IR 05000285/2007006
| ML071340250 | |
| Person / Time | |
|---|---|
| Site: | Fort Calhoun  |
| Issue date: | 05/08/2007 |
| From: | William Jones NRC/RGN-IV/DRP |
| To: | Ridenoure R Omaha Public Power District |
| References | |
| IR-07-006 | |
| Download: ML071340250 (19) | |
Text
May 8, 2007
SUBJECT:
FORT CALHOUN STATION - NRC SPECIAL INSPECTION REPORT 05000285/2007006
Dear Mr. Ridenoure:
On April 9, 2007, the U.S. Nuclear Regulatory Commission (NRC) completed a special inspection at your Fort Calhoun Station. The inspection examined your activities and root cause evaluation following the failure of two safety related ABB circuit breakers. The enclosed special inspection report documents the inspection findings which were discussed on April 9, 2007, with Mr. David Bannister, Plant Manager, and other members of your staff.
The enclosed special inspection report documents examined activities conducted under your license as they relate to safety and compliance with the Commissions rules and regulations and with the conditions of your license. In accordance with 10 CFR 2.390 of the NRC's "Rules of Practice," a copy of this letter, and its enclosure, will be available electronically for public inspection in the NRC Public Document Room or from the Publicly Available Records component of NRCs document system (ADAMS). ADAMS is accessible from the NRC Web site at http://www.nrc.gov/reading-rm/adams.html (the Public Electronic Reading Room).
Sincerely,
/RA/
William B. Jones, Chief Engineering Branch 1 Division of Reactor Projects Docket: 50-285 License: DPR-40
Omaha Public Power District
- 2 -
Enclosure:
NRC Inspection Report 05000285/2007006 w/Attachment 1: Supplemental Information Attachment 2:
Special Inspection Charter
REGION IV==
Docket:
50-285 License:
DPR-40 Report:
05000285/2007006 Licensee:
Omaha Public Power District Facility:
Fort Calhoun Station Location:
Fort Calhoun Station FC-2-4 Adm.
P.O. Box 399, Highway 75 - North of Fort Calhoun Fort Calhoun, Nebraska Dates:
February 20 through April 9, 2007 Inspectors:
M. Peck, Senior Resident Inspector L. Willoughby, Resident Inspector Accompanied By:
B. Correll, Reactor Inspector Approved By:
William B. Jones, Branch Chief, Engineering Branch 1 Division of Reactor Safety
Enclosure
SUMMARY OF FINDINGS
IR 05000285/2007006; 02/20-04/09/2007; Fort Calhoun Station, Special inspection to evaluate
ABB circuit breaker failures.
This report documents special inspection activities conducted by a special inspection team consisting of one senior resident inspector, one resident inspector and one region-based reactor inspector. The significance of most findings is indicated by their color (Green, White,
Yellow, Red) using Inspection Manual Chapter 0609, "Significance Determination Process."
Findings for which the significance determination process does not apply may be Green or be assigned a severity level after NRC management review. The NRC's program for overseeing the safe operation of commercial nuclear power reactors is described in NUREG-1649,
"Reactor Oversight Process," Revision 3, dated July 2000.
No findings of significance were identified.
REPORT DETAILS
OTHER ACTIVITIES
4OA5 Other Activities
1. Special Inspection Scope
The NRC conducted a special inspection to evaluate the risk significance and corrective actions associated with two safety-related ABB circuit breaker failures at the Fort Calhoun Station (FCS). On January 25, 2007, the raw water Pump AC-10B power supply breaker failed during the start sequence. The licensee determined the apparent cause of the failure was a broken linkage arm connecting the circuit breaker to the mechanism operated contact switch. The licensee replaced the linkage arm and returned the pump to service. On February 8, 2007 the raw water Pump AC-10C supply breaker failed during the start sequence. The second breaker failure was also the result of a broken linkage arm. Both linkage arms failed at the same location. Due to the common cause aspect of these failures, the licensee declared the two remaining raw water pumps inoperable. The licensee replaced the failed linkage arm and the linkage arms on the two remaining raw water pumps. Failure of two linkage arms raised a concern for a potential common mode failure of safety-related applications powered from 4160 volt ABB circuit breakers. Other potentially affected FCS safety-related components included the low pressure safety injection pumps, the motor-driven auxiliary feedwater pump, the normal and alternate feeder breakers to the 4160 volt busses, and bus feeder breakers. Non-safety related buses and components, supplied by 4160 volt ABB circuit breakers, may also have been affected.
