FirstEnergy's Davis-Besse nuclear power plant will be ready to restart in July now modifications to reactor head control rod drive mechanism nozzles have been completed, but it will have to work at a lower core temperature and on a shorter operating cycle.
|Davis-Besse: set for July restart (Image: NRC)
Signs of cracking were spotted around several of the control rod drive mechanism (CRDM) nozzles, during a scheduled refuelling and maintenance outage at the Ohio plant earlier this year. FirstEnergy Nuclear Operating Company (Fenoc) carried out extensive tests and has now announced that modifications have been carried out to 24 of the 69 nozzles, each of which penetrates the reactor vessel head, using approved methods including robotic welding.
The nozzles are made of material now known industry-wide to be susceptible to stress cracking, and Fenoc says its investigations have indicated that such cracking is accelerated by elevated reactor head temperatures. The company intends to address this by lowering the reactor core temperature by reconfiguring the fuel assemblies, and also shortening the next operating cycle. The nozzles will then undergo more intensive inspections and testing during the unit's planned 2012 outage. Finally, a new reactor head with nozzles made of a different alloy that is less susceptible to cracking is due to be installed in 2014.
Before the plant can restart, the reactor head components need to be reassembled, fuel assemblies reloaded and other pre-restart tests carried out. A special inspection team from the US Nuclear Regulatory Commission (NRC) is also continuing to monitor activities related to the nozzle modifications.
Davis-Besse, a 877 MWe pressurized water reactor (PWR), underwent a replacement of its reactor pressure vessel head in 2002, after severe CRDM nozzle cracking occurred following an employee's cover-up of missed inspections. Since 2003, all US PWR licensees have been under orders from the NRC to carry out specific inspections of the vessel head and associated penetration nozzles based on their susceptibility to primary water stress corrosion cracking. Reactor vessel heads have been replaced or are scheduled for replacement at various PWRs in the USA.
Researched and written
by World Nuclear News