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Performance Assessment of the Geological Disposal of Spent Fuel in a clay layer


Hitherto the performance assessments, which have been carried out within the CEC's R&D programme on radioactive waste management like PAGIS and PACOMA, considered mainly waste types which result from the reprocessing of spent fuel. However for technical reasons the recycling cannot be repeated more than three cycles and the economical justification of the reprocessing becomes debatable because of the relatively low cost of fresh uranium. It is therefore reasonable to consider the direct disposal of uranium oxide and/or mixed oxide spent fuels as a realistic option. The main objective of the study is the evaluation of the radiological consequences of the geological disposal of spent fuel in a hypothetical repository located in the Boom clay layer at the Mol site.
Hitherto the performance assessments of geological disposal in clay layers considered essentially waste forms resulting from the reprosessing of the spent fuels. The low prics of fresh uranium and te use of mixed oxides (MOX) fuel mean that the direct disposal of spent fuel is becoming a realistic option for the nuclear fule cycle.

The inventories of the various spent fuel types that can be ex[ected to arise from the Belgian nuclear programme are estimated. Available repository concepts for the disposal of reprocessing waste are adapted to allow the placement of the large spent fuel containers. Little information is available about the behaviour of spent fuel in the conditions prevailing in clay, or about the long term behaviour of spent MOX fuels in general.

Many models have been developed. However the modelling of the essential release processes requires the development of a specific near field model whuch takes into account the different corroaion rates of the various componets of the disposed spent fuel assemblies and the radiolytic oxidation of the uranium oxide matrix. An examination of the methodologies indicates that the calculations of this first assessment of spent fuel disposal in clay should focus on the analysis of the normal evolution scenario. Deterministic as well as stochastic calculations are performed.

An essential conclusion of the study is that the calculated maximum dose rate is about one order of magnitude higher that one estimated for the case of high level waste disposal. On the other hand, the maximum dose is still one order of magnitude lower than the one estimated for the case of the disposal of the iodine captured at the reprossing plant. Recommendations are formulated for rhe R&D programmes as well as for more advanced performance assessments.
Work programme

Data collection
- Spent fuel inventories by calculations of radionuclide inventories of UO2 and MOX spent fuels from PWR reactor burn ups of 33 and 45 MWd/kg with :

.repository concept: the existing repository concepts are adapted to allow for the disposal of spent fuel assemblies,
.near field processes and characteristics : the near field model should take into account the typical processes describing the release of radionuclides from the spent fuel,
.far field and biosphere data (data collected for PAGIS and PACOMA will be updated).

- Adaptation of the methodology developed within the EVEREST project to the case of spent fuel disposal

- Models and computer codes (a new near field model is needed)

- Deterministic calculations will be carried out to evaluate individual dose rates and collective doses.

- Stochastic calculations including sensitivity and uncertainty analyses

- Conclusions

Funding Scheme

CSC - Cost-sharing contracts


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