ML031260489

March 2003 Davis-Besse Reactor Head Damage, NRC Update

ML031260489
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Site:	Davis Besse
Issue date:	03/31/2003
From:	NRC/RGN-III
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Download: ML031260489 (8)

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March 2003 This is the seventh periodic update on the NRC response to the reactor vessel head damage at the Davis-Besse Nuclear Power Station. The updates will be available at public meetings of the NRC Davis-Besse Oversight Panel which is coordinating the agencys activities related to the damage. Each update will include background information to assist the reader in understanding issues associated with the corrosion damage.

NRC News Briefs Davis-Besse Upcoming Milestones Davis-Besse at Senate Hearing Here are some of the principal work activities remaining The Senate Environment and Public Workss to be completed at Davis-Besse before the utility seeks Subcommittee on Clean air, Climate Change, authorization from the NRC to resume operation of the and Nuclear Safety, held an NRC oversight plant.

hearing on February 13. The head corrosion at Davis-Besse was a major topic of discussion n Performing the integrated leak rate test to during the hearing. A video archive of the determine if the reactor containment, which was hearing is available on the internet at: cut open to move the new and old reactor vessel http://epw.senate.gov/audio-visual_media_108 heads, meets NRC requirements for

.htm "leak-tightness."

NRC Briefs Governor Taft n Performing a special seven-day test of the reactor and reactor cooling system to assure On February 27, Jim Dyer, the Region III there are no leaks, particularly from the tubes Regional Administrator, Jack Grobe, Davis- which pass through the bottom of the reactor.

Besse Oversight Panel Chairman, and others The reactor will not be started up for the test; of the NRC staff briefed Ohio Governor Bob heat added through the operation of the reactor Taft on the agencys response to the head cooling pumps will be sufficient to approach corrosion at Davis-Besse. normal operating temperature and pressure.

n Completing containment sump screen Fuel Has Been Loaded modifications designed to increase the area of the sump strainers in the reactor containment to On February 26, FirstEnergy completed ensure that they do not become clogged by loading the 177 fuel assemblies into the debris which might collect at the bottom of the reactor at Davis-Besse. NRC inspectors containment in certain accident conditions.

observed the core reloading process; they did not identify any significant issues. The reactor n Completing safety culture and safety conscious may not be started up without NRC work environment survey and assessment authorization. designed to measure the level of attention to safety issues among plant operators and NRC Reports issued managers.

Two inspection reports were issued recently n Resolving all items on the NRCs Restart Check and posted to the NRCs web site at List and seeking NRC approval for restart.

http://www.nrc.gov - select Davis-Besse from

the key topics menu. The reports are on the news and correspondence page.

The System Health Assurance report, (Report No. 50-346/02-13) issued February 26, documents the NRCs review of the licensees plans, procedures and implementation of the System Health Assurance Plan.

The inspectors found that the plans and procedures were well designed and rigorously implemented. The inspectors reviewed two of the 36 anticipated review reports completed by the licensee at the close of the inspection and will review the remaining reports as they are completed.

A second inspection report, (Report No. 50-346/02-014) issued February 26, discusses the in-depth design and capability inspection of three plant safety systems - Service Water, High Pressure Injection and 4160 Volt Electrical Distribution Systems. This inspection identified four findings of very low safety significance. The licensees and the NRCs reviews of significant safety systems identified a significant number of deficiencies in calculations, analyses, and testing which will require resolution prior to restart.

Ongoing and Planned NRC Inspections The NRC has a series of inspections planned before making a decision on whether the Davis-Besse plant may resume operations. Several of these inspections include:

n Management and Human Performance, Phase II (Safety Culture) - This inspection will focus on the work underway to survey and assess the safety culture among the staff at Davis-Besse - how the management and workers will deal with safety concerns.

n Radiation Protection - This ongoing inspection is examining the Davis-Besse program for radiation protection for workers at the plant as well as for the general public. The inspection is an outgrowth of earlier inspections of worker radiation exposures and minor radioactive contamination that was found in offsite locations.

n Corrective Action Team Inspection - This inspection looks at the effectiveness of the corrective action program at Davis-Besse - how the utility finds and fixes problems.

n Reactor Vessel Test (Normal Operating Pressure/Normal Operating Temperature) - Monitoring the plants test of the reactor vessel and associated piping to assure there are not leaks in the system.

n Integrated Containment Leak Rate Test - Monitoring the testing by the plant staff of the reactor containment to assure that it meets the NRC requirements for leak-tightness of the containment.

