Radiation Measurements and Shielding Calculations for Spent Fuel Casks

Abstract

Radiation measurements were carried out on two kinds of newly developed NFT-type casks, the NFT-22B and the NFT-14P, in order to evaluate the cask shielding design safety margins and supply benchmarks for cask shielding calculation. The measurements involved placing about 100 sets of small dosimeters on and 1m away from the cask surface. The time integral dose equivalents for 2 to 3 weeks were measured. And gamma ray energy spectrums at some measurement points were also measured by a Nal scintillation counter. The measured dose equivalent rates were 1/2 -1/20 lower than the values estimated by the shielding design method which took the radiation source strength of the spent fuels loaded into the casks into consideration, confirming that the shielding design method used for the casks was conservative enough. The causes of the conservatism in the shielding design were also investigated. Furthermore the effects of changes in hydrogen and B4C concentrations in epoxy resin used for neutron shielding in the NFT casks were evaluated.