Euradwaste '08 - EU Bookshop - Europa

though with a slow rate. The data indicate that only some hundred μm of the fuel will corrode in
1 million years.
Whether or not radiolytic effects are detrimental to spent fuel stability depends to a large extent
on the time of container failure. In the accidental scenario of container failure within the first
centuries after disposal, radiolysis may prevail. In this case, bromide concentrations in
groundwater might make the stabilizing hydrogen effect insignificant.
It was long thought that radiolysis will provide oxidizing conditions at the fuel surface, even in
overall reducing environments. This has motivated various studies of secondary phase formation
of U(VI) solid phases like phosphates or studtite in NF-PRO. It has now been shown that in
hydrogen saturated environments reducing conditions will remain dominant directly at the fuel
surface. Therefore, only U(IV) containing alteration products are expected to form.
5. Remaining uncertainties and recommendations for the future
5.1 Vitrified waste
The integrated tests and modelling have shown that the key mechanisms allowing a qualitative and
semi-quantitative description of the glass dissolution processes are known. The application to specific
disposal sites will have to take into account the specific characteristics of the disposal design
and host rock characteristics. Glass dissolution in cement based near-field conditions has not been
studied in NF-PRO. The related phenomenological description and process understanding still
needs more work than for the other systems at neutral pH. In all reference concepts, the overpack is
expected to last much longer than just the thermal phase, and thus will provide a non-negligible
safety reserve. The formation of metallic corrosion products does not fundamentally change the
glass dissolution, because sorptions on metallic corrosion products and on bentonite are similar.
The current models predict faster glass dissolution as long as the magnetite layer is not saturated
with silica, but the total sorption capacity of the magnetite is too low to have an important effect on
the total glass life time. Detailed modelling of the interactions between glass and corrosion products
is not yet possible, but a detailed description would most probably not lead to significantly shorter
life time predictions. Moreover its application in the safety assessment may be not straightforward
because the formation of a thick corrosion product layer is unlikely. Instead, iron rich clay minerals
are expected to be formed in the absence of glass [5]. The long term effect of metallic corrosion
products is probably negligible, because of saturation of the sorption sites by silica. Furthermore,
site densities of magnetite used in the experimental studies are expected to be much higher than in
reality, since dense layers of very little porosity will be formed. In the reference source term model,
the maximum dissolution rate is applied as long as the magnetite layer is not saturated. Omitting
this magnetite layer (i.e. replacing it by a bentonite layer) would imply a longer glass life time.
Hence the reference source term model is conservative in this respect. One should take this into account
when setting future research priorities. Nevertheless, the programme has suggested two potential
long term effects of metal corrosion that may be worthwhile further investigation: (1) the
precipitation of iron rich silica phases partially replacing the magnetite layer (this may include also
coprecipitation phenomena), and (2) a local pH increase due to actively corroding iron.
Apart from this, a better characterization of the long term dissolution processes and measurements
in conditions more representative for specific disposal designs are still necessary.
The programme has given further evidence that retention of specific radionuclides in secondary
phases is likely to contribute to the overall safety, but this process is not included in the reference
source term model. Including it would require much more research in this area, taking into account
the different conditions depending on the disposal site and design.
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