Euradwaste '08 - EU Bookshop - Europa

in the European coordinated action project MICADO. This project assesses uncertainties governed
by the divergence between the various models and the experimental databases, and uncertainties in
predictions that arise by comparing the outcomes of the various models.
The waste form is of course only one of the barriers within a disposal concept characterised by the
superposition of multiple partly interdependent barriers. Barrier functions concern the establishment
of favourable geochemical conditions, limiting water access to the waste, limiting the transfer of
contaminants to the geosphere etc. The relative role of the waste form within these barrier systems
has been illustrated in the integration research component of NF-PRO [1]. The calculations show
that, compared to the absence of any barrier, the presence of a suitable waste form in its container
and the low solubility of key radionuclides already reduce hypothetical dose contributions at the
near-field/geosphere interface by up to five orders of magnitude. The importance of the waste form
depends on the stability of the waste form and on the disposal concept. In a disposal concept in
which the transfer of contaminated groundwater to the biosphere takes longer than the time of degradation
of the waste form, the waste form stability will have less significance than when nearfield/biosphere
transfer rates are fast. On the other hand, if, as it appears today, a waste form is stable
for hundreds of thousands or even for more than a million years, then it will provide important
safety margins and it sustains the safety case in any disposal concept.
2. Methodology
The methodology of the waste form oriented research within NF-PRO consisted in combining bibliographic
surveys with modelling and experiments, to obtain missing data in integral near-field environments
for various host rock formations, and the evolution with time. Integration with the other
research components (processes in the engineered barrier and in the engineering disturbed/damaged
zone) was attempted by a performance assessment oriented approach. The programme started from
a review of previous projects on glass and spent fuel and considering the material choices, boundary
conditions and scenarios fixed by the other research components of NF-PRO. Work on glass was
organised in a work package oriented on experiments, and another one on the geochemical modelling
of the observed interactions. Work on spent fuel was organised in a work package dealing with
the evolution of spent fuel under normal and early failure scenarios, focussing mainly on the instant
radionuclide release fraction (IRF), and a work package concerning the behaviour of the fuel matrix
under near-field conditions. The methodology of the still ongoing MICADO project consists in
bringing together model developers and potential users, experimenters and people with overall system
understanding to (1) select models (2) to select and review a common spent fuel/UO2/MOX
experimental database, (3) to apply the models with or without re-parameterisation to the repository
relevant part of the database, and assess deviations and interdependence of the model parameters,
(4) to evaluate all relevant information and approaches for spent fuel performance assessment, including
uncertainty propagation to the overall safety analyses, and assess simplification strategies to
translate detailed mechanistic models into overall system codes, and (5) to provide an independent
regulator point of view on the appreciation of the effects of documented model uncertainties on predictive
uncertainties for the repository safety.
3. Results
We provide only a summary of the main results. For more details, we refer to reference [2].
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