Conceptual design study of helium cooled fast reactor in the "feasibility study" in Japan

Okano, Yasushi; Naganuma, Masayuki  ; Ikeda, Hirotsugu; Mizuno, Tomoyasu; Konomura, Mamoru

Conceptual design of gas-cooled fast reactor (GFR) have been studied for selecting applicable combinations of coolant, fuel material and configuration, and, balance of plant, as a part of feasibility study in Japan. Large-scale He-cooled GFR, employing mixed nitride fuel and achieving a high core outlet temperature of 850C, is recognized to achieve attractive features as a future nuclear reactor system. Three fuel configurations are considered and compared in their core and safety performances; one is horizontal-flow cooling fuel assembly (F/A), another is hexagonal matrix block F/A, and the last one is sealed pin bundle F/A. The horizontal-flow and matrix block F/A cores show nearly the same neutronics performances on discharge burnup around 120GWd/ton, breeding ratio above 1.1, and, core cooling performances under depressurization condition without control rod scram or auxiliary core cooling system (ACCS) actuations; whereas around 30% smaller quantity of fissile Pu required is a merit for matrix concept. The sealed pin bundle F/A core potentially shows attractive neutronics performances on discharge burnup about 141GWd/ton with breeding ratio of 1.27, although rapid control rod scram and ACCS actuations are indispensable for core cooling under depressurization accident conditions.