Analysis of the pressure tube failure at Pickering NGS A unit 2

About noon on the 1st August 1983, the pressure tube in fuel channel G16 of the Pickering NGS A unit 2 reactor developed a critical through-wall crack and failed by fast fracture after 342 days of continuous full power operation. Following removal of the fuel, a TV inspection inside the fuel channel revealed an axial crack in the bottom of the pressure tube approximately 2 metres long, in which two fuel pencils were lodged. After extracting the fuel pencils, the fuel channel was removed and shipped to the Atomic Energy of Canada Limited's Chalk River Nuclear Laboratories for detailed examination to determine the cause of failure. Examination of failures normally takes a course of looking at the fracture and gradually refining the work into finer detail to determine the actual origin of the failure. In this case, several other aspects also needed to be examined. The position of the garter spring was very important, as was examination of the calandria tube, which was subsequently removed. During the inspection several other fuel channels in Pickering A, Bruce A and NPD reactors were inspected and some removed for further assessment at CRNL. All these aspects came together to outline the cause and mechanism of failure. The following gives a very brief review of the salient features of the examination of Zircaloy 2 and zirconium-niobium pressure tubes and the implication for operation of subsequent reactors which have zirconium-niobium pressure tubes