Euradwaste '08 - EU Bookshop - Europa

Advances in research on vitrified high-level waste
The dissolution of vitrified waste is generally described as a two-stage process involving a high initial
dissolution rate, during which near-field materials become increasingly saturated with silica released
from the waste glass, followed by a low residual dissolution rate. Prior to NF-PRO, glass dissolution
models and parameters were mostly based on experimental observations obtained from
relatively simple subsystems, for example the subsystem glass-claywater. Under NF-PRO, integrated
experiments were conducted to investigate glass dissolution processes in more realistic nearfield
conditions. Different vitrified waste glasses (SON 68 and blended Magnox glass) were
brought in contact with various compacted near-field materials including the bentonite buffer (Volclay
KWK) and corrosion products (magnetite) released from metallic repository components. The
experimental matrix allowed for simulating different stages in the evolution of the near-field system.
Conditions representative for the long-term evolution were reproduced by adding amorphous
silica to saturate silica sorption sites on the corrosion products and the clay buffer. Experimental
results obtained by NF-PRO confirm that, after the saturation of the aqueous phase adjacent to the
glass with silica, the glass dissolution rate decreases by several orders of magnitude. It was found
that corrosion products from metal-based near-field components may retard the decrease in glass
dissolution rate as they provide additional adsorption sites for dissolved silica. Experimental results
were used to calibrate and to improve glass corrosion models in the presence of corrosion products
and clay: a satisfactory agreement between the modelling and experimental results was obtained.
In addition to research on the key mechanisms affecting glass dissolution under integrated nearfield
conditions, NF-PRO investigated various factors potentially influencing the mobility of radionuclides
released from vitrified waste. In particular, the effect of dissolved carbonate on the release
of rare earth elements and uranium was investigated and new data were obtained on the uptake
of Eu in newly formed clay minerals secondary phases such as powellite and calcite.
Advances in research on spent nuclear fuel
An important area of research dealt with by NF-PRO concerns the investigation of key processes
affecting the release of radionuclides from spent nuclear fuel under geological disposal conditions.
In spent fuel, radionuclides are inhomogeneously distributed. Consequently, the release of radionuclides
from spent fuel is typically described by two terms:
The Instant Release Fraction (IRF), which corresponds to the fraction of the radionuclide inventory
that segregated partially from the UO2 or MOX matrix during irradiation in the reactor.
This fraction is located in parts of the fuel with low confinement properties, in particular
at the grain boundaries and in the fuel microstructures. The IRF is anticipated to be immediately
released in repository conditions upon contact with groundwater following breaching of
the spent fuel canister/cladding.
The fraction of the radionuclides that are embedded in the spent fuel matrix and which will
dissolve slowly in contact with groundwater.
In previous Community–supported Programmes, a substantial amount of work has been performed
to increase knowledge on the Instant Release Fraction. As part of the project "Spent fuel Stability
under Repository Conditions" (SFS) (Fifth Framework Programme), a model was developed to describe
and to quantify the IRF fraction [6]. Work by NF-PRO has built on the outcome of SFS and
has addressed a number of remaining uncertainties and open questions. The following key results
were obtained by NF-PRO regarding the Instant Release Fraction:
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