Condition of feeders at Embalse Nuclear Generating Station

'Full text:' Since 1986, wall thickness measurements have been carried out on the outlet feeder pipes at the Embalse Nuclear Generating Station during every scheduled outage. The measured wall thickness data were used for monitoring the actual condition of the feeders and to evaluate their wall thinning rates, and also to detect any unexpected change in their wall thinning rate. Flow assisted corrosion is considered to be a plausible cause of feeder wall thinning. In some of the other CANDU plants the number of feeders visited for wall thickness measurements was significantly increased when a higher-than-expected wall thinning rate was discovered. Of the 380 outlet feeders in the Embalse Nuclear Generating Station wall thickness measurements of the first bends have been carried out at least once until now. From the evaluation of the measurements all of them are projected to achieve the Design Life of 30 years (or 24 Full Power Years, taking into account a gross capacity factor of 80%) if their wall-thinning rates remain at the current values. Over the years the wall thickness measurements were also performed in the second bends and in the bends close to the main headers. In the bends close to the main headers, the measurements show the wall-thinning rates to be much lower than in those close to the end fittings. Based on the Operating Experience (OPEx) in other CANDU Stations of detecting cracks in the feeder bends, and on the recommendation of the principal designer (AECL), an additional new task was started in the Embalse Nuclear Generating Station in the 2002 scheduled outage to detect possible cracks in the feeders. In that opportunity, 96 outlet feeders' first bends were visited. In the 2004 inspection campaign that followed 106 new bends were selected. For this selection, the criterion followed was to inspect all type-6 outlet bends. This type was given the priority, as it appeared to have the highest risk based on the inspection results, and on the geometric-and-construction features. In the same 2004 campaign, finally a total of 321 bends were visited. These included the 106 first outlet bends (except one, because of access restrictions) from the original program plus additional first outlet and inlet bends as well as second outlet bends. From these two campaigns, all first outlet bends of all types have been inspected, and no crack signals were found. Some details are presented in the paper covering the wall thickness measurement and crack inspection programmes, the evaluation of the results and the conclusions drawn from the evaluation. (author)