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1. Introduction
The problem of the radiotoxicity of long-lived radioactive wastes produced in various nuclear
fuel cycles is important from the viewpoint of ecological impact of these cycles. Separation of the
most important nuclides and their extraction from storage with subsequent transmutation permits to
reduce radiologic danger of remaining wastes in storage. The removal of nuclides with increased
decay heat power from storage permits to ease requirements to heat removal systems of long-term
storages. Quantitative comparison of the radiologic characteristics of minor actinides produced in
various fuel cycles is also of interest.
Changes in radiotoxicity and decay heat power of actinides from spent thorium-uranium nuclear
fuel of VVER-1000 type reactors at storage during 100 000 years are investigated in this paper.
The radiotoxicity RT i
of each nuclide i by air or by water is determined by the ratio:
RT i
= A i
/MPA i
where A i – activity of considered amount of a nuclide i, MPA i – represents the maximum permissible
activity of this nuclide by air or by water according to radiation safety standards. Total radiotoxicity
is equal to a sum of radiotoxicities of all nuclides taken in those amounts in which they are contained
in the considered mix of nuclides. For the calculations, data of MPA accepted in Russia [1] were
used. For the calculations of the decay heat power, the contributions from alpha-, beta- and gamma –
radiations [2] were taken into account.
2. Calculation results
Total radiotoxicity of actinides in air and in water and the contributions of most important
actinides in total radiotoxicity at storage of spent thorium-uranium fuel of a VVER-1000 type reactor
during 100 000 years are presented in Tables 1 and 2. The content of actinides in spent thoriumuranium
fuel was calculated for the neutron spectrum created by basic uranium fuel in a VVER type
reactor. The data correspond to burn-up of basic uranium fuel, 44 kg of fission products per 1 tonne
and subsequent cooling during 3 years. The fresh fuel was a mix of thorium with an addition of 3.3%
233
U. Isotopes of thorium, uranium, and more heavier nuclides were taken into account.
Total decay heat power of actinides and the contributions of the most important actinides at
storage of spent thorium-uranium fuel during 100 000 years is given in Table 3. Decay heat power, as
well as the radiotoxicity, corresponds to the content of actinides in 1 tonne of unloaded fuel.
The data presented show that radiotoxicity of actinides of spent thorium-uranium fuel during the
first 100-300 years is determined by the nuclide 232 U and its daughter nuclides, first of which is 228 Th.
The half-life of 232 U is 68.9 years, of 228 Th, 1.9 years. The subsequent daughter nuclides in the decay
chain of 232 U after 228 Th are short-lived. Among other actinides, the most important are 238 Pu and 234 U.
Their contribution is 1-2 order lower. The appreciable contribution in radiotoxicity in air introduces
also 233 U. The contributions from 239 Pu, 240 Pu, 241 Pu, 241 Am, 232 Th are 4 order lower, that of 232 U. At
100 years storage, the total radiotoxicity in air decreases 2.4 times, than that of in water – 2.6 times, at
1 000 years – 410 times and 280 times. After 1 000 years storage, the 232 U with daughter nuclides
gives no contribution to radiotoxicity. It is determined by 234 U, 230 Th. After 3 000 years there is an
increase of radiotoxicity because of accumulation of 226 Ra, 229 Th, 230 Th.
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