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.

The discharge of cooling water from the Hanford reactors introduce radioactive contaminants to the Columbia River. These materials may subsequently bring about exposure to human populations either through the direct use of the water for sanitary purposes or transfers of the radioisotopes into the food chains. It is therefore desirable to keep to a minimum the amounts of radioisotopes released to the river.