Template:Davis-Besse Reactor Vessel Head Degradation

Davis-Besse Reactor Vessel Head Degradation

Published: May 4, 2017

Updated: May 22, 2017

Author: Ashley LaFountain

Topic Editor: Andy Jorgensen

Topics: Nuclear Energy, Nuclear Power Reactors, Major Accidents

The Davis-Besse Nuclear Power Station (DBNPS) is located in Oak Harbor, Ohio approximately 30 miles east of Toledo, Ohio. The plant is currently owned and operated by First Energy Nuclear Operating Corporation. Construction began on September 1^st, 1970 and the plant synchronized to the grid in 1977. DBNPS is a pressurized light water reactor ^[1] that produces a net of approximately 908 Megawatts of power. Recently, the plant completed the license renewal process and received permission from the NRC to extend its operating license to April 22^nd of 2037.

While performing inspections in accordance with Nuclear Regulatory Commission (NRC) Bulletin 2001-01 “Circumferential Cracking of Reactor Pressure Vessel Head Penetration Nozzles,” axial cracking on three Control Rod Drive Mechanisms (CRDM) was discovered. While performing repairs to CRDM nozzle three on March 6, 2002 the nozzle itself was displaced when mechanically disturbed.

When this occurred, plant personnel investigated the reactor pressure vessel environment surrounding nozzle three. Upon investigation, the ultrasonic testing revealed large amounts of degradation and wastage from boric acid used to control the reaction within the reactor. The reactor pressure vessel specifically the reactor head is clad in 3/8” of stainless steel. However, the center of the reactor head is carbon steel which is very susceptible to boric acid corrosion, especially at higher temperatures. It was revealed that the wastage only left the 3/8” stainless steel liner as a barrier between the reactor coolant system (2250 psi) and the containment atmosphere. The area was approximately the size of a football as seen below. If the stainless steel reactor pressure vessel lining would have completely ruptured this would have initiated a loss of coolant accident (LOCA). That being said, there are multiple fission product barriers including zirconium alloy fuel cladding, the reactor coolant system pressure boundary, and the containment vessel. In this case, only the reactor coolant system pressure boundary was at risk of being breached. Therefore, the degradation posed no significant public health risk.

Nozzle #3 Area Cut Away From Reactor Head (Source: Nuclear Regulatory Commission via Wikimedia Commons)

The resulting outage making repairs to the reactor pressure vessel and other systems at Davis-Besse proved to be expensive and time consuming. The reactor was not able to resume operation until March of 2004.

The primary reason for the degradation to the Reactor Pressure Vessel (RPV) was due to Primary Water Stress Corrosion Cracking (PWSCC) of the CRD nozzle. Alloy 600 materials have been known to be vulnerable to PWSCC and the plant has continued to mitigate alloy 600 metals within the plants primary systems. Also, inadequate inspections of the RPV prevented earlier detection of the nozzle cracking, subsequent leakage, and corrosion specifically during the 1998 and 2000 refueling outages.

Regarded as a “serious nuclear safety incident”, this incident has substantially impacted public perception. Even right after the event took place, the local newspaper, the Port Clinton News Herald, released an article entitled, “NRC raps D-B for not finding reactor corrosion sooner”. In this article, the NRC chastised Davis-Besse workers for missing a number of opportunities to discover the boric acid degradation. The plant received additional criticism for not further investigating radiation air filters that were clogging with boric acid deposits in 1999. While the employees knew there were boric acid deposits, they thought the source was something else. Specifically, the staff at Davis-Besse went from changing the air filters from monthly to every other day. In regards to the air filters, senior metallurgical engineer for the NRC, Mel Holmberg noted, “This was a missed opportunity to identify leakage in the reactor head".

The reactor head replacement was installed in 2002 and was acquired from a partially completed nuclear power plant in Midland, Michigan that was never used. However, the current reactor head was installed in 2011 with a different alloy, Alloy 690, which is less susceptible to PWSCC and was manufactured by Areva and stored in France prior to being shipped to Davis-Besse for installation. Before being permitted to restart the plant in 2004, the Station was required to undergo an independent evaluation of the safety culture at Davis-Besse to address safety concerns before restarting. Specifically, some of the changes included improved and stricter guidelines for structures systems or components (SSCs) with the potential to crack or corrode, additional training for plant inspectors, and monitoring of the plants’ inspection programs.

In all, since the 2002 incident, Davis-Besse has made significant strides to improve safety standards. Together, Davis-Besse and Ohio's other nuclear power plant, Perry Nuclear Power Plant, continue to produce 90% of Ohio’s carbon-free energy.

Citation:

LaFountain, A. (2017, May 22). Davis-Besse Reactor Vessel Head Degradation. Retrieved from: Davis-Besse Reactor Vessel Head Degradation

Template:Davis-Besse Reactor Vessel Head Degradation

Davis-Besse Reactor Vessel Head Degradation

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