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3.1 Vitrified waste
The simplified reference source term for vitrified waste at the start of NF-PRO was the r0-rr model.
In this model, glass dissolution is described to occur in two stages: a first stage of high dissolution
rate (r0), which lasts up to the saturation of the metallic overpack products with silica, and a second
stage of low residual dissolution rate (rr or rres), which lasts until the glass is completely dissolved.
The processes at the basis of this reference source term model were studied in a programme with
three components
(1) Glass-water interaction, covering the determination of some basic parameters of glass dissolution
and the effect of dissolved carbonate on the release of rare earth elements and U.
(2) Radionuclide immobilisation in secondary phases.
(3) Validation of key mechanisms of glass dissolution in integrated near-field conditions.
We present the main results for each of these components.
Determination of basic parameters of glass dissolution
The dissolution rates r0 and rres are well known for the R7T7 glass (corresponding to the SON68
standard reference glass), but this was not the case for the UK “blended Magnox-UO2” glass, which
contains more magnesium and aluminium than the R7T7 glass. These parameters were determined
within NF-PRO. The blended Magnox-UO2 glass behaves less favourable than the R7T7 glass:
both the forward rate and the residual rate are higher for the blended Magnox-UO2 glass. The reason
for the higher residual rate is probably the formation of secondary magnesium phases, triggering
the glass dissolution. The lower stability of the blended Magnox-UO2 glass was confirmed by
the integrated tests, which are discussed further. The reference source term model can now be applied
for the blended magnox-UO2 glass, provided sufficient data are available concerning another
important parameter, i.e. the exposed surface area of the glass.
Effect of dissolved carbonate on the release of rare earth elements and uranium
Long-term dissolution tests with glass GP WAK1 in synthetic Opalinus and Konrad clay pore water
solutions showed no clear effect of carbonates on the release of rare earth elements and uranium
from the glass in the carbonate concentration range of 73-98 mg/l, but this conclusion cannot be extrapolated
as such to higher carbonate concentrations. Crystalline secondary phases were observed,
such as powellite, barite, calcite, CaSO4 and clay-like Mg(Ca,Fe) silicates, but there were no distinct
uranium phases in the gel. This means that carbonate concentrations do not have much impact
on the radionuclide retention in the gel. This should be taken into account if radionuclide retention
is considered explicitly in the source term model. In the reference source term model used for NF-
PRO, radionuclide retention is not considered explicitly. In this case, retention in the gel contributes
to the safety margin.
Role of radionuclide immobilisation in secondary phases
Under conditions typical for a deep geological nuclear waste repository, secondary alteration phases
are formed during dissolution of the waste glass, once their solubility limit has been reached. Radionuclides,
which have been released from the waste matrix may co-precipitate with these secondary
phases and form thermodynamically stable solid solutions, such as amorphous gel layers and
various crystalline phases (see previous paragraph). It was found that trivalent actinides (Am, Pu,
Cm) can be structurally incorporated into the host minerals powellite, calcite and clay minerals,
forming solid solutions. The quantitative understanding of this solid solution formation has im-
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