Circumferential Cracking of the End Cap Welds - Case Analysis

Abstract

A number of fuel bundles defected during 1984 in the Bruce-3 reactor due to circumferential cracking in the end cap weld zone of outer fuel elements. A COG-funded program has been started to study defect mechanisms and thresholds, and review fuel element specification. Methods of mechanical testing, metallography and stress analysis used in the program are described, and factors studied so far discussed. Mechanical tests include internal pressurization with gas, and loading with an expanding mandrel. Both methods produce the same type of cracking as observed in the in-reactor end cap weld defects. The program has identified the main parameters governing the cracking of the end cap welds. Combining the stress analysis of fuel power histories in-reactor, of axial and radial interferences between pellets and the cladding, and of the load systems used in mechanical tests, makes it possible to assess fuel operational limits (in-reactor) on the basis of out-reactor tests.