The US Nuclear Regulatory Commission has issued action letters to 40 nuclear power plant operators with pressurised water reactors (PWR), confirming commitments to resolve concerns regarding flaws in certain welds in the reactor coolant system by the end of the year.

The remaining 29 PWR plants in the country have either completed requisite actions already or do not have welds susceptible to these flaws, the NRC said.

The move followed the discovery of flaws in pressuriser welds at the Wolf Creek reactor near Burlington, Kansas.

The orders ensure more aggressive monitoring of leaks and a lower leak rate threshold for plants to shut down, and also commit operators to conduct ultrasonic inspections of some welds containing Alloy 82 and Alloy 182.

The first incidence of these flaws occurred in a US reactor in 1993 but the size and nature of the Wolf Creek flaws led the NRC to conclude that the industry must accelerate its planned actions for the remainder of the affected plants in the PWR fleet.

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