Measurement of Gamma Dose Rates on Packages Loaded With Spent Fuel Assemblies

After the loading of spent fuel assemblies in a cask, and prior to shipment, measurements are performed to determine the maximum dose rates around the package and the conveyance. These maximum dose rates shall comply with regulatory limits specified in the regulation. The measurements are also compared with predicted values calculated with the spent fuel characteristics prior to the loading. Consignees are also required to perform dose rate measurements on the packages and conveyances. The measurements should therefore be conservative, while being as accurate as possible, with a good repeatability. The choice of a device for the measurement of gamma dose rates must be done carefully, as several important factors may have a strong influence on the result of the measurement, such as the reference spectrum for the calibration, the size of the sensible volume of the detector, the integration time, etc. The spectrum of emission of spent fuel assemblies may vary according to the type of fuel (enriched U or MOX), the enrichment, the burn-up, and the cooling time. Furthermore the spent fuel assemblies usually generate gamma photons of high energy. The device should be able to measure accurately the photon emission on a wide energy range. This paper presents an ORANO / EDF survey on some devices designed for the measurements of gamma radiations on spent fuel packages, including an inter-comparison with measures on ORANO TN casks loaded with EDF fuel. It appears that some devices may strongly overestimate the gamma dose rates due to the presence of high energy secondary gamma photons.