CRITICALITY AND DOSE UPTAKE BENEFITS FROM NEW PACKAGE DESIGN

Packages used by Sellafield Ltd for the on-site and off-site transport of cans of fissile materials have been in service for a number of years. These were designed against the relevant regulations and requirements at the time for the anticipated materials to be transported, using extant technology. An increased range of Pu and U compounds need to be transported including those from reprocessing operations and residues from experimental and decommissioning operations. Some of these will be of relatively high burnup compared to historic arisings while others will have been treated into new wasteforms to prepare them for long term storage or disposal to a geological disposal facility. There are obvious financial and operational benefits in higher package payloads for both the facility and transport phases of operation. There are, however, a number of challenges to overcome to achieve this for the wide range of materials in question. A new transport package has been designed by International Nuclear Services (INS) to address these. From a criticality perspective the primary enhancement of the new package is the inclusion of a high-integrity multiple water barrier. This gives the potential for less restrictive criticality limits on package payload, with a resultant reduction in the overall number of package movements. Package mass limits are also dependent on a number of other factors relevant to meeting IAEA Transport Regulations as well as plant safety requirements. Higher mass limits, coupled with higher burnup materials, require the package body to have an improved shielding capability to keep dose rates within acceptable levels and to reduce neutron interaction within arrays of packages. Similarly consideration has been given to how the package is loaded and unloaded to reduce the associated dose uptake. The paper will present a more detailed account of the features of the package design relevant to criticality safety, shielding and dose assessment and give examples of the improvements that have been made.