NRC confirms weld action

[NRC, 12 March] The licensees of 40 pressurized water reactors will raise levels of vigilance concerning reactor coolant system (RCS) welds. The US Nuclear Regulatory Commission (NRC) has issued Confirmatory Action Letters (CALs) confirming the licensees' commitment to put in place "more timely inspection and [weld] flaw prevention measures, more aggressive monitoring of RCS leakage, and more conservative leak rate thresholds for a plant to shut down to investigate a possible [coolant water] leak." The measures should be put in place and welds inspected during an outage before the end of 2007. If no outage is scheduled this year, they must justify an extended schedule to the NRC. The USA's 29 other PWRs either do not feature the type of weld in question or have already taken the required action. The concerns are centred on welds containing Alloy 82 and Alloy 182, used to weld together alloys like Inconel 600 and 601 as well as dissimilar metals such as carbon steel and stainless steel. The steps were taken after the discovery of certain flaws in the welds of the pressuriser at the Wolf Creek plant, which "were repaired and did not affect the safe operation of the plant." The CALs are an interim measure while the American Society of Mechanical Engineers updates its Boiler and Pressure Vessel Code, which will subsequently be reviewed and incorporated into NRC requirements as appropriate.