Examination of Variable Frequency Drive Transformer Copper Cooling Line Failures from Nuclear Power Plants

Abstract

The findings from a failure examination on copper cooling lines from nuclear power plant transformer cooling systems are discussed in this article. The failures occurred prior to regular service at two different nuclear power plants during installation and qualification testing. A failure examination including—light optical microscopy, scanning electron microscopy, metallography, and microhardness—was conducted on the fractured lines. The fractures predominately had fatigue striation morphologies, but were found to have initiated in areas exhibiting intergranular cracking, secondary cracking, and surface pitting. Additionally, evidence of phosphorus rich particles was found in the IG crack regions. The failures are hypothesized to be the result of outside diameter environmentally initiated cracking, assisted by residual stresses from brazing, and propagated by fatigue.