Many changes were made in the recent upgrade of the experimental fast reactor JOYO to the MK-III design. The core changes which made to achieve a fourfold increase in irradiation capacity are the introduction of a second enrichment zone, an increase in core radius and a smaller decrease in core height. Performance tests done at low power, which focus on the neutronics characteristics, are presented. These tests include nuclear instrumentation system response, the approach to criticality and excess reactivity evaluation, control rod calibrations performed several ways, and isothermal temperature coefficient evaluation. The measurements are emphasized but comparisons with calculated predictions are included. The design predictions are consistent with the performance test results, and all technical safety specifications are satisfied. The JOYO MK-III core will provide enhanced irradiation testing capability, as well as serving as a test bed for improving fast reactor operation, performance and safety.

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