Concept of radiation dose rate sensitivity applicable to spent fuel shipping casks. Definition of concept and application to evaluation of array effect obtained upon arranging spent fuels in different burnup states within same shipping cask.

For shipping spent fuels of very high burnup, the normal procedure is to place the fuels of highest burnup in the center of cask, and to surround them with those of lower burnup. The actual arrangement of these fuels within the cask is currently determined individually at the time of loading, based on experience.The present paper proposes a simple method for quantitatively evaluating the effect brought on the radiation dose rate at cask surface by differences in the arrangement of spent fuel inside the cask. The proposed method is based on a concept embodying “sensitivity to change in radia-tion source strength shown by the radiation dose at cask surface”. This concept permits quick and reliable evaluation of an arrangement of spent fuels in a cask. The concept is formulated by means of a quantity termed "sensitivity coefficient of radiation dose rate", which coefficient can be easily calculated.The present method provides a means of finding an ideal array for loading spent fuels in several -including high- burnup states, once the upper limits of fuel burnup and radiation dose rate at cask surface are given.The validity of the present method is verified by a trial application to analysis of the measured radiation dose rate distributions around an actual shipping cask.