NEUTRON SPECTRA AND DOSIMETRIC QUANTITIES AT TRANSPORT CASKS FOR REACTOR SPENT FUEL AND VITRIFIED WASTE

Several types of dual purpose casks are used for transport and interim storage of spent fuel assemblies (SFA) and vitrified high-active waste (HAW). Some of these CASTOR-type casks have been stored in the German interim storage facility at Gorleben in recent years. Generally, radiation exposure of personnel during transportation and storage of spent fuel casks is dominated by neutrons. The forthcoming implementation of the new radiation weighting factor for protection quantities, as well as the new quality factor for the operational quantities as a result of the ICRP 60 recommendation and European Directive in national regulations, will in particular affect the radiological protection during transportation and storage of spent fuel. Therefore, comprehensive measurements of neutron spectra and dose rates were performed at four different cask types, two loaded with spent fuel and two with high-active vitrified waste. Neutron fluence spectra and dose rates were measured by means of a Bonner multisphere spectrometer. The paper briefly describes the experimental method and evaluation procedure. Measured spectra for various locations, types of casks, and inventory are discussed, and compared to calculation~ with commonly used radiation transport codes. By folding the spectral distributions with the corresponding conversion coefficients, area doses and effective doses for the old and new quantities were determined and their ratios are given. The neutron area doses around spent fuel casks will increase by approximately 50 % and the effective doses by more than I 00 % with the new conventions. This may have practical consequences, if design values of transport casks and transport limits remain unchanged. On the other hand, the former conservatism between measured doses and the primary limiting effective doses decreases significantly.