Performance Assessment of Geological Disposal of High-level Radioactive Waste in a Plastic Clay formation
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Performance Assessment of Geological Disposal of High-level Radioactive Waste in a Plastic Clay Formation Jan Marivoet, Xavier Sillen, Dirk Mallants and Peter De Preter1 Waste & Disposal Dept., SCK•CEN, Boeretang 200, 2400 Mol, Belgium 1 NIRAS/ONDRAF, Kunstlaan 12, 1210 Brussels, Belgium ABSTRACT In Belgium the possibilities to dispose of high-level waste in the plastic Boom Clay formation has been studied since 1975. Consequently many results of the site characterisation programme are already available. Various performance assessments have been carried out dealing with the disposal of high-level waste arising from reprocessing and with direct disposal of spent fuel. The performance assessment consists of two main steps: scenario development and consequence analyses. The scenario analysis is based on a catalogue of features, events and processes (FEPs) having the potential to influence the behaviour of the repository system. Two main groups of scenarios are distinguished. The normal evolution scenario, including a number of variants, treats the FEPs that are fairly sure to take place. Altered evolution scenarios focus on FEPs having a probability of occurrence lower than one but that might influence the performance of the repository system. For the impact analyses, a robust concept approach was introduced, which focused the analyses on a limited number of well-characterised barriers and processes. The impact analyses are complemented with sensitivity and uncertainty analyses based on deterministic and probabilistic approaches.
INTRODUCTION In Belgium about 60 % of the electricity is produced in nuclear power plants. The first two nuclear reactors became operational in 1975 and the last one in 1985. The total installed capacity of the seven nuclear reactors is about 5.5 GW(e). The reactors are expected to be operated for 40 years. The amount of fuel needed for the scheduled nuclear programme is about 5000 tonnes of uranium. Contracts for the reprocessing of 630 tU have been concluded with the French company COGEMA. The Pu that is recovered in the reprocessing plant is used to produce 67 tonnes heavy metal (tHM) of mixed oxide (MOX) fuel, which is burned in two Belgian nuclear reactors together with standard uranium oxide (UOX) fuel. At present, a moratorium has been decided on future reprocessing. Consequently the reprocessing of the remaining 4160 tHM of UOX fuel is unlikely. A study on potential host formations for the disposal of high-level radioactive wastes carried out in 1974 has indicated that only clay and shale formations could be considered in Belgium. The Belgian Nuclear Research Centre SCK•CEN started in 1976 a limited research programme on the possibility to dispose of high-level waste in the Boom Clay Formation under the nuclear site of Mol-Dessel. The research programme has grown considerably since the creation of the Belgian Radioactive Waste Management Agency NIRAS/ONDRAF in 1982. The underground research laboratory located in the Boom Clay became operational in 1984; many results are available from
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