Core performance tests for the JOYO MK-III upgrade

Many changes were made in the recent upgrade of the experimental fast reactor JOYO to the MK-III design. The core changes which were made to achieve a four-fold increase in irradiation capacity includes the introduction of a second enrichment zone, an increase in core radius and a decrease in core height. Performance tests done at low power, during the rise to power, and at full power, which focus on the neutronics characteristics, are presented. These tests include the nuclear instrumentation system response, the approach to criticality and excess reactivity evaluation, control rod worth calibrations, isothermal temperature coefficient evaluation, the calibration of the nuclear instrumentation system with reactor thermal power, and the burn-up reactivity coefficient. The measurements and comparisons with calculated predictions are shown. 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 serve as a test bed for improving fast reactor operation, performance, and safety. Through the performance test evaluation, the core characteristics of a small size sodium cooled fast reactor with a hard neutron spectrum are clarified.