n Safety System Design Reviews - The utilitys Systems Health verification program and earlier NRC inspections had found potential design questions that needed to be resolved. This inspection will look at the effectiveness of the design reviews and the resolution of any issues found.

n Boric Acid Corrosion Management Program - This inspection will focus on the utilitys program for controlling boric acid leakage and its possible effects.

n Reactor Coolant System Integrated Leakage Program - The utility has installed new equipment for detecting small amounts of leakage from the reactor cooling system. This inspection will evaluate the system and the utilitys plans for using the data from the equipment.

n Containment Sump Modification - Reviewing the installation of new screens for the containment sump, which increase the surface area to avoid possible clogging of the screens during certain accident Next NRC Davis-Besse Oversight Panel Meetings - 2 p.m. and 7 p.m. - Tues., April 15, Camp Perry - Highway 2 west of Port Clinton OH

conditions.

n Assessment of Backlog Issues - Evaluating the work Davis-Besse plans to defer until after the plant has resumed operations or to be performed during future outages. This review will consider the appropriateness and safety of the proposed deferrals.

n Restart Assessment Team - As the utility nears the point where it will seek NRC authorization for restart, this team inspection will thoroughly review the readiness of the plant and the plant staff to resume plant operations safely and in compliance with NRC requirements. The inspection findings will be considered by the NRC Oversight Panel in making its recommendation to the Regional Administrator on possible restart.

Red Safety Significance of Davis-Besse issues The Nuclear Regulatory Commission staff has issued its preliminary finding that performance deficiencies at the Davis-Besse Nuclear Power Plant which led to the cracking of the control rod drive tubes and the resulting corrosion damage to the reactor vessel head were of high safety significance.

Under its safety significance determination process, NRC officials classify inspection findings at nuclear power plants as being one of four colors which delineate increasing levels of safety significance, beginning with "green" and progressing to "white," "yellow" or "red." A red category indicates high safety significance.

Under the agencys Reactor Oversight Process, a red finding would result in strong actions by the NRC to improve plant performance, including development of a performance improvement plan by the utility and follow-up NRC inspections.

In the case of Davis-Besse, however, the NRC implemented its procedure for plants in extended shutdown with performance problems and established a special NRC Oversight Panel to coordinate the agencys response to the performance problems which led to the reactor vessel head damage.

The NRC documents reviewing its preliminary assessment of the safety significance will be available on the NRCs web site: http://www.nrc.gov - select Davis-Besse from the key issues menu. The documents will be posted on the News and Correspondence page.

Background:

What Happened at Davis-Besse In March 2002 plant workers discovered a cavity in the head or top of the reactor vessel while they were repairing control rod tubes which pass through the head.

The tubes, which pass through the reactor vessel head, are called control rod drive mechanism nozzles.

Cracks were detected in 5 of the 69 nozzles. In three of those nozzles, the cracks were all the way through the nozzle, allowing leakage of reactor cooling water, which contains boric acid.

Corrosion, caused by the boric acid, damaged the vessel head next to Nozzle No. 3, creating an irregular cavity about 4 inches by 5 inches and approximately 6 inches deep. The cavity penetrated the carbon steel portion of the vessel head, leaving only the stainless steel lining. The liner thickness varies somewhat with a minimum design thickness of 1/8 inch. Subsequent examination by Framatome, FirstEnergys contractor, found evidence of a series of cracks in the liner, none of which was entirely through the liner wall.

Earlier indications of the problem: Through-Wall Cracking of Nozzles in France and at the Oconee Nuclear Power Station in South Carolina In the early 1990s control rod drive mechanism nozzle cracking was discovered at a nuclear plant in France.

These cracks penetrated the nozzle wall along the length of the nozzle (referred to as axial cracking). In 1997 the NRC issued Generic Letter 97-01 to gather information on the inspection activities for possible cracking in the control rod drive mechanism nozzles in plants in the United States. Subsequently, through-wall circumferential cracks -- around the nozzle wall -- were discovered in two control rod drive mechanism nozzles at the Oconee Nuclear Power Station, Unit 3, in 2001. While axial cracking had been found at several other plants and repaired, circumferential cracking had not been seen before. Circumferential cracking is more significant because it could lead to complete separation of the nozzle and a resulting loss-of-coolant accident.

After the Oconee discovery, the NRC issued Bulletin 2001-01, requiring all pressurized water reactor (PWR) operators to report to the NRC on structural integrity of the nozzles, including the extent of any nozzle cracking and leakage and their plans to ensure that future inspections would guarantee structural integrity of the reactor vessel boundary. The NRCs Bulletin instructed nuclear power plants with similar operating history to Oconee Unit 3, including Davis-Besse, to inspect their reactor vessel head penetrations by December 31, 2001, or to provide a basis for concluding that there were no cracked and leaking nozzles.

