MIXING OF PACKAGE DESIGNS: NUCLEAR CRITICALITY SAFETY

International transport regulations incorporate rules for mixing package designs during transport, including transit storage. Permissible combinations of packages are based on transport indexes. In the paper, two main types of flaws in the rules for mixing package designs have been defined and investigated. (I) Reduced leakage of neutrons from a group of packages. A package which has a high reactivity value in the centre of an array may have a low value on the outside. For another type of package, the opposite may be true. A special case is a non-fissile package. (2) Increased neutron coupling between packages. Sometimes safety depends on the extent of neutron absorption in the packaging materials. Similar packages can be almost entirely isolated from each other. Mixing of such packages with others can reduce the neutron-absorbing effect of the packagings. Flaws are the result of physical relations that can be understood without the need for complicated calculations. However, calculations of a few examples have been made. The findings of the study are that the rules for the mixing of package designs are questionable. General application of the rules may lead to a considerable deterioration in safety. Another conclusion is that a thick layer of concrete, lead or iron on one side of a cube-shaped configuration of packages may provide a lower level of safety than water on all six sides. In the long term, a change in the transport regulations is recommended in order to give due consideration to those cases in which a mixing of package designs would not provide an adequate level of safety.