Investigation of Neutron Reaction Behavior in Water-cooled FBR with MOX Fuel

A water-cooled FBR with MOX fuel is under development using current LWR technologies and its design targets are to achieve a BR value, which is a ratio of a fissile plutonium content in a spent fuel to one in a fresh fuel, larger than 1.0 and to obtain a negative void reactivity. This study has been performed to clarify the reasons why the water-cooled FBR has high sensitivities for core specifications. The water-cooled FBR must reduce an X/HM, which is a ratio of hydrogen or deuteron to heavy metal atomic number densities in a unit core volume, to about 0.2 for light water cooling case and 1.0 for heavy water case to attain the required breeding performance. Although the water-cooled FBR is a fast reactor, typical neutron energy values are about 0.4 keV for the former case and about 13 keV for the latter case, and such relatively soft neutron spectra compared with the sodium-cooled FBR one give significant differences on nuclear performance. The water-cooled FBR has a positive void reactivity due to the dynamic reaction rate spectrum changes from normal to voided conditions, named neutron spectrum shift effect. The spectrum shift gives a negative void reactivity in depleted UO2 fuel, so the optimization of the fuel arrangements among the MOX fuel reduces the void reactivity.