FirstEnergy Nuclear Operating Company requested an extension of the inspection deadline until its refueling outage beginning March 30, 2002, and provided the technical basis for its request. The NRC did not allow the plant to operate until March 30, but agreed to permit operation until February 16, provided that compensatory measures were taken to minimize possible crack growth during the time of operation. The NRC was unaware that nozzle leakage or corrosion had occurred at Davis-Besse when it agreed to the February 16 date.

Boric Acid Corrosion Control Procedure The water that circulates through a pressurized water reactor to cool the nuclear fuel contains a low concentration of boric acid. This borated water can potentially leak through flanges, pump and valve seals, and other parts of the reactor cooling system and cause corrosion.

The NRC has taken steps to make sure that PWR operators are aware of and pay attention to the corrosion boric acid can cause in certain environments:

• In 1986-89, the NRC issued a series of documents, called generic communications, informing PWR licensees that boric acid can corrode and damage steel reactor components.

• The NRCs Generic Letter 88-05 requested PWR operators to implement a program to ensure that boric acid corrosion does not lead to degradation of the reactor cooling system components. All nuclear power plants with PWRs, including Davis-Besse, reported to NRC that the Boric Acid Control Procedures had been established and would be implemented.

Barriers Built into Nuclear Plants to Protect Public Health and Safety

The design of every nuclear power plant includes a system of three barriers which separate the highly radioactive reactor fuel from the public and the environment. The Davis-Besse reactor head damage represented a significant reduction in the safety margin of one of these barriers, the reactor coolant system.

The reactor coolant system, however, remained intact, as well as the other two barriers, the fuel and the containment.

1. Fuel Pellets and Rods The first barrier is the fuel itself. The fuel consists of strong, temperature-resistant ceramic pellets made of uranium-oxide. The pellets are about the size of a little finger-tip. They retain almost all of the highly radioactive products of the fission process within their structure.

The pellets are stacked in a rod made of a zirconium alloy. At Davis-Besse, each fuel rod is about 13 feet long. The rods are assembled into bundles, with each assembly containing 208 rods. The reactor core contains 177 fuel assemblies. Any fission products which escape from the pellets are captured inside the cladding of the rod, which is designed to be leak-tight. Small pin hole leaks do occasionally occur, however, and the operating license requires leakage monitoring and contains limits on the maximum allowable leakage of radioactive materials from the fuel rods.

2. Reactor Coolant System The second barrier is the reactor coolant system pressure boundary. The reactor core is contained inside the reactor pressure vessel, which is a large steel container. Thick steel pipes supply cooling water to the reactor and carry away the heated water after it passes through the reactor core. The pressure vessel, the connected piping, and other connected components make up the reactor coolant system pressure boundary. At Davis-Besse, the reactor coolant system contains about 60,000 gallons of cooling water, circulated by four large pumps at a rate of about 360,000 gallons per minute.

This system is designed to be leak-tight at operating conditions which include a water temperature of 605E F and a water pressure of 2,150 pounds per square inch. The operating license contains limits on the maximum allowable amount of leakage from the system, and it specifies requirements for monitoring any leakage. If a leak is identified as being through any solid wall of the system (reactor vessel, cooling pipes or other components) continued operation of the plant is prohibited, no matter how small the leak rate.

3. Containment Building The third barrier is the containment building. This is a large cylindrical building which contains the entire reactor coolant system. None of the piping that contains the high-temperature and high-pressure reactor coolant water extends outside the containment building. The containment is a 1 1/2 inch thick steel cylinder, rounded at the top and bottom, which is designed to be leak-tight. This steel structure is surrounded by a reinforced concrete shield building, which is the round building visible from the outside of the plant. Its walls are 2 to 3 feet thick.

NRCs Response to Vessel Head Damage The NRC responded to the vessel head degradation with a series of actions, some specific to Davis-Besse and others aimed at other PWR plants. The agency began a review of its regulatory activities as well.

Davis-Besse On March 12, 2002, the NRC initiated an Augmented

Inspection Team to examine conditions that led to the head degradation and on March 13, 2002, the NRC issued a Confirmatory Action Letter to Davis-Besse documenting a number of actions the plant needed to implement for the unit to be allowed to restart. On April 29, 2002, the NRC established an Oversight Panel under the Agencys Manual Chapter 0350, to coordinate and oversee NRC activities necessary to address repairs and performance deficiencies at the plant in order to guarantee that it can operate safely. The plant will not restart until the NRC is satisfied that plant operators have met all necessary safety requirements.

