The Fermi age of thermal neutrons in a lattice containing both water and graphite in some sort of weighted average of the age in water and in graphite independently, with correction for volumes of non-moderating elements or voids. The correct weighting function has been in question during design calculations for the NPR. This paper presents a very simple and direct approach to the problem, resulting in a weighting equation which seems to be theoretically sound. Unfortunately, simple theories cannot be guaranteed to give good results in complicated systems; nevertheless, it appears that the weighting dunction derived here is to be preferred over methods involving empirical relationships which are of doubtful validity in the NPR geometry.

Nuclear safety specifications for storage, banding, or transfer of enriched uranium slugs depend upon slug arrangement and geometry, the presence of a moderator such as water or wood, and the presence of other fissionable materials and reflectors. Specifications are given for enriched I and E uranium slugs, solid uranium slugs of two types, and aluminum-uranium alloy slugs.