Euradwaste '08 - EU Bookshop - Europa

question certain assumptions, as well as providing, eventually, a larger set of studies that aid in
reaching scientific consensus on various issues. Unlike the national safety cases, EU Projects such
as NF-PRO do not attempt to achieve completeness in the sense of covering all relevant areas at an
appropriate level of depth. Instead, the project scope is a natural outcome of the respective interests
of the organizations involved and the project must seek to develop appropriate shared goals and
also accept the inability to tackle the completeness question, which is best left to the safety cases of
the national programs that may put the NF-PRO results to further use.
One of the distinguishing features of NF-PRO also lies in the communication that has been established
in the project between groups doing detailed process investigations (the four hexagons on the
sides in Figure 1) and those doing broader assessments of disposal systems (i.e. Performance Assessment)).
Here again one sees the different roles played by those who are discipline-focused and
seek ever greater detailed understanding, which complements the perspective of those who try to
provide a balanced synthesis of information that clarifies how all the repository components and
associated processes interact.
3. NF-PRO findings from a PA perspective
The main findings of NF-PRO seen from the perspective of PA are summarised here. Specialists in
the various scientific disciplines may have different views from PA specialists regarding the significance
of various project achievements, but from the PA perspective, it is considered that both
the quality of the scientific contributions and the impact on the system performance are relevant and
these determine the emphasis presented here. It is emphasized that the summary of findings and recommendations
for future work must be considered as guidelines that need to be evaluated in the
context of the particular disposal system in question and that the absence of a comprehensive total
system PA analysis in the project (due to the NF focus), requires that other considerations may also
influence the focus and even the necessity for R&D in a given area. The details of the work are discussed
in Grambow et al. [8], Arcos et al. [9], Villar et al. [10], Aranyossy et al. [11], and Johnson
et al. [12].
4. Release of radionuclides from the waste matrix
HLW
Studies performed within NF-PRO have improved understanding of the impacts of corrosion products
and clay on the long-term dissolution rate of HLW. These studies indicate significant transient
impacts on the rate (the rate may be initially similar to the high initial rate in distilled water), but a
number of results suggest that the long-term rate will nonetheless be very low. During this transient
only a relatively small fraction of the glass would dissolve before the residual rate is attained. The
details of the transient are of interest to the extent that understanding something about the reactions
involved may help to explain better the processes that control the long-term rate.
The initial or transient glass dissolution rate (about 100 to 1000 times higher than the long-term or
residual rate) is often used in assessment calculations, where it is applied to the entire duration of
the dissolution process and is referred to as a “pessimistic” value or is used in a sensitivity analysis
context. In the context of communicating the results of safety assessments, it appears that it would
be consistent with the findings to use two dissolution phases accounting for the initial rate and then
the residual rate. Use of the initial rate for the entire duration can be considered solely as an illustration
of how the system behaves if the waste form dissolves very rapidly, i.e. as an illustration of
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