Core Modification to Improve Irradiation Efficiency of the Experimental Fast Reactor Joyo

Core modification has been investigated to further increase the core burnup and to improve the irradiation efficiency of the experimental fast reactor Joyo. This modification enables the core to accommodate more irradiation test subassemblies that have lower fissile material contents compared with the driver fuel. The design calculations showed that the replacement of the radial reflector elements made of stainless steel with those made of zirconium alloy or nickel-based alloy is effective in improving neutron efficiency. The irradiation test capacity can be increased by reducing the number of control rods based on a reevaluation of the design margin in the control rod worth calculation. The design calculation results show that these modifications, without any change in fuel specification, will be useful for conserving driver fuels and enhancing the irradiation capability of Joyo.