A Criticality Perspective on Multiple Water Barriers   for Fissile Material Transport Packages

New packages have been identified and are under development by INS to facilitate the re?disposition of fissile materials under the NDA’s programme of managing the UK’s historic nuclear legacy. The development of new fissile material transport packages is an increasingly complex and costly business, and much time is spent on ‘future?proofing’.   The IAEA regulations stipulate performance requirements for the behaviour of the package under both normal and accident conditions. According to Paragraph 677 of the IAEA transport regulations (2009 Edition), for fissile material package designs, the criticality safety assessment must assume that water leaks into or out of all void spaces within the single package. This is irrespective of the integrity of the package following the IAEA standard tests, unless ‘special features’ to prevent such leakage of water are incorporated into the package design. These special features include Multiple high standard Water Barriers (MWB).   Water is an efficient moderator of neutrons, i.e. it possesses the ability to reduce the energy of fast neutrons down to thermal energies, which will then have an increased probability of causing further fissions. Hence the requirement to assume the addition of water into a “dry” package can significantly increase the reactivity of a system and reduce the mass of fissile material required to form a critical assembly. If water ingress can be prevented, criticality safety will be guaranteed for the particular package design.   The new packages developed by INS for irradiated and unirradiated fissile materials will feature MWB technology which will enable safe transport of optimised payloads. The paper presents, from a criticality viewpoint, examples of the advantages of package designs incorporating MWBs for transport of a variety of fissile materials.