The performance of NSF in BWR operating conditions

NSF is a zirconium alloy in the zirconium-tin-niobium-iron family. The nominal alloy content is 1 % tin, 1 % niobium, and 0.4 % iron. The initial targeted component for NSF in boiling water reactors (BWRs) is the fuel assembly channel. A BWR fuel channel encases the fuel bundle such that boiling is confined within the channel while the water remains solid outside the channel. The BWR fuel channel also defines the gap between the bundles where the cruciform control blade inserts and withdraws to control the criticality of the core. The key performance requirements of the BWR fuel channel are dimensional stability (including both shape and thickness) and adequate mechanical properties to withstand the applied stresses. The distortion mechanisms that affect dimensional stability are fluence gradient-induced bow, shadow corrosion-induced bow, and elastic and creep bulge. Although NSF is known to have improved irradiation growth characteristics compared with both Zircaloy-2 and Zircaloy-4 that translates to increased dimensional stability in fuel channels, the BWR corrosion performance, tensile properties, and irradiation creep performance are less well known. This paper reports on the physical metallurgy of NSF, oxide thickness and hydrogen content after in-reactor operation, mechanical properties (both irradiated and unirradiated), in-reactor creep response, and irradiation growth and bow of NSF channels.