Circumferential cracking of the endcap weld region of PHWR CANDU type fuel elements

A number of PHWR CANDU type fuel elements irradiated to a burnup of ∼ 21 MW d/Kg U failed due to stress corrosion cracking in the endcap weld region. Stress corrosion cracking occurs when irradiation-embrittled material experiences high tensile stress in the presence of a corrosive internal environment provided by fission products. For this reason there is a need to control the stress and strain distribution in the endcap weld region as one of the key factors that affect the initiation and propagation of the cracks in this region. Aiming to achieve this objective a calculation approach has been implemented by linking a global behaviour fuel element computer code (ROFEM 1B) and a general purpose finite element computed code (TEPSAC). Calculated distributions of the stresses correlate well with the location and direction of the cracks observed in irradiated fuel elements. This kind of results are of prime importance in fuel behaviour analysis and were utilized in the improvement of the endcap weld specification. Also the paper summarizes some of other obtained results pointing out the method versatility. (Author) 7 Figs., 7 Refs