The belgian experience with scenario development in performance assessments

In early assessments of geological disposal in the Boom Clay layer at the Mol site, the scenario selection was based on fault tree analysis and expert judgement. In more recent assessments a systematic approach to scenario identification and selection was applied following the recommendation of the NEA report on scenario development of 1991. Starting from the NEA FEP list of 1991, to which a few FEPs specific for disposal in clay were added, a catalogue of FEPs relevant for geological disposal in the Boom Clay formation at the Mol site was drawn up. The considered FEPs were screened by applying a number of elimination criteria. The retained FEPs were than partitioned into two groups: those that are certain or about certain to occur are considered within the 'normal evolution scenario', the remaining less probable FEPs can lead to an 'altered evolution scenario'. The FEPs of the second group are than classified according to the state of the repository system to which they belong by considering which component of the repository system is affected by the FEP. This allows to take some FEPs together within one altered evolution scenario. Finally eight altered evolution scenario are identified for further analysis within the PA. (authors)