Alloy design and characterization of a recrystallized FeCrAl-ODS cladding for accident-tolerant BWR fuels: An overview of research activity in Japan

Following the severe accident at the Fukushima Daiichi nuclear power plant in 2011, recrystallized FeCrAl-ODS claddings have been developed in Japan as an accident-tolerant fuel (ATF) to mitigate the extent of oxidation reactions with hot steam. This paper presents an overview of the alloy design and the process used to manufacture the recrystallized cladding, together with a analysis of the applicability of these alloys as BWR fuel cladding and a summary of the results of simulated severe accident performance. A synergistic effect of co-addition of 12 mass% Cr and 6 mass% Al was found to suppress steam oxidation by a stable Al2O3 scale formation and to inhibit a brittle αʹ precipitate after aging. The incorporation of 0.4 mass% Zr greatly improved not only the high temperature strength by replacing coarse Y-Al oxide particles with finer Y-Zr particles, but also the oxidation resistance at temperatures above 1673 K. These excellent performances are associated with the excess oxygen content. It was verified that core excess reactivities affected by the increased neutron absorption by Fe, Cr, Al can be compensated by reducing the thickness to half that of Zircaloy cladding, while maintaining mechanical integrity. A simulated design basis Loss of Coolant Accident (LOCA) event with assessment of post-LOCA ductility confirmed that FeCrAl-ODS cladding provided a greater safety margin along with exceptional high temperature strength and a significantly higher integral LOCA burst temperature compared with wrought FeCrAl alloy (C26M) and Zircaloy. An analysis using the MAAP 5.05b SA code indicated that melting of the Zircaloy core could be slightly accelerated due to release of the huge amount of exothermic reaction heat in the case of small amount of coolant injection. In contrast, the water injection always acts toward cooling the FeCrAl-ODS core.