The NRC determined that this event met several of the deterministic criteria described in Management Directive 8.3, NRC Incident Investigation Program, and represented sufficient risk significance to warrant a special inspection. The special inspection team reviewed the circumstances related to the ABB circuit breaker failures and assessed the effectiveness of the licensees actions for resolving these issues. The team also assessed the effectiveness of the immediate actions taken and the notifications made by the licensee in response to the January 25, and February 8, 2007 breaker failures.
The team used NRC Inspection Procedure 93812, Special Inspection Procedure, to conduct the inspection. The special inspection team reviewed procedures, corrective action documents, and design and maintenance records for the equipment of concern.
The team interviewed key station personnel regarding the event, reviewed the root cause analysis, and assessed the adequacy of corrective actions. A list of specific documents reviewed is provided in Attachment 1. The charter for the special inspection effort is provided as Attachment 2.
2. Overview of ABB Circuit Breaker Design, Degraded Conditions, and Failures
Omaha Public Power District (OPPD) modified the FCS safety and non-safety related 4160 volt electrical distribution breakers in 1995. This modification involved adapting replacement ABB circuit breakers for use in existing General Electric bus cubicles.
OPPD contracted with ABB Combustion Engineering Nuclear Power, Windsor, Connecticut, to modify the existing General Electric breaker trucks to accommodate the replacement ABB breakers. The replacement breakers were expected to last 20 years and perform 10,000 open/close cycles. OPPD purchased the modification and replacement ABB breakers as Class 1E, safety-related components. The modification included replacement of the existing General Electric Mechanism operated contact switch linkage arms with a 0.3125-inch nominal diameter, 12-inch long off-set cold rolled carbon steel rod.
The team concluded that the January 25 and February 8, 2007, raw water pump breaker failures were due to inadequate design of the mechanism operated contact switch linkage arm used to adapt the ABB circuit breakers to the General Electric cubicles during the 1995 period. Both linkage arms failed at the same location (Figure 1) due to reverse bending fatigue. The number of operating cycles on the failed breakers were 1222 cycles, for raw water Pump AC-10B, and 1728 cycles for raw water Pump AC-10C. Replacement arms, manufactured to the same design specifications, also failed during laboratory testing due to reverse bending fatigue. The replacement arms, however, underwent additional stress cycles prior to failure. The inspectors concluded that the failure could have been predicted based on the linkage arm geometry, the static and cyclic stress induced on the component by the breaker and mechanism operated contact switch spring, and material used in the construction.
Figure 1 - Failed Mechanism Operated Contact Linkage Arm
The mechanism operated contact switch is aligned with the breaker by a mechanical linkage arm and changes position as the breaker either opens or closes. The mechanism operated contact switch is located in the breaker cubicle and is not physically part of the breaker (Figure 2). The broken linkage arms resulted in failure of the raw water pump discharge valves to open during the pump starting sequence on January 25 and February 8, 2007. The normally-closed discharge valve receives an open signal after the pump breaker closes. The valve open-signal is generated when the mechanism operated contact switch changes position. The broken linkage arm resulted in the failure to complete the circuit.
Figure 2 - Linkage Arm Installed in Modified 4160 Volt Breaker
3. Effectiveness of ABB Breaker Maintenance and Monitoring Programs
The team concluded OPPD maintenance and monitoring programs were sufficient to maintain the ABB breakers within their designed configuration and operating parameters. No linkage arm failures occurred at FCS prior to January 25, 2007.
