05000354/LER-2008-001
| Hope Creek Generating Station | |
| Event date: | |
|---|---|
| Report date: | |
| Initial Reporting | |
| 3542008001R00 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
General Electric— Boiling Water Reactor (BWR/4) Reactor Recirculation System, Motor Generator Set — {AD/MG} *Energy Industry Identification System {EIIS} codes and component function identifier codes appear as {SS/CCC}
IDENTIFICATION OF OCCURRENCE
Event Date/Time: June 17, 2006 Discovery Date/Time: January. 19, 2008 11:52
CONDITIONS PRIOR TO OCCURRENCE
Hope Creek was in Operational Condition 1 with reactor power at 100%. No structures, systems, or components were inoperable that contributed to the event.
- DESCRIPTION OF OCCURRENCE On January 19, 2008, the 18-month Technical Specification surveillance requirement 4.4.1.1.3 was performed to demonstrate operability of the reactor recirculation pump motor set {AD/MG} scoop tube mechanical and electrical stops. The over-speed set points are required to be less than or equal to 109 percent and 107 percent, respectively of rated core flow. Hope Creek sets the stops at the beginning of each cycle following the calibration of the Jet Pump Loop Flow summers. This sequence ensures that accurate flows are used to set the stops. Typically, calibration of the Jet Pump Loop Flow summers is performed two weeks after completion of the initial power ascension following the refueling outage to allow for xenon equilibrium to be reached. The surveillance to demonstrate operability of MG set stops is then performed. Subsequent to RF14, the calibration of the Jet Pump Loop Flow summers was performed on January 17, 2008 followed by the MG stop set surveillance on January 19, 2008. The surveillance was performed within the 18-month surveillance frequency. The position was that no other timing requirements needed to be satisfied. This interpretation was consistent with known General Electric and industry practices.
The "A" and "B" MG set stops were found non-conservatively high ("A" mechanical stop was found at 112.7% and "B" mechanical stop was found at 113.9%). The mechanical stops are important because they are used in the core reload design analysis and in relevant Updated,Final Safety Analysis Report (UFSAR) Chapter 15 (Accident Analysis) events. In accordance with the surveillance procedure, the mechanical and electrical stop set points were adjusted to their proper settings.
An eight-hour notification was made for this event under the provisions of 10CFR50.72(b) (3)(v)(D) "Any event or condition that at the time of discovery could have prevented the fulfillment of the safety function of structures or systems that are needed to: (D) Mitigate the consequences of an accident.
The potential impact of this condition is a function of the reactor recirculation pump increasing flow transients (Reference UFSAR 15.4.5). If the analyzed transients were to have occurred, the potential existed that the Safety Limit Minimum Critical Power Ratio (SLMCPR) or the fuel rod thermal mechanical limits could have been exceeded under the most limiting
- postulated conditions. An evaluation of the potential impact determined that for the time . period (June 17, 2006 to January 19, 2008) when the settings were non-conservative, the recirculation increasing flow transient would not have resulted in any fuel limits being exceeded.
SAFETY CONSEQUENCES
The potential impact of this condition is a function of the reactor recirculation pump increasing flow transients (Reference UFSAR 15.4.5). If those analyzed transients were to have occurred, the potential existed that the.Safety Limit Minimum Critical Power Ratio (SLMCPR) or the fuel rod thermal mechanical limits, respectively assuring avoiding centerline melt and 1% cladding plastic strain, could have been exceeded under the most limiting postulated conditions.
An evaluation of the potential impact of the improper settings determined the recirculation flow increasing transients would not have resulted in any fuel limits being exceeded. The initial core power distribution conditions required at the beginning of a recirculation maximum flow event to ensure no fuel limits are exceeded during the event are enforced by the core flow dependent Minimum Critical Power Ratio (MCPRf) and Linear Heat Generation Rate (LHGRf) power distribution limits. The required MCPRf and LHGRf are selected based upon the Technical Specification MG set scoop tube mechanical stop overspeed setting. The MCPRf and LHGRf were selected based upon the 109% setting. This evaluation determined the MCPRf and LHGRf required for the as-found condition of the settings and compared this requirement to the actual MCPR and LHGR operating conditions during the time period when the settings were non-conservative. The actual MCPR and LHGR operating conditions have been confirmed to have been within the initial conditions required to ensure no fuel limits would have been exceeded if a recirculation increasing flow transient had occurred with the non-conservative settings. Therefore there are no safety consequences to this event.
CAUSE OF OCCURRENCE
An apparent cause evaluation was conducted.
