Reactor head nozzle modifications complete

Davis-Besse reactor nozzle repairs involve robotic welding and enhanced inspections on the reactor head to address stress corrosion cracking identified during a refueling outage, with NRC oversight and revised operations lowering core temperature.

Davis-Besse Nuclear Power Station officials told the Nuclear Regulatory Commission during a public meeting last night in Port Clinton, Ohio, 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 February 28.

Davis-Besse has been operating safely and reliably, despite earlier problems at the plant, 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, echoing TVA safety changes adopted to address safety culture across the industry.

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.

Work activities scheduled to be completed before restart include: reassembly of the reactor head components, reload fuel assemblies and other restart readiness testing.

Elsewhere in the sector, a lapsed water discharge permit prompted additional reviews at a TVA reactor during related oversight activities.

The NRC Special Inspection Team continues to monitor activities related to the nozzle modifications, similar to oversight of the Browns Ferry safety plan at TVA facilities.

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