Euradwaste '08 - EU Bookshop - Europa

nuclear fission in the reactors. The different neutron spectra in fast reactors or ADSs can have some
influence on the generated amount of some fission products; this explains the somewhat higher dose
due to 135 Cs at 3 million years for fuel cycles A3 and B1 in the case of disposal in granite. The calculations
indicate that the amount of iodine that is going into the repository, which depends on the
fraction of spent fuel that is reprocessed, strongly influences the maximum dose. The higher disposal
density, which can be considered in the case of disposal of HLW from advanced fuel cycles,
can result in a small decrease in the calculated dose from radionuclides (e.g. Se, Zr, Tc, Pd, Sn) for
which the release is controlled by a solubility limit. Very long-term doses, i.e. after a few million
years for disposal in clay, are calculated to be lower in the case of advanced fuel cycles, because
smaller amounts of actinides are present in the HLW arising from these fuel cycles.
Table 2 shows that the fraction of the disposed radiotoxicity that is released into the environment
over a 10 million years period ranges between 10 -6 and 10 -7 . These values confirm the excellent performance
of a geological disposal system.
On the basis of the analysis reported above, maximum doses associated with future high-level waste
repositories could strongly depend on the applied iodine management. If iodine is captured at future
reprocessing plants and immobilized in matrices for which the stability is comparable to today's
borosilicate glasses, doses due to these iodine-wastes would still dominate total doses resulting
from a repository in which the main long-lived waste types, i.e. SF, HLW, ILW and iodine-waste,
arising from a fuel cycle would be disposed of; the resulting peak doses for fuel cycles with reprocessing
of the spent fuels would even be higher than for the reference fuel cycle A1. As a consequence,
if the iodine were to be captured at the reprocessing plants, it would be necessary to develop
extremely stable waste matrices, possibly in combination with long-lived containers, for conditioning
and packaging the iodine waste.
Although the ILW contains relatively high amounts of mobile fission and activation products such
as 14 C, 36 Cl and 129 I, the maximum ILW dose is, in the case of disposal in clay (see Fig. 4), calculated
to be about one order of magnitude lower than the maximum HLW dose; in the disposal concept
considered, the diffusive transport of radionuclides through the 40-m-thick natural clay barrier
ensures that the releases of mobile radionuclides from the host clay formation into the surrounding
aquifers are spread over several tens of thousands of years. An important radionuclide occurring in
ILW is 14 C, of which considerable amounts were calculated to be generated in a fast reactor (due to
activation of impurities in structural metals) or an ADS (because of nitride fuel). The 14 C peak is
clearly visible on the ILW dose curves for disposal in clay at about 20 000 years. The development
of low activation materials might reduce the generated amount of 14 C. Considering sorption of C on
the cementitious materials used in the near field of the repository or on minerals of the clay host
formation would also reduce the 14 C doses calculated in the evaluations.
Regarding calculated doses in the variant human intrusion scenario, Figure 5 shows that only for 3
HLW types, arising from the advanced fuel cycles B1 and B2, does the dose in the geotechnical
worker scenario reduce to less than the dose associated with an intrusion into a Cigar Lake uranium
ore body within a 1 million years time scale. For the other HLW and SF types the calculated doses
to a geotechnical worker remain (much) higher than the dose associated with an intrusion into the
Cigar Lake ore body. Consideration of the intervention levels of 10 and 100 mSv, which were proposed
by ICRP [13] when human intrusion could lead to doses to those living around the repository
site, can be used to contextualise the dose values calculated for the various SF and HLW types. Applying
a simple criterion of ranking when the calculated dose for each SF and HLW type is bounded
by the ICRP intervention levels allows a ‘ranking’ of the high-level waste and spent fuel types. It
should be noted that the applicability of intervention levels to human intrusion doses is questionable
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