G C Goodnight Consulting, Inc. Goodnight Consulting, Inc. G C G - - PowerPoint PPT Presentation

Nuclear Safety Culture and the Davis-Besse Head Event

A Presentation For

Bangkok, Thailand

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42395 Ryan Road, Suite 112-650 Ashburn, VA 20148 USA

Voice: +1 -703-729-2247 Fax: +1-703-729-8053 WWW.GoodnightConsulting.com

Goodnight Consulting, Inc.

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USA

WWW.GoodnightConsulting.com

Goodnight Consulting, Inc.

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Chuck Goodnight

IAEA Mission THA4015 09 01: Safety Culture and Integrated Management System Roadmap Development

17-21 October 2011

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Presentation Outline

• Background
• Davis-Besse Head Event
• Relationship to Safety Culture
• Safety Culture Overview

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• Key Aspects of Safety Culture
• Summary

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• Davis-Besse Nuclear Power Station

– Operator: FirstEnergy Nuclear Operating Co. – Designer: Babcock &Wilcox – Summer Net Capacity: 893 MWe – Location: Oak Harbor, Ohio, USA

Davis-Bess Nuclear Plant Information

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– Commercial Operation Date: 31 July 1978

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• During a previous inspection, cracks were identified at the base of a Control

Rod Drive Mechanism (CRDM) nozzle on the reactor vessel head

• Information about the cracking, including images, were sent to a contracted

metallurgy subject matter expert (SME) for evaluation

• The evaluation did not identify the potential for cooling water penetration of

the cracks, which would ultimately allow boric acid corrosion

Davis-Besse Head Event – Overview

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the cracks, which would ultimately allow boric acid corrosion

• DB engineering staff accepted the subject matter expert’s evaluation
• During the next fuel cycle, the cracks expanded, permitting the leakage of

primary coolant water, and the subsequent corrosion of the carbon steel reactor pressure vessel head (RPV) head by boric acid in the water

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• Additional boron was added due

to the change in water chemistry

• The leaks continued, and

additional boron was added

• In the next refueling outage, an

Davis-Besse Head Event – Overview (Continued)

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• In the next refueling outage, an

inspection of the vessel head revealed an 8 inch (~20 cm) wide hole in the vessel head that was a result of the boric acid corrosion

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• During the inspections in

the prior refueling outage, boric acid deposits were visible in several areas around the vessel head

Davis-Besse Head Event – Overview (Continued)

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• “Through-Wall Cracking Detected - On February 16, 2002, in response to

Bulletin 2001-01, the Davis-Besse Nuclear Power Station (DBNPS), located in Oak Harbor, Ohio, began a refueling outage with the intent to perform work that included remotely inspecting the VHP nozzles from underneath the head focusing on the control rod drive mechanisms (CRDM). The licensee found that three CRDM nozzles had indications of through-wall axial cracking. Specifically, the licensee found these indications in CRDM nozzles 1, 2, and 3,

Davis-Besse Head Event - Detection

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Specifically, the licensee found these indications in CRDM nozzles 1, 2, and 3, which are located near the top of the RPV head.”

Source: NUREG/BR-0353, Rev. 1, August 2008 (Davis-Besse Reactor Pressure Vessel Head Degradation: Overview, Lessons Learned, and NRC Actions Based on Lessons Learned):

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• “The repair of these nozzles was performed remotely from underneath the
• head. On March 6, 2002, the licensee terminated the repair process on CRDM

nozzle 3 to determine the cause of unusual equipment operation, and removed the machining apparatus from the nozzle. During the removal, the nozzle tipped in the downhill direction until it rested against an adjacent CRDM. If structurally sound, the surrounding steel should have held the nozzle in place.”

Davis-Besse Head Event - Repair Attempts

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Source: NUREG/BR-0353, Rev. 1, August 2008 (Davis- Besse Reactor Pressure Vessel Head Degradation: Overview, Lessons Learned, and NRC Actions Based on Lessons Learned)

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• “The licensee investigated the condition of the RPV head surrounding CRDM

nozzle 3. The investigation included removing the CRDM nozzle and removing boric acid deposits from the top of the RPV head. Upon completing the boric acid removal on March 7, 2002, the licensee conducted a visual examination of the area and identified a large cavity in the RPV head on the downhill side of CRDM nozzle 3. The corrosion was caused by borated water that leaked from the reactor coolant system onto the vessel head through cracks

Davis-Besse Head Event - Investigation

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that leaked from the reactor coolant system onto the vessel head through cracks in the nozzle and the weld that attached nozzle 3 to the RPV head. The remaining thickness of the RPV head in the wastage area was found to be approximately 3/8 inch.”

