Abstract
A series of critical experiments and analyses have been made on cylindrical assemblies fueled with 20% enriched uranium metal in the JAERI fast critical facility. The first assembly contains in the core 10.8% of stainless steel and 6.5% of void by volume, and is provided with a natural uranium blanket 30 cm thick. Based on a number of correction experiments, the critical mass of the assembly was determined to be 94.9±0.3 kg of 235U. With a reduced blanket thickness of 10 cm, the critical mass was found to increase by 16.4 %. When one quarter of the enriched uranium metal was substituted by graphite, the critical mass increased by 36.6%. Comparison of these results with theoretical calculations indicated that the criticality of the assemblies can be well estimated by means of the Sn approximation with ANL-635 set.
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