By Scott DiSavino March 8 (Reuters) - 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) said Friday. 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. DECISION TIME "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.