Criticality safety of disposition of nuclear fuel to be discarded.

Criticality safety of disposition of nuclear fuel to be discarded. In this contribution, some fundamentals of disposal criticality safety will be explained, such as safety requirements for the criticality, safety assessment of a final repository, consistency of the safety requirements for the preclosure period with the safety requirements for the postclosure period of the repository the fundamental dependence of time periods of conformance 017 the nuclice inventories stored in the final repository and the changes in these inventories due to radioactive decay and due to possible impacts on these inventories caused by longterm changes in the geological, geohydrological and geochemical conditions at repository site probability of occurrence of criticality events respect to sonic present cask designs to be used for disposal and the waste acceptance requirements to be met by these designs and necessity, for furnishing proof of ensuring safety of the repository against potential criticality. The problems to be investigated in disposal criticality safety analysis for estimating the probability of occurrence of criticality events and the consequences of such events with respect to the safety, reliability and performance of the repository are outlined. Investigation of these problems requires the development of models describing the complex mechanisms (scenarios) which may result in criticality under repository conditions, the complex and diverse types of criticality excursions under repository conditions, and the impact of these excursions on the safety, reliability and performance of the repository. The development of such models requires the concerted cooperation of many special disciplines. It is therefore proposed to set up a national "Final Disposal Project" working group linking up still closer those groups, institutions and organizations cooperating already in researching on final disposal issues and including criticality analysis experts and all the other experts required. One of the objectives of this working group is to give consideration to a risk-informed approach which balances the risks from all operations, facilities and factors associated with the preparation and handling of waste packages as well as the operating and the postclosure period of the repository such that the total risk is optimally, ALARP (as low as reasonable practicable). This approach includes balancing the risks from potential criticality excursions under repository conditions in the postclosure period against the risks resulting from safety measures that have to be taken in the preclosure period for ensuring physically safe prevention of a criticality event in the postclosure period.