A Comparison of Dose Rate Measurement and Assessment that enabled the Capacity of a Spent Fuel Flask to be increased

BNFL use spent fuel flasks to transport LWR fuel from reactors in Europe and elsewhere to Sellafield, Cumbria, UK for reprocessing. The initial dose rate assessment for a spent fuel flask for a particular European BWR fuel, carried out using the MCBEND Monte Carlo code [1], indicated that only 15 of the possible 16 compartments within the multielement bottle (MEB) could be used, without exceeding the regulatory criteria [2]. The dose rate is dominated by the neutron component, even though a substantial neutron shielding cover is attached to the flask. The assessment was based on pessimistic assumptions regarding neutron multiplication within the flask and indicated that the top compartment should not contain a fuel assembly. Consequently for the Transport Submission, the assessment was repeated with a steel displacer (to displace the same quantity of water as a fuel assembly) within the top compartment, in order to determine the peak dose rates when 15 assemblies were transported. The displacer is a hollow steel structure of square cross section. Subsequently, a series of special dose rate measurements was carried out on shipments of flasks carrying 15 fuel assemblies. The purpose of these measurements was to provide further validation of the code and thus enhance the confidence that may be placed in the assumed safety margins for shipments already assessed. When the measurements were compared with calculation, it was confirmed that the calculated dose rates were unduly pessimistic. The degree of pessimism suggested that 16 fuel assemblies could indeed be carried in the flask, with more than adequate margins of safety.