Performance of End Cap Welds in 37-Element CANDU Fuel

Abstract

Circumferential cracking of the end cap welds, and power ramp cracking of the sheath are two types of defects that have been encountered in operating 37-element CANDU fuel. Results of the studies to identify defect mechanisms and determine defect thresholds of circumferential cracking are presented. Also discussed are similarities and differences in conditions for power ramp cracking of the sheaths. Among the factors that can help in preventing both types of defects, the most important so far seem to be: Preserving the as-manufactured texture of Zircaloy tubing, Reducing access to the cladding, of fission products that can cause stress corrosion cracking, Keeping the inside surface of the weld zone under compression and reducing the level of stresses in all crucial areas.