Criticality safety in the storage of spent power reactor fuel

Under a complete nuclear fuel cycle the criticality safety of spent fuel would be a very simple problem. Under this situation the small number of spent fuel assemblies existing at any given time could be easily handled by providing sufficient storage that interaction between assemblies would be negligible. The large number of fuel assemblies which now have to be stored has given rise to some difficult questions. The assemblies are stored in sufficiently close packed arrays that interaction between assemblies becomes of major importance. The neutron absorption by structural materials and by specially added neutron poisons is included in the evaluation of the system neutron multiplication. Two sets of critical experiments are available for the benchmarking of criticality calculations. All of the data and techniques are available to allow such high density storage to be safely carried out. However, continued storage of spent fuel is only a temporary expedient while we await the decisions which will lead to permanent solutions