The physical position of the stops did not change between the last performance of the surveillance test (June 17, 2006) and the January 19, 2008 performance of the test. Therefore, the non-conservative as found settings were due to a change in the correlations between the pump speeds and core flow (i.e.
change in Recirculation System hydraulic behavior). The following causes contributed to this occurrence.
® Alteration in Operational Practices in Cycle 14 Involving the Matching of Recirculation Loop Flows.
Prior to � 2006, Hope Creek interpreted "Recirculation Loop Flow" (in TS 3.4.1.3) to refer to "Recirculation, Loop Drive Flow". Following an initial failed attempt in Cycle 14 to set the MG stops, the procedure used to set the stops was revised. As part of the revision process, it was identified that "Recirculation Loop Flow" should have been associated with "Jet Pump Flow" to align with the LOCA analysis. The need to alter operational practices was identified on June 14, 2006. The alteration of the operational practice was initiated at the setting of the MG set stops on June17, 2006. The initial response was to blend the two practices and match both Recirculation Loop Drive Flows and Jet Pump Loop Flows within TS 3.4.1.3 requirements. It was not until early August 2006 that the change was completed and'Jet Pump Loop Flows were being consistently matched by the Operating Shifts.
During this transition period, the correlations began to show the behavior change. Once the Jet Pump Loop Flows were being consistently matched, the correlations stabilized and reflected the-change in Recirculation System hydraulic behavior.
�■9331..121SINIM.� With matching Jet Pump Loop Flows, both Recirculation Loops were capable of pumping the same Recirculation Loop Drive Flows with Significantly lower speeds (i.e. more efficient). This behavior change may have been related to hydraulic behavior in the lower plenum of the reactor vessel between.the Jet Pump discharges and the core inlet. With matching Jet Pump,Loop flows, there is less radial flow between the structures in the lower plenum and possibly a lower backpressure for the Recirculation Loops. The magnitude of the behavior change may have been due to the degree of hydraulic asymmetries between the Recirculation Loops prior to the replacement of the "A" Recirculation Pump in RF14. Based on investigations performed as part of the cause evaluation, the impact of altering the operational practices was not predictable via industry benchmarking or an operating experience search.
- Change in Recirculation System Hydraulic Behavior Not Promptly Detected The change in Recirculation System hydraulic behavior was retrospectively evident in August 2006 when the implementation of the alteration in operational practices was largely completed. However, the Recirculation System trending program did not detect the change. Data was reviewed with a short-term focus. Data changes on a daily basis were interpreted as "noise". The change following the operational practice alteration was not recognized.
0 Significance of Change in Recirculation System Hydraulic Behavior Not Understood The interpretation was, as long as the Recirculation MG Stops were set within the required frequency (at least once per 18 months), no other actions were required. This interpretation was consistent with knOwn General Electric and industry practices.
There is a lack of guidance on when to perform the Technical Specification Surveillance Requirement 4.4.1.1.3 with respect to a refueling outage or plant change potentially impacting the maximum Total Core Flow. There is no guidance on the conditions for trending data or extrapolating future performance.
Improved Standard Technical Specification (NUREG-1433) does not include a similar requirement for .
Recirculation M/G Set Stops so it could not be used to clarify Hope Creek Technical Specificdion Surveillance 4.4.1.1.3.
PREVIOUS OCCURRENCES
A review of previous reportable events at Hope Creek was performed to determine if a similar event had occurred.
10/08/2000 — Hope Creek, LER 2000-010-000 Reactor Recirculation Pump Motor Generator Set Scoop Tube Mechanical and Electrical Stop Over speed Set points Found Outside of Technical Specification Limits. This LER described the "B" recirculation pump motor-generator set stops as being set non conservatively high. Further investigation discovered that an error was made in the calibration of the stops and the Technical Specification required set point values for the stops were never exceeded. The LER was retracted via PSEG Nuclear Letter LRN-01-0027 dated February 7, 2001.
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CORRECTIVE ACTIONS
The mechanical and electrical stop set points were adjusted.to their proper settings.
The Recirculation System trending program will be reviewed to identify any near term changes needed to ensure compliance with TS Surveillance Requirement 4.4.1.1.3. Appropriate changes will be implemented.
The Recirculation System trending program will be benchmarked against industry best practices. Appropriate changes in the program will be implemented.
The Technical Specification BASES will be reviewed. Revisions will be made as appropriate to provide guidance on the performance of, and compliance with TS Surveillance Requirement 4.4.1.1.3.
COMMITMENTS
This LER contains no commitments.