Plating End Fittings to Reduce Hydrogen Ingress at Rolled Joints in CANDU Reactors

Abstract

Zr-2.5Nb pressure tubes in CANDU nuclear reactors absorb hydrogen at a low rate from the primary heat transport water circulated through the tubes. Extra hydrogen is picked up at the rolled joints connecting them to out-of-core steel piping. This enhanced ingress may contribute to pressure tube cracking at incorrectly assembled joints. The risk of pressure tube failure has been decreased by ensuring correct joint assembly, and could be further decreased by reducing hydrogen ingress at rolled joints. In this paper, progress toward using plated end fittings to reduce rolled-joint hydrogen ingress is reviewed. During experiments designed to understand the mechanism for rolled-joint hydrogen ingress, it was found that chromium plating the end fitting, where it contacted the pressure tube, could reduce hydrogen ingress. As a result, a program was initiated to develop plated end fittings for use in CANDU reactors. There are several aspects to this development. Full-scale plated joints are tested for reduction in rolled-joint hydrogen ingress, and for compliance with CANDU design criteria. Experiments are also conducted to determine the role of a plating in reducing ingress, and to develop improved coatings. Results are promising for end fittings plated in the rolled joint region with 18 pm of chromium. Hydrogen ingress is between 30% and 60% less at chromium-plated joints in the short term, and possibly more than 95% less in the long term, than at standard joints. Also, plated joints can comply with design criteria for leak tightness, pull-out strength and residual stresses. Nevertheless, there is room for improvement. The benefit of plating should be increased to an 80% reduction in hydrogen ingress to provide 30 years protection for pressure tubes at rolled joints. In addition, some joint designs may require better leak tightness. Improvements are being sought with alternate plating materials and procedures.