Officials from the Davis-Besse Nuclear Power Station in Ohio have told the Nuclear Regulatory Commission that modifications to the reactor head control rod drive mechanism nozzles are complete and the plant will be ready to return to safe and reliable operations in July.
Modifications of 24 of the 69 nozzles having small cracks were completed during the past two months using industry-proven methods, including robotic welding. The flaws were identified during a comprehensive inspection of the reactor head as part of a scheduled refueling outage that began 28 February 2010.
Davis-Besse has been operating safely and reliably since 2004 with an unused, replacement reactor head obtained from a cancelled Midland, Michigan, plant.
An extensive investigation by Davis-Besse plant engineers indicated that elevated reactor head temperatures accelerated the cracking of the nozzle material. This material is known industry-wide to be more susceptible to stress cracking. To impede future nozzle cracking, plant operators will lower the temperature of the reactor core by reconfiguring the fuel assemblies and shorten the next operating cycle.
During the planned 2012 outage, the nozzles will undergo more intensive inspections and testing. A new reactor head, with nozzles made of a different alloy that is less susceptible to cracking, will be installed in 2014.
"Our successful modifications, along with a revised operations plan to minimize stress cracking on these nozzles, will support the continued safe and reliable operation of the plant," said Barry Allen, Davis-Besse site vice president.
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