How the Davis-Besse Reactor Head Replacement Will Be Reviewed

As I wrote on yesterday’s post, installing a 180-ton reactor head at the Davis-Besse plant in Ohio is no small feat for the power company that owns the plant and for the NRC, which is monitoring and verifying the safety of the installation.

First, NRC inspectors are reviewing the calculations necessary to make sure that a 180-ton reactor head can be moved safely to the reactor building, that the cranes are qualified to bear this tonnage and that the floor in the area of containment where the head will stand before being placed on the reactor vessel can withstand such weight.

To get the old head out and the new head into containment, a hole will be cut in the containment and shield buildings after all the fuel is removed from the reactor. NRC inspectors will be observing major steps of this operation to make sure that structural and safety issues are properly handled.

A runway system and transfer cart will be installed through this opening and cranes positioned to transfer the heads. NRC inspectors will monitor the actual transport of the old head out and the new head into containment.

NRC inspectors will review radiological controls to make sure workers and the public are safe as the old head is transported from containment to a special storage facility on plant property. They will verify that appropriate security measures are taken throughout the entire process. Inspectors will verify that the shield building, which is designed to protect the containment building against potentially damaging flying debris, is properly restored.

In addition, they will closely monitor post-installation tests to verify that the plant can operate safely when it is returned to service. The coolant system will be tested for leaks; the control rod drive mechanisms will be tested to make sure they can drop into the core to shut down the reactor within a specified amount of time; a pressure test will be performed to verify the containment building does not leak and x-rays taken to test the welds that close the opening.

Inspectors will verify that the shield building, which is designed to protect the containment building against external hazards, is properly restored so it can perform this function after the opening is closed.

After NRC inspectors are satisfied that the reactor can be returned to service safely and the plant resumes operations, the reactor vessel will be subject to the NRC head inspection regime in place. The NRC will continue to review inspection results to ensure the continued safe operation of the Davis-Besse plant.

Viktoria Mitlyng
Sr. Public Affairs Officer
Region III

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

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
%d bloggers like this: