Core design consideration for reduction of the fast neutron fluence in large fast reactors

Kawashima, Katsuyuki; Ikegami, Tetsuo

The fast neutron fluence plays the major role in limiting the higher fuel burnup in large fast reactor cores with austenitic stainless steel cladding and duct. Therefore, the core design considerations are performed to reduce the peak fast fluence to average burnup ratio (PFB ratio). The important findings include as follows: (1) Key design parameters are defined which are related to the PFB ratio, and the formula for evaluating the PFB ratio of the fuel is established as: PFB ratio power peaking factor/fissile enrichment irradiation time / core averaged irradiation time. (2) Available design methods to reduce the PFB ratio include the flux shaping by the operating control rods and the decrease of irradiation time due to the multiple batch refueling scheme. (3) These design methods are applied to the 1000 MWe homogeneous core design, resulting in the 15-18% reduction in the PFB ratio without significant penalties on the other core characteristics such as peak power density and discharge burnup.