Replacement of the reactor vessel head will start in October 2011
FirstEnergy (NYSE:FE) Nuclear Operating Company's Davis-Besse 893 MW PWR at Oak harbor, OH, near Toledo, will enter a fuel outage in October 2011, which is earlier than called for in the usual outage schedule.
During this extraordinary outage, a new reactor head with new control rod channels, or nozzles, will installed at the plant. The nozzles are actually tubes that guide the control rods through the reactor's lid and into the core.
FirstEnergy had originally scheduled the lid replacement for 2014. However, inspections revealed new cracking in the reactor's second lid much earlier than expected by engineers and the NRC. The NRC and the utility determined an earlier replacement date was a better solution than trying to extend the life of the current head which was installed in 2004.
Davis-Besse returned the reactor to service last July after modifications to the reactor head control drive mechanisms. It was shut down in February 2010. Changes were made to 24 of 69 nozzles that were found to have small cracks. The changes included the use of robotic welding. The reactor lid is 17 feet across. It is a one piece casting which is nearly 7 inches thick. It contains 69 holes for control rods of which eight are spares.
The cracking occurred in the reactor lid as a result of elevated reactor core temperatures which plant engineers say accelerated the cracking of the nozzle material. The problem occurs at other reactors as well. Cracks in the nozzles have the potential to cause leaks and uncontrolled releases of radioactivity.
Second lid needs replacement
This is the second time the nozzles, or control rod guides through the reactor head at Davis-Besse, have been found to have developed cracks. The current lid, its second, is from a Michigan reactor which was never used.
To prevent future cracking until second lid can be replaced, the reactor temperature will be lowered below the 600F range that caused the cracking in the first place.
Also, the reactor's fuel assemblies will be reconfigured placing the newer, and hotter fuel assemblies away from the center of the reactor to distribute heat more evenly inside the core.
Todd Schneider, a spokesman for First Energy, told this blog that cracks occurred in nozzles at in the center of the reactor lid which is where the hottest temperatures were experienced by the materials.
· See this multi-part graphic from the Cleveland Plain Dealer which illustrates the elements of the nozzles and the reactor lid.
· See this photo from the NRC of the nozzles in place with one removed.Next steps for the core
The corrective measure, Schneider said is that, "newer fuel assemblies burn hotter so they will be positioned away from the center. Older assemblies in their second or third cycle, which produce less heat, will be moved to the center."
The reactor has 177 fuel assemblies and about one-third of them will be changed out during the next fuel outage. Until then, moving the fuel assemblies around keeps the reactor in revenue service.
During the 2012 fuel outage period, a new reactor lid manufactured by AREVA will be installed with nozzles that have 15% chromium in the steel. The old nozzles had 10% chromium. The higher percentage of the new metal alloy is expected to be more resistant to high temperatures.
FirstEnergy did not disclose how long the reactor outage to replace the lid would take or what it would cost including the price of the lid itself. The U.S. does not have the manufacturing capability to make new reactor lids which is why it came from AREVA.
Davis-Besse was completed in 1978 by Babcock & Wilcox and is currently applying for a 20-year license extension from the NRC.
Update 03/10/11: NRC spokeperson Victoria Mitlyng told this blog that reactor lid replacement is common in the U.S for a variety of reasons. However, there isn’t a single list with all of the reasons for the replacements.
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