Euradwaste '08 - EU Bookshop - Europa

elease, strong radiation, pressure rebuilding, etc.) after repository closure; as long as this safety
function is effective, no release of radionuclides from the waste form can occur;
slow release: after container failure, groundwater comes in contact with the conditioned waste
and degradation of the waste matrix and leaching of radionuclides will start; various physicochemical
processes, such as corrosion resistance of the waste matrix, precipitation, sorption or
co-precipitation will strongly limit radionuclide releases into the buffer;
retardation: radionuclides dissolved in the groundwater in contact with the waste will start to
migrate through the buffer and the host formation; because of the very low hydraulic conductivity
of bentonite and clay of the host formation (in the case of disposal in clay), groundwater
flow in the repository's barriers is about negligible and radionuclide transport will be mainly
diffusive; furthermore, many radionuclides will be sorbed onto minerals of the buffer and the
host formation; retardation delays the releases and drastically limits the amounts of radionuclides
that are released into the environment per unit of time; many radionuclides will have decayed
before reaching an aquifer, from where they can reach the human environment.
2. Methodology
The methodology adopted for analysing the impact of advanced fuel cycle scenarios on geological
disposal consisted of two steps: (1) an analysis of the adaptations needed to accommodate the new
waste streams in the disposal concept, and (2) an assessment of the radiological consequences of the
radioactive waste repository.
It was first verified whether the available repository concepts are applicable for the disposal of the
main high-level and long-lived waste types arising from the advanced fuel cycles. Several waste
characteristics were taken into consideration such as volume, composition, thermal output, gamma
and neutron emission, retrievability aspects, and criticality; it appeared that the MOX SF arising
from fuel cycle A2 is the most difficult-to-handle waste type in the Red-Impact waste inventory.
Variants of the disposal concepts allowing the accommodation of MOX SF have already been developed
by the waste agencies. Consequently, the HLW types arising from the advanced fuel cycles
do not require considerable adaptations to the available repository concepts, but the dimensions of
the disposal galleries can be adjusted to the thermal output of the new heat-generating waste types.
The analyses reported in Section 3.1 will thus focus on the influence of the thermal output of the
HLW and SF on the dimensions of the geological repository.
The second step of the analyses consists in an assessment of the radiological impact of the geological
disposal of SF, HLW, and intermediate-level waste (ILW) arising from the considered fuel cycles.
In a safety assessment of a geological repository a representative set of possible evolution scenarios
is considered. The assessments made in the framework of the Red-Impact project focused on
the radiological consequences in the case of the expected evolution scenario (this scenario is also
called normal evolution or reference scenario, and is associated with radionuclide transport in
groundwater); this scenario assumes that the engineered and natural barriers of the repository system
will function as expected. Additionally, in order to illustrate the impact of the reduction of the
actinide inventory of the waste, we also considered a variant human intrusion scenario - this assumes
that a geotechnical worker handles a core taken from a borehole drilled through the repository
and that the core contains fragments of the disposed waste.
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