By Scott DiSavino
March 8 U.S. power company Entergy Corp
must deal with a potential issue with the metal in the reactor
vessel at its Palisades nuclear plant in Michigan by 2017 or
shut the reactor, the U.S. Nuclear Regulatory Commission (NRC)
The NRC said the plant is safe but estimates that by 2017
the metal in the aging reactor vessel could reach a regulatory
limit for handling something called pressurized thermal shock
due to years of radiation, temperature and pressure stresses.
Pressurized thermal shock is a phenomenon common to
pressurized reactors like Palisades that could occur in a rare
accident scenario in which a large amount of cold water has to
be injected into the reactor resulting in its rapid cooling that
could challenge vessel integrity.
"This is a generic issue for all pressurized water reactors,
and Palisades complies with all pressurized thermal shock
regulations," Entergy said in an email.
The NRC said it will hold a webinar on March 19 to discuss
pressurized thermal shock and how it relates to Palisades.
"The reason we are holding the webinar is because there is a
lot of alarming information out there about this phenomenon
leading to the imminent shattering of the reactor vessel, which
is not the case," NRC spokeswoman Viktoria Mitlyng told Reuters.
The 793-megawatt (MW) Palisades, which entered service in
1971, is located on the shore of Lake Michigan in Covert
Township about 120 miles (193 km) west of Lansing, the Michigan
state capital. In 2007, the NRC renewed the reactor's original
40-year operating license for an additional 20 years until 2031.
This issue could be important for Palisades as energy
analysts have already mentioned the reactor as one of the older,
smaller, single unit plants in the Midwest region, which has
seen Dominion Resource Inc's decision to retire the
Kewaunee reactor in Wisconsin later this year due in part to
weak natural gas and power prices.
As a reactor vessel ages the metal becomes less ductile - it
tends to bend less and becomes more brittle - as it is bombarded
by neutrons and other forces, Mitlyng said.
Mitlyng said pressurized thermal shock could occur in the
unlikely event of a severe accident.
"But the NRC has to plan for the unlikely," she said.
As an example of an unlikely severe accident, Mitlyng said,
a large pipe could break, forcing the operator to fill the
vessel with emergency water supplies from storage tanks or Lake
Michigan. The lake water is about 40 degrees F (4 C), while the
vessel operates at about 550 degrees, she said.
Mitlyng said if you had a massive amount of cold water
coming into the hot vessel, there is more of a risk - albeit
small - that the integrity of the vessel could be challenged.
"Once Palisades reaches that limit in 2017, if they don't
take steps to address the issue, the plant would have to shut,"
she said, noting that the NRC's embrittlement limits are
conservative to protect the public safety.
She could not immediately say which other reactors were
approaching their embrittlement limits - just that Palisades is
one of the plants closest to its limit.
To address the issue, Mitlyng said Entergy would have to
provide evidence the vessel would remain safe beyond 2017.
She said they could make some physical changes to the vessel
or offer data under an alternative rule on embrittlement limits.
If Entergy wants to use the alternative embrittlement rule,
she said the company would have to perform inspections and map
out and characterize any flaws in the reactor vessel.
Entergy said its current analysis requires the company to
perform additional inspections in 2013 and submit an updated
evaluation to the NRC in the spring of 2014.
Mitlyng said it will take the NRC about three years to go
through all the data before making a decision.