Regulators: chance of Palisades vessel break resulting in radiation release – one in a million

Mar 20, 2013

During an online public meeting Tuesday night, federal nuclear regulators reiterated their belief that the Palisades nuclear power plant in Covert, Michigan, near South Haven is safe.

The Nuclear Regulatory Commission hosted the meeting to talk with the public about the strength of the vessel that contains the nuclear reactor and fuel. Radiation, high pressure and temperatures over long periods of time make the metal vessels in all pressurized water reactors more vulnerable at nuclear plants.

Palisades is the oldest nuclear power plant in the state, and it’s got one of the most brittle reactor vessels in the country. Older nuclear plants like Palisades have some copper in the mostly steel vessel; later designs have stronger steel, regulators said.

Mark Kirk is a Senior Materials Engineer in the Office of Nuclear Regulatory Research at the NRC.

“It’s unquestionably true that Palisades, one of the welds in Palisades, is one of the most embrittled in all of the plants operating in the US,” Kirk said. “Even so, Palisades continues to operate in compliance with the relevant NRC rules.”

By 2017 the plant’s vessel will become too brittle to legally operate.

In order to keep operating beyond 2017, the company that owns Palisades, Entergy Corporation, is going to have to prove its aging vessel won’t crack in a rare emergency scenario called pressurized thermal shock. At least, the company has to prove the risk of that happening is low enough that it meets NRC rules.

Kirk said the chance of an emergency scenario leading to pressurized thermal shock is between 1 in 1,000 and 1 and 10,000  - depending on which of the plant’s safety systems was compromised if such a situation occurred.

Kirk said that scenario has played out only once in the US, in 1978, at the Rancho Seco Nuclear Generating Station in California. That plant is no longer operating.

The Rancho Seco plant in California was closed after a public vote in 1989.
The Rancho Seco plant in California was closed after a public vote in 1989.
Credit Climate Watch

However, Kirk said the risk of a pressurized thermal shock event leading to a through-wall crack that could release radiation is much smaller; more like 1 in 1,000,000.

The company has several options to continue operating beyond 2017. The company has indicated it will begin tests in the fall to help determine next steps for the plant.

The plant was recently upgraded from one of the worst safety performance levels of any plant in the country following a series of problems in 2011.