Transport index evaluation for low neutron leakage arrays -- The sphere and shell model

It is often desirable to have a package for fissile material shipment certificated for specified fissile and nonfissile mass limits but without restrictions on the disposition of the materials inside the containment vessel. Reasoned arguments are presented which indicate that, provided there are no significant quantities of moderating material in the containment vessel, then analyzing a package with a sphere and shell model for the fissile material will bound the multiplication factor. That is, any other disposition of this same material in the same shipping container and for the same array size will be less reactive. Calculational results are presented which support this argument. 3 figs