Source: NUREG/BR-0353, Rev. 1, August 2008 (Davis- Besse Reactor Pressure Vessel Head Degradation: Overview, Lessons Learned, and NRC Actions Based on Lessons Learned)

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• “This thickness consisted of the thickness of the stainless steel cladding on the

inside surface of the RPV head, which is nominally 3/8 inch thick. The stainless steel cladding is resistant to corrosion by boric acid, but it is not intended to provide structural integrity to the vessel. Failure of the stainless steel cladding would have resulted in a loss-of-coolant accident (LOCA). The LOCA would have resulted in actuation of the plant’s emergency systems.”

Davis-Besse Head Event - Investigation (Continued)

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Source: NUREG/BR-0353, Rev. 1, August 2008 (Davis-Besse Reactor Pressure Vessel Head Degradation: Overview, Lessons Learned, and NRC Actions Based on Lessons Learned)

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• The hole was approximately

6 inches long x 8 inches wide (~ 15 x 20 cm)

• The corrosion went

completely through the carbon steel vessel head depth (6.63 inch/ 17 cm)

The Extent Of The Damage To The Reactor Vessel Head Was Unprecedented

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depth (6.63 inch/ 17 cm)

• The corrosion was stopped

by the stainless steel liner

• f the vessel head

(0.375 in/ .0.94 cm)

Source: NUREG/BR-0353, Rev. 1, August 2008 (Davis-Besse Reactor Pressure Vessel Head Degradation: Overview, Lessons Learned, and NRC Actions Based on Lessons Learned)

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• Safety culture can be defined as “all personnel having a questioning attitude

such that anything unusual is challenged to ensure there is not a safety consequence”

• INPO’s definition of Safety Culture is “An organization's values and behaviors

– modeled by its leaders and internalized by its members – that serve to make

Relationship to Safety Culture

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– modeled by its leaders and internalized by its members – that serve to make nuclear safety the overriding priority” (INPO, 2004)

• The IAEA’s definition of Safety Culture is “Safety culture is that assembly of

characteristics and attitudes in organizations and individuals which establishes that, as an overriding priority, nuclear plant safety issues receive the attention warranted by their significance” (IAEA, INSAG 4, 1991)

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• “What if this metallurgy analysis is incorrect and the cracks grow – what might

be the consequences of such an event, and what should be done to mitigate the risk of such an event?” (engineering, coordinating with operations and maintenance)

• “Are additional or more frequent inspections appropriate?” (engineering and
• perations)

Where Did Davis-Besse Staff Have A “Loss” of Safety Culture?

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• perations)
• “Why is the water chemistry level changing such that additional boron is

required to maintain the chemistry balance?” (chemistry, operations, and potentially reactor engineering due to changes in reactivity management)

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• “Where is the boron going, if the levels are lower than normal, and additional

boron is required?” (chemistry, operations, and engineering)

• Why is a higher level of boric acid being removed from the supply chain

inventory (supply chain/materials management)

Where Did Davis-Besse Staff Have A “Loss”

• f Safety Culture? (Continued)

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• Everyone is personally responsible for nuclear safety.
• Leaders demonstrate commitment to safety.
• Trust permeates the organization.
• Decision-making reflects safety first.

INPO’s Principles Of A Strong Safety Culture

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• Nuclear technology is recognized as special and unique.
• A questioning attitude is cultivated.
• Organizational learning is embraced.
• Nuclear safety undergoes constant examination.

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• The Davis-Besse Head Event could have been avoided had their been a strong

safety culture at the plant, and across the operating organization’s leadership

• The consequences were BOTH good and bad:
• Bad Consequences:

– The Davis-Besse plant came very close to suffering a Loss of Coolant Accident (LOCA) – The event cost the operating organization almost $300 Million in direct costs, not including the loss of power sales for the next two years while the plant was inoperable.

Summary

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the loss of power sales for the next two years while the plant was inoperable.

• Good Consequences:

– Nuclear safety culture came into sharper focus around the world – All other U.S. commercial nuclear reactors conducted extensive vessel inspections, focusing

• n vessel penetration areas

– Many PWR plants were able to identified vessel head penetration cracking, and were able to safely monitor crack progression prior to vessel head replacement

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