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Challenges of assessing long-term performance of nuclear waste matrices in
repository near-field environments – insights from the NF-PRO and MICADO
projects
Summary
Karel Lemmens 1 , Bernd Grambow 2 , Kastriot Spahiu 3 , Yves Minet 4 ,
and Christophe Poinssot 4
1 SCK·CEN, Belgium
2 SUBATECH, France
3 SKB, Sweden
4 CEA-Marcoule, Nuclear Energy Division, France
For the first time, waste form behaviour (borosilicate glass and spent fuel) has been
studied under near-field conditions in a fully integrated fashion. The results obtained on
glass in the NF-PRO project confirm the fundamental understanding of the dissolution
mechanism also for very compacted near-field conditions in presence of material
boundaries such as glass/iron corrosion product and glass/clay. NF-PRO has also reduced
important uncertainties in the long-term performance of spent fuel under near-
field conditions. A quantification of the remaining uncertainties in parameters and models
is under way in the MICADO project. It has been shown that the instant release fractions
(IRF) will not increase with time. Radiolysis effects were shown to be less important
for fuel dissolution than previously thought. Although NF-PRO has improved our
phenomenological understanding of the glass/SF dissolution processes, it is still difficult
to reliably predict waste form performance under near-field constraints.
1. Introduction
In a geological repository for high-level vitrified waste and spent fuel, the waste matrix represents
the first engineered barrier isolating the disposed waste from the biosphere. The mechanisms of dissolution
of waste forms in groundwater are very complex. The analysis of experimental data is even
more complex since the rate controlling process changes with time and geochemical boundary conditions.
Dissolved silica plays a key role in the glass dissolution process. The presence of near-field
materials will influence dissolved silica concentrations and the timing of changes in rate limiting
steps. In the case of spent fuel, the waste matrix has a complex structure. Both slow release from the
matrix and fast release from the gap and grain-boundaries must be considered. Redox conditions
and radiation are of critical concern for spent fuel stability.
In the Fifth Framework Programme (FP5), the main emphasis has been on the integration of data
from experimental studies into modelling. Waste form related work performed in NF-PRO (FP6)
focussed on remaining open questions, in particular on those issues that entail significant uncertainties
with respect to the source term in an integrated assessment, considering the thermal, chemical,
hydrological and mechanical evolution of the near-field. The quality of key experimental data and
the different approaches and underlying hypotheses for spent fuel performance are being evaluated
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