The company made modifications of 24 of the 69 nozzles having small cracks during the past two months using methods such as robotic welding. The flaws were identified during an inspection of the reactor head as part of a scheduled refueling outage that began February 28, 2010.
The plant engineers of Davis-Besse, which has been operating since 2004 with an unused, replacement reactor head obtained from a cancelled Midland, Michigan, plant, found out through an investigation that elevated reactor head temperatures has hastened the cracking of the nozzle material, which is known 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.
Before restarting the power station, the plant operators will reassemble the reactor head components, reload fuel assemblies and conduct other restart readiness testing.
The company said that during the planned 2012 outage, the nozzles will undergo more 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.
The NRC Special Inspection Team will continue to monitor activities related to the nozzle modifications.
Barry Allen, site vice president of Davis-Besse, said: “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.”
