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New rule for US pressure vessels

Wednesday, 3 October 2007

Risk of damage to reactor pressure vessels from pressurized thermal shock has been found to be less than previously thought. The US Nuclear Regulatory Commission has developed a new more realistic rule that nuclear power plant operators could choose to follow.

Risk of damage to older reactor pressure vessels frompressurized thermal shock (PTS) has been found to be less than previouslythought. The US Nuclear Regulatory Commission (NRC) has developed a new morerealistic rule that nuclear power plant operators could choose tofollow.

Pressurized water reactors (PWRs) use pure light water as both moderator and coolant in the reactor's primary circuit, which transports heat from the reactor core to generate steam for power generation. During operation, the reactor pressure vessel is typically at about 330 degrees C, and contains the water at pressures of around 155 kilograms per square centimeter. PTS can occur when the reactor is rapidly cooled while still pressurized. It could lead to cracking of the vessel's steel and, potentially, failure with very serious safety consequences.

Barry Elliot of the NRC told World Nuclear News that an example of an event that could cause thermal stresses great enough to cause PTS would be the rapid injection of cold water to the reactor vessel during a loss of coolant accident.

Research carried out at the Oconee 1, Beaver Valley 1 and Palisades PWRs has indicated that the overall risk of PTS-induced vessel failure after 60 years of operation is "much lower" than previously estimated. The USA's nuclear plants were typically designed for 40-year lifespans, but the over-engineering practiced in the nuclear industry has mean that many have been licensed by the NRC to operate for up to 60 years. New reactors built today would be expected to enjoy lifespans of at least 60 years by design.

The safety body has developed a new "more realistic" rule based on the research which PWR operators in the USA could voluntarily adopt for determining the probability of vessel failure during a PTS event. Operators following the rule would have to perform additional detailed analyses of the results of periodic reactor vessel inspections.

The proposed rule is open for public comment during the next 75 days.

Further information

US Nuclear Regulatory Commission

WNA's Nuclear Power Reactors information paper