The modification vendor completed prototype qualification testing of the modified breaker in accordance with ANSI C37.59 prior to installation. Mechanically aged breakers were cycled 2000 times while housed in a switchgear cell. The vendor chose the 2000 cycle test scope to demonstrate mechanical endurance based on the recommended service interval. ABB concluded this testing demonstrated satisfactory performance of the mechanism operated contact switch linkage. The qualification testing did not include operation of the breaker in the "test" configuration.
When the breaker is racked vertically into the switchgear and in the "open" position, the top of the breaker mechanism operated contact switch plunger is spaced about 0.125-inch from the end of the linkage arm. The breaker may be test cycled without engaging 4160 volt power to the load. In this "test" configuration, the breaker is racked down and partially withdrawn from the cubicle. An offset metal "test flag" is installed to bridge the vertical gap between the mechanism operated contact switch plunger bolt on the top of the breaker and the end of the switchgear mechanism operated contact switch linkage arm. Testing conducted by OPPD revealed higher than normal radial and inertial loads on the plunger bolt on top of the breaker when the breaker was cycled when the "test flag" was installed. The radial load, caused by the horizontal offset between the mchanism operated contact switch and the breaker plunger, resulted in frictional loading of the plunger bolt as it was driven through the metal guide bushing at the top of the breaker. The mass of the test flag, which is about the same as that of the moving parts in the mechanism operated contact switch linkage system during normal operation, doubled the normal inertial load. Breaker timing tests demonstrated closure times of about 30 milliseconds and opening times of about 15 milliseconds.
These breaker cycle durations resulted in between 10 and 100 gram acceleration on the breaker plunger, "test flag," and mechanism operated contact switch plunger. The licensee concluded that these inertial loads were significant contributors to the fatigue failure of the linkage arms.
Improperly adjusted test flag linkage may have also contributed to the cycle fatigue of the linkage arms during breaker cycling. Insufficient clearance could result in the test flag exerting excessive force against the mechanism operated contact switch housing and too great of a gap could increase the number of linkage arm stress cycles each time the breaker position was changed. OPPD personnel observed a "jack hammer" affect on the original General Electric "test flags resulting in very noisy operation. The licensee's measurements of the static mechanism operated contact switch spring force supported the conclusion that minor variations in the travel of the mechanism operated contact switch plunger would result in large changes in the compression forces of the mechanism operated contact switch plunger spring and linkage arms. The team concluded that no other breaker maintenance issues were precursors to the failed linkage arms.
After the February 8, 2007, failure, OPPD began informally tracking the operating data (number of cycles) when transferring linkage arms from one breaker to another. The licensee plans to establish a formal tracking method which will be incorporated into station procedures.
4. Root Cause and Extent of Condition Review
On February 24, 2007, OPPD completed a revised operability determination concluding that ABB breaker linkage arms with less than 1000 breaker cycles were operable. The licensee concluded that between 335 and 579 stress cycles had occurred prior to fracture failure of the two broken linkage arms following the onset of initial surface cracking. The licensee based this conclusion on an evaluation of scanning electron microscope images of the fracture surface. Based on an estimated 10 stress cycles per breaker cycle, the licensee concluded that a linkage arm failure would not be expected to occur before 34 breaker cycles following the onset of a surface crack. OPPD examined the linkage arms that had been removed from the remaining two raw water pumps, with 1671 and 1688 breaker cycles each, the auxiliary feedwater breaker linkage arm, with 759 breaker cycles, and the low pressure coolant injection breaker linkage arm, with 755 breaker cycles. Nondestructive examination of these linkage arms indicated that no initial crack formation had occurred at any of the high stress points.
On March 15, 2007, RSI Materials Engineering completed laboratory testing and evaluation of old and newly manufactured linkage arms. The test scope included mechanical fatigue, metallurgical failure analysis, a comparative analysis of replacement rods with existing rods, and Weibull Analysis using existing operational data. The testing confirmed that the failure mechanism was reverse bending fatigue. Based on the Weibull Analysis, OPPD concluded that no linkage arm failures were likely to occur prior to 1000 breaker cycles (97.5 percent confidence).
