Preferential Water Ingress into a Dry Spent Fuel Cask

Water ingress into a fissile system, such as may occur during transportation of spent fuel casks, or in storage casks, may result in a significant increase in the keff of that system, which, if it were not accounted for in the abnormal operating or accident analysis of the cask, could potentially result in an inadvertent nuclear excursion with severe nuclear and radiological consequences.

This paper will provide the results of an analysis of a gradual increase in water level in spent fuel casks and will discuss the way the keff of the system responds to such an increase. It will also provide the results of a preferential water ingress analysis, which sought to answer the question that if there is change in the keff of the system accompanied by a fractional increase in the amount of water in the cask, which one of the four water ingress pathways/channels would have the greatest effect on the keff. The water ingress pathways in question, and their respective unit numbers in brackets, are fuel rod (Unit 1), burnable poison rod (Unit 2), instrumentation tube (Unit 3) and cask air gap (Unit 9). This analysis will also determine which of the four channels has the highest sensitivity coefficient and will compare the effects of material composition on the keff when water is either freshwater or seawater.