Results of an investigation of fuel element outer annulus enthalpy imbalance are reported and analytical results of suggested methods to limit this imbalance are presented. Also included are comparisons of calculated and experimental enthalpy imbalance ratios along with other quantities. Results indicate that tightening fuel tolerances and clearances can substantially reduce the present outer annulus imbalance thereby increasing power capability. A basic pre-requisite to reduction of clearance between fuel support diameter and process tube diameter is a test establishment of the minimum allowable clearance compatible with charging requirements. Matching fuel elements to process tube diameters will also reduce the imbalance but is in general not feasible at the present time.

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Demands for cobalt-60 have been relatively small as long as the principal application were for radiography and gaging. The accumulated production in the entire United STates is less than ten megacuries, although the Hanford plant is capable of producing about 200 megacuries every year using uranium enriched to 0.95 percent, and greater amounts with higher enrichments. Two new developments brighten the prospects for large-scale utilization of cobalt-60. First is the possible use of cobalt-60 as a heat-producing radioisotope. Although shielding weights make cobalt-60 unattractive for space applications, the general size of thermal applications can be appreciated by noting that 32 meacuries of cobalt-60 would be required to provide the 500 kw of isotopic heat which NASA reports they will need by 1980; terrestrial applications may be an order of magnitude larger. This application of cobalt-60 has been pushed primarily by Savannah River and utilization of high activity cobalt-60 (200-500 curies/gram) as a heat source is being investigated by them under MFC 5.