Analysis and evaluation studies on nuclear and thermaL design of Joyo MK-III next generation core

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The Joyo experimental fast reactor is being upgraded with MK-III core to improve the irradiation capability. The initial MK-III core fuel being manufactured with MOX powder with equal parts of uranium and plutonium. The uranium has 18% U enrichment with uranil nitrate. Due to the JCO accident, uranil nitrate is not available in Japan. The first two fuel campaigns for the MK-III core will use JNC plutonium processed in France so the fuel composition will match the initial fuel. However, this plutonium is limited and future plutonium imports have many unresolved problems and may not be available for MK-III fueling. Assuming that the 3 fuel campaign in 2010 will use MOX from reprocessing plant as an alternate, a core and thermal design of Joyo MK-III next generation core has started. The MOX powder extracted from the reprocessing plant will decrease the U enrichment of fabricated fuel pellets. To reach criticality with this fuel, the active core height and plutonium content of fuel should be increased. With 20% enriched uranium, the core design requires 34% plutonium content and 55cm fuel height. With 5% enriched uranium for LWR, the core design requires 37% plutonium content and 60cm fuel height. Neutron flux decreases with increasing core height and increases with increased plutonium content so the two effects can cancel each other out and the maximum fast neutron flux is as high as that of MK-III standard core. While the effect on control rod characteristics due to increasing the fuel height is small, the decrease of effective delayed neutron ratio eff causes increased control rod reactivity described in the cent of unit. The absolute values of Doppler coefficient and power coefficient increase. The absolute value of sodium void reactivity decreases but it is still negative. The maximum fuel temperature meets the thermal design standard value based on the decreased fuel melting point due to the increased plutonium content.