Friday, December 9, 2011

NRC Challenged on Plant Corrosion

Repeated stress corrosion cracking of stainless-steel components at a Michigan nuclear plant has a congressman asking federal regulators why they continue to allow use of the metal in such applications. A Nuclear Regulatory Commission investigation into a service water pump failure that recently shut down the Palisades Nuclear Plant has traced the problem to the same intergranular stress corrosion cracking (IGSCC) that caused the same pump to fail in 2009. NRC The same service water pump failed twice for the same reason in two years at Palisades Nuclear Plant, near South Haven, MI. The plant’s license was renewed in 2007, through 2031. In an Inspection Report issued Nov. 29, the NRC classified the discovery as a so-called “White” finding, indicating an issue “with low-to-moderate increased safety significance.” However, the NRC issued no violations, despite what one congressman says is a long history of such problems at multiple facilities. ‘Well-Known History’ “I am concerned that this failure is the latest in a string of similar incidents at Palisades and other nuclear power plants over the last two decades, and may be related to the continued use of types 410 and 416 martensitic stainless steel (410SS and 416SS), which are used in components of SW pumps at nuclear power plants,” Rep. Edward Markey (D-MA), senior member of the House Energy and Commerce Committee, wrote last week to NRC chairman Greg Jaczko. “The pumps are critical to reactor safety, and I am concerned that despite a well-known history of problems related to the vulnerability of these metals to intergranular stress corrosion cracking (IGSCC), they continue to be used and continue to cause failures in U.S. nuclear power plants.” Markey says the metals’ vulnerability to IGSCC has been documented by the NRC and in scientific and industry operating literature. Embrittlement Problems The NRC’s finding says that the plant operator, Entergy Nuclear Operations Inc., “failed to take into consideration significant operating experience from as early as 1993 and as late as 2010 that linked IGSCC susceptibility of 410 and 416 stainless steels to temper embrittlement.” Clearly, then, Markey says, making the information available to plant operators has not been sufficient to ensure the safe use of these metals. So why not issue relevant requirements? The lack of mandates is “particularly alarming” since a 2007 NRC Information Notice characterized the pump cracking problem as difficult to detect, even with testing, he says. “Operating experience also shows that pump shaft failures and coupling failures can result in sudden total loss of flow before standard performance monitoring techniques alert plant staff to the impending failure,” the 2007 NRC Notice states, according to Markey. Operator Report Entergy reported that the pump was inoperable for a month before it failed. The report notes that the 416SS metal was installed in a 2007 design specification change. “Personnel involved in the design change process did not have sufficient metallurgical knowledge,” the report says. “Palisades did not obtain an adequate technical review by personnel with expertise in metallurgy.” The report says that the choice of 416SS did not account for the corrosive Lake Michigan environment. Questions and Requests Markey requested that NRC provide a number of documents by Jan. 6. That includes a list of all plants that use 410SS and 416SS components and a list of their uses. Noting that NRC moved in 2005 to increase inspections at plants that used another metal (steel Alloy 600) susceptible to degradation, he asks: “Will you initiate a similar review for the 410SS and 416SS steels used in pump components? If not, why not?” Finally, he asks what actions the NRC will take to address the integrity of the 410SS and 416SS pump components now in use. “If no such actions are planned,” he adds, “why not?” Concrete Problems The steel corrosion issue arose just days after the NRC issued a system-wide warning on the risk of concrete cracking and degradation at its facilities. In that case, the agency was responding to a problem at the control building of the Seabrook Station Nuclear Plant in New Hampshire. The concrete problem was laid to Alkali-Silica Reaction-induced (ASR) degradation, which occurs when alkalis—usually from cement—react with certain types of silica in the aggregate, in the presence of moisture.

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