INFLUENCE OF THE ACCIDENT BEHAVIOUR OF SPENT FUEL ELEMENTS ON CRITICALITY SAFETY OF TRANSPORT PACKAGES – SOME BASIC CONSIDERATIONS

For each design of a package for the transport of spent nuclear fuel the criticality safety under routine, normal and accident conditions of transport has to be shown. Under accident conditions of transport the neutron multiplication is substantially affected by the condition of the fuel elements and the basket. On the other hand, the impact behaviour of spent fuel elements is not completely known and therefore conservative assumptions are taken for criticality safety assessment. A prudent choice of such assumptions based on a good knowledge of the relevant parameters significantly simplifies the proof of compliance. The paper starts from recalling the main principles for maintaining subcriticality in a spent fuel transport package. Then these principles are examined systematically regarding possible changes in the arrangement of the fissile material (pin pitch expansion, pin slippage, release and recollection of fissile material). Based on a generic package design the parameters with the maximum influence are identified for each change. Furthermore, the paper gives criteria for determination of the relative importance of the different changes and directions for questions that should be answered in a safety demonstration.