DOSE RATE ASSESSMENT FOR THE TRANSPORT OF VITRIFIED RESIDUES ON BOARD PNTL SHIPS USING NEW FEATURES AVAILABLE IN THE MCBEND COMPUTER CODE

Pacific Nuclear Transport Limited (PNTL) ships transport vitrified residues, produced at reprocessing plants in Europe, to the storage facility in Japan. The vitrified residues containers are transported in suitably designed shielded flasks (casks). This paper describes an improved radiation dose rate assessment methodology, which has been used for a maximum loading of these flasks on board one of the ships. The method is also applicable for the transport of spent fuel. The MCBEND Monte Carlo computer code [1] has been used for determining both gamma and neutron dose rates. This represents an improved calculational route, compared to previous assessments, which used approximate point-kernel methods to assess ship loadings. The availability of novel routines in MCBEND9E enables Monte Carlo techniques to be used for the first time, for a ship fully loaded with flasks. Monte Carlo methods produce accurate values of dose rates. The attenuation of radiation through the walls of the flasks is considerable, and to model the radiation source within up to 20 or more flasks on the ship is an extremely difficult calculation for Monte Carlo methods. The problem was surmounted by performing one MCBEND9E calculation for a single flask, with all of the information about particles leaving the flask being stored in an annular leakage file. This information was then used as the source data for each flask in a second MCBEND calculation for the whole ship, which included geometric models of all the flasks. The automated route, providing the ability to transpose source data submitted for a single flask in this way, has not been available until MCBEND9E was written. Dose rates at points in the accommodation areas and regularly occupied working spaces have been calculated. The gamma and neutron contributions were summed to determine the worst case position on each deck. The most onerous dose rates for centre-line and off centre-line positions have been presented. By using MCBEND Monte Carlo calculations, the analysis confirms that optimised loadings of vitrified residues flasks (and potentially also spent fuel flasks) can be carried on board PNTL Ships without exceeding the dose rate criteria, thus ensuring the safety of the crew and the public. The method demonstrates there is potential to carry more flasks, without exceeding the dose rate criteria. Using pointkernel techniques relied on experiment to theory comparisons to justify the method and needlessly restricted the number of flasks capable of being transported in a single shipment.