OPPD concluded the root cause of the linkage arm failures was a less than adequate process for identification of critical interface/operating configurations when specifying the replacement breaker procurement requirements. Specifically, OPPD failed to identify the usage of the "test flag" as a critical constraint during the 1994 breaker modification procurement. The OPPD purchase contract for the breaker modifications required "the circuit breakers, with interrupter, are otherwise identical to and interchangeable with the existing General Electric Magna-Blast air-magnetic circuit breakers." The purchase specification did not include use of the "test flag" or a requirement to perform production testing with the "test flag." Use of the test flag increased the linkage arm stress, aggravating the reverse bending fatigue.
Contributing Causes:
- The linkage arm fabrication process used die-cut rather than machine rolled threads. Use of the die-cut fabrication process increased the effect of high stress loading on the linkage arms
- Use of the "test flags" at FCS was not adequately controlled to ensure that critical gaps are maintained Corrective Actions:
- Evaluation of material change for mechanism operated contact switch linkage arms to obtain reliable operation
- Develop replacement schedule or strategy to address rod replacement needs
- Revise procedure for "test flag" installation
- Provide administrative control for tracking mechanism operated contact switch linkage arms The team concluded OPPD's corrective actions were adequate.
The inspectors evaluated the consequences of a linkage arm failure on other safety-related loads power by the 4160 volt ABB breakers. The mechanism operated contact switch linkage arm was common to all twenty-three 4160 volt ABB safety-related breakers. The raw water pumps were adversely affected because the mechanism operated contact switch was required to open the discharge valve during the start sequence. Failure of the mechanism operated contact switches on the breakers supplying the 4160 volt/480 volt in-house transformers (T1B-3A, T1B-3B, T1B-3C, T1B-4A, T1B-4B and T1B-4C) would trip and lock-out the 480 volt bus feeder breaker.
A reset of the mechanism operated contact switch contacts would then be required to close any of these breakers. The licensee determined the breaker cycle count for each of the six 4160 volt/480 volt in-house transformer breakers was less than 225 cycles. A mechanism operated contact switch failure in the breakers suppling reactor coolant pump motors could result in the failure of the associated undervoltage relays. This could result in a diesel generator overload during a loss of off-site power event. The licensee determined the breaker cycle count for each of the reactor coolant pump motors was 253 cycles or less.
The team verified that OPPD correctly applied Technical Specification requirements associated with the linkage arm failures. The licensee applied the Raw Water Pump Technical Specification Action requirements following the January 25 and February 8, 2007 failures during the pump start sequence. OPPD also declared the two remaining raw water pumps inoperable on February 8, 2007 after discovering the potential for a common mode failure of the breaker linkage arms. The licensee appropriately recognized this as a condition outside of the Technical Specifications, requiring a six-hour reactor shutdown (as described in NRC Event 43157). OPPD subsequently retracted the Event Notification after nondestructive examination of remaining raw water pump breaker linkage arms did not show crack initiation.
The team performed a review of pertinent industry operating experience and potential precursors to the FCS breaker failures, including the effectiveness of licensee actions taken in response to any operating experience. The team concluded no nuclear operating experience related to ABB circuit breaker linkage arm failures was available.
When contacted, ABB responded that no technical bulletins on the modification had been issued. The team has confirmed that no generic issues exist related to the ABB circuit breakers/mechanism operated contact switches. The team has not identified any other safety related uses of the ABB modification or other linkage arm failures.
4OA6 Meetings, Including Exit
On April 9, 2007, the results of this inspection were presented to Mr. David Bannister, Plant Manager, and other members of his staff who acknowledged the findings. The inspectors confirmed that the supporting details in this report contained no proprietary information.
SUPPLEMENTAL INFORMATION
KEY POINTS OF CONTACT
Licensee Personnel
- G. Cavanaugh, Supervisor Regulatory Compliance
- H. Faulhaber, Division Manager, Nuclear Engineering
- J. Herman, Manager, Engineering Programs
- E. Matski, Compliance
- S. Miller, Superintendent, Systems Engineering