Under the magnifying glass: Davis-Besse’s Reactor Vessel Head Replacement by Moderator

Under the magnifying glass: Davis-Besse’s Reactor Vessel Head Replacement by Moderator

Davis-Besse is getting a brand new reactor vessel head this fall -- its third since 2002. And we’re going to invest more than 400 inspection hours to make sure the replacement is done right, and safely, for the workers and the public.

New reactor heads have been installed at 36 out of 69 pressurized water reactors in the U.S.

This head replacement, however, is a major milestone because of the history at Davis-Besse. This is the location where, in 2002, a football-sized cavity was discovered in the reactor vessel head. Because of corrosion found on the head, only a thin stainless steel liner remained between the reactor and the containment building.

The NRC responded to this event by completely overhauling its regulations to require more rigorous examinations of reactor vessel heads.

The damaged head was replaced with a similar head manufactured for another plant that never started operating. It went into service in 2004, after the NRC allowed Davis-Besse to restart.

The replacement was a temporary measure. (A brand new head made from a different metal that is much less susceptible to corrosion was originally supposed to be installed in 2014.)

However, the first replacement head developed several small cracks in an unexpectedly short period of time. The cracks were discovered during NRC-required inspections in 2010. Unlike the degradation found in 2002, these cracks did not challenge the overall integrity of the head and demonstrated that the NRC’s new inspection program worked to identify cracks before they could result in significant head degradation or leakage.

The cracks in the replacement head were repaired. But the NRC and the plant’s owner, FirstEnergy, had extensive discussions about how long the repaired head could remain in service given the uncertainties associated with the unexpected cracking. As a result, FirstEnergy decided to replace the reactor vessel head in October 2011.

This brings us to this brand new reactor vessel head manufactured in France from an alloy that is much less susceptible to corrosion than the two previous heads. The process to install this new head began this week after the reactor shut down on October 1.

The NRC will be there every step of the way. In fact, our reviews started in July, when we began out inspections to verify that the new reactor vessel head was made in accordance with our standards and requirements. NRC resident inspectors at the plant and specialists in metallurgy, health physics, security, and other areas from the NRC Region III Office in Lisle, Ill., are reviewing calculations, procedures and work plans and will directly observe the most significant activities associated with head replacement and post-installation testing. The results of these inspection activities will be documented in the Resident Inspector Quarterly Inspection Report.

Tomorrow, I’ll outline describe how the 180-ton head is actually replaced – no small feat – and how the NRC will conduct its inspections.

Viktoria Mitlyng

Sr. Public Affairs Officer

NRC Region III