Euradwaste '08 - EU Bookshop - Europa

4. Discussion
Tests with dissolved 238 Pu in H2 containing solutions without a UO2 surface (homogeneous
�-radiolysis) confirm literature results at much higher H2 concentrations, i.e. that no consumption
of oxidants occurs in the bulk solution in absence of a catalysing surface.
Tests under -radiation with spent fuel and UO2(s) in H2 saturated low ionic strength NaCl
solutions with 10 -4 or 10 -3 M Br - have contributed to the understanding of the importance of
fuel surface processes in these conditions. H2 considerably lowers the amount of oxidants
produced by �-�-radiolysis by reacting with the oxidising OH-radical to produce water and a
reducing H-radical. Br - ions block the beneficial effect of H2 in the bulk solution.
Presence of Br - ions can result in a decrease, or increase, or have no influence on the production
of H2O2 (usually an oxidant) by �-radiolysis in dilute groundwaters. This should be
studied further in the future, since �-radiolysis is expected to dominate at the time of fuel
contact with groundwater in the normal evolution scenario.
Important progress was achieved on the modelling of the radiolysis effects at the fuel/nearfield
interface, by integrating a modified radiolytic model into a transport code and accounting
for the presence of H2 via the corrosion potential of the UO2 matrix. An important result
is that in H2 saturated media, redox fronts at the fuel surface break down, because the rate of
consumption of H2 by radiolytic or catalytic reactions is much lower than the rate of its
transport to the surface.
The study of the oxidative dissolution rates of UO2 in phosphate containing solutions shows
that the enhancing effect of phosphate as compared to carbonate may be larger in solutions
containing both ligands, due to the absence of secondary phases.
The study of the waste form dissolution processes and the exchange of information with the other
research components of NF-PRO have allowed a better description of the expected evolution scenarios
and the associated safety margins. The findings provided no compelling arguments to fundamentally
change the reference source term models. The underlying mechanisms are better understood
now, though, and missing elements have been better identified.
The results show that the glass reference source term model and the underlying expected evolution
scenario is conservative for several reasons that were less evident at the start of NF-PRO:
1. The reference source term models implicitly assume very short overpack life times, necessary
to overcome the thermal phase. In reality, the life time of the overpack will be much
longer, as suggested in [4]. Depending on the disposal design, the expected time for complete
overpack corrosion can be some tens of thousands up to 250 000 years or more. Even
if the overpack is locally perforated, intruding water will be trapped in a confined volume.
This means that the release of radionuclides by the waste form will start much later or be
slower than assumed in the reference evolution scenario and source term models.
2. The glass source term model assumes that the glass dissolves at the forward rate as long as
the magnetite is not completely saturated and does not take into account the saturation of the
magnetite by Si from the clay.
3. The glass source term model does not take into account the possibility that the dissolved
iron may diffuse at least partially into the clay to be sorbed or integrated into secondary
phases, rather then form a compact magnetite layer at the interface with the glass [5]
The life time calculations for vitrified waste have shown that the silica sorption on a magnetite layer
can shorten the life time of the glass at most by a few centuries. When we compare this short effect
with the long expected overpack life time, it is likely that the overall effect of the overpack will be
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