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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 γolume ， 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::tO.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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