 2.5 times more and radiotoxicity of actinides of thorium fuel is about 3.5 times less than that of usual uranium fuel.

Conclusion

Recommendations for uranium and plutonium spent fuel could be done to perform chemical separation of plutonium, americium, curium before long-term controllable storage. Americium should be separated after 50-70 year of storage for sufficient conversion of Pu-241 in Am-241. Cm-244 decays almost completely after 100 years. Extracted americium (possibly, with long-lived curium isotopes) should be directed to transmutation and plutonium should be reused. The separation of actinides is also effective to reduce decay heat power.

In thorium spent fuel, the overwhelming share of radiotoxicity is determined by U-232. It is obvious that the repeated use of thorium fuel will be accompanied by accumulation of radiotoxicity. For a one-fold use of thorium fuel with deep U233 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 obtained by extraction and transmutation of plutonium fraction (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

[1] Radiation Safety Standards (NRB-99). Minzdrav of Russia. Moscow, 1999.