Generic On March 18, 2002, the NRC issued Bulletin 2002-01, instructing PWR licensees to report on the condition of their head, past incidents of boric acid leakage and the basis for concluding that their boric acid inspection programs were effective. All plants sent their responses and indicated that no evidence of extensive corrosion of reactor vessel heads was found at these plants. On August 9, 2002, the NRC issued Bulletin 2002-02 advising PWR operators that more stringent inspection techniques may be necessary to detect head penetration nozzle cracks. Visual examination of reactor vessel heads and nozzles may need to be supplemented with other inspection techniques, such as the use of ultrasound, electric currents and liquid dyes. In October, the agency also requested PWR licensees to provide additional information on their boric acid inspection program with greater detail than initially covered in the responses to Bulletin 2002-01.

NRC Davis-Besse Oversight Panel An NRC Davis-Besse Oversight Panel was created to make sure that all corrective actions, required to ensure that Davis-Besse can operate safely, are taken before the plant is permitted to restart and that Davis-Besse maintains high safety and security standards if it resumes operations. Should the plant restart, the Oversight Panel will evaluate if Davis-Besses performance warrants reduction of the NRCs heightened oversight and, if so, recommend to NRC management that the plant return to a regular inspection schedule. The panel was established under the agencys Manual Chapter 0350.

The panel brings together NRC management personnel and staff from the Region III office in Lisle, Illinois, the NRC Headquarters office in Rockville, Maryland and the NRC Resident Inspector Office at the Davis-Besse site. The eight-member panels chair and co-chair are John Grobe, a senior manager from Region III and William Dean, a senior manager from NRC headquarters.

As part of determining if plant corrective actions are adequate to support restart, the Oversight Panel will evaluate FirstEnergy's return to service plan, which is divided into seven areas of performance that the utility calls "building blocks." A series of NRC inspections are being performed to verify the company is taking proper actions in each of the seven areas. These reviews will include the work by the FirstEnergy staff and, in addition, the NRC staff will perform independent inspections in each of the "building block" areas.

Issues to be resolved in order for Davis-Besse to restart The NRC Oversight Panel will only consider recommending that Davis-Besse resume operations when the plant has demonstrated its readiness to operate safely. Key elements will include:

• Davis-Besse management and personnel properly understand the technical, organizational, programmatic and human performance problems that led to the extensive degradation of the plants reactor vessel head.

• Davis-Besse enhances programs for operating the plant safely, detecting and correcting problems, controlling boric acid corrosion, and is fostering a more safety-conscious environment among plant managers and workers.

• Davis-Besse improves the performance standards of its managers and workers, including their ownership of the quality of work products and the safety focus of decision-making.

• The replacement of the vessel head is technically sound and all reactor components are inspected, repaired as necessary, and demonstrated to be ready for safe operation.

• Plant safety systems inside and outside containment are inspected, repaired as necessary, and have been confirmed to be ready to resume safe operation of the plant.

• Plant operators demonstrate appropriate safety focus and readiness to restart the plant.

• Any organizational or human performance issues resulting from the ongoing investigation conducted by the NRCs Office of Investigations are addressed.

• All licensing issues that have arisen as a result of the reactor head replacement have been resolved.

• Resolution of radiation protection issues associated with the radiation exposure to workers during steam generator work and the particle contamination found in offsite locations.

• Modification of the strainer system for the containment sump, which would be the source of cooling water for recirculation in the event of a loss-of-coolant accident.

What Happens If the Plant is Allowed to Restart If the facility is permitted to restart, the NRC Oversight Panel will continue to monitor plant activities and operations until panel members are confident that the root cause(s) of the problem have not recurred. Should FirstEnergy achieve that performance level, the NRC Oversight Panel would recommend to NRC management that responsibility for the plant oversight be transferred back to the Region III line organization for monitoring under the Reactor Oversight Process. The panel would then cease to exist. Should FirstEnergy not demonstrate sustained improved performance, the panel will recommend appropriate regulatory actions.

Public Participation in the Process The NRCs experience is that members of the public, including public officials and citizens, often raise questions or provide insights that are important to consider. If you have questions or want to provide information or a point of view, please contact us. For feedback on this newsletter, contact Viktoria Mitlyng 630/829-9662 or Jan Strasma 630/829-9663 (toll free 800/522-3025 - ext -9662 or -9663). E-mail:

opa3@nrc.gov. Extensive information about the Davis-Besse reactor vessel head damage and the ensuing activities is available on the NRC web site: http://www.nrc.gov - select Davis-Besse under the list of key topics.