05000412/LER-2002-003
| Beaver Valley Power Station Unit No. 2 | |
| Event date: | 2-22-2002 |
|---|---|
| Report date: | 09-19-2002 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(i) |
| 4122002003R01 - NRC Website | |
U.S. NUCLEAR REGULATORY COMMISSION
PLANT AND SYSTEM IDENTIFICATION
Westinghouse-Pressurized Water Reactor (PWR) Reactor Trip System Instrumentation (JC)
CONDITIONS PRIOR TO OCCURRENCE
Unit 2: Mode 5 at 0 % power Unit 1: Mode 1 at 100 % power There were no systems, structures, or components that were inoperable at the start of the event that contributed to the event.
DESCRIPTION OF EVENT
Design Engineering was conducting a review of historical performance data for Overtemperature Delta Temperature (OTDT) reactor trip function used in the Reactor Protection System (RPS) at Beaver Valley Power Station (BVPS) Unit No. 2. On February 22, 2002, while the unit was shutdown for a refueling outage, the review revealed that the procedure methodology for lead/lag testing did not provide correct data for time constant measurements. The procedures that calibrate the OTDT Temperature (T)-Average lead/lag cards contained an incorrect test setup caused by a calculation error. These errors caused incorrect OTDT T-Average dynamic compensation in all three OTDT protection channels.
The incorrect OTDT T-Average lead and lag times caused the OTDT time constant Tau 4 (lead time) for all three Reactor Coolant System loops to not comply with the BVPS Unit 2 Technical Specification 3.3.1.1 Reactor Trip System Instrumentation setpoints. BVPS Unit 2 Technical Specification Table 3.3-1 requires a Tau 4 value of greater than or equal to 30 seconds as stated in the Unit 2 Licensing Requirements Manual Section 4.1, Core Operating Limits Report (COLR).
Technical Specification requirements. This discrepancy did not apply to BVPS Unit 1.
As a result of the above finding, an extent of condition review was initiated at BVPS Units 1 and 2 involving RPS dynamic time response testing. During this extent of condition review, an inconsistency was noted between the approved scaling methodology and the test methodology utilized in the maintenance surveillance procedure for the Overpower Delta Temperature (OPDT) channel for BVPS Unit No. 1. Subsequent review by Engineering on March 7, 2002, confirmed that the procedures utilized to calibrate the OPDT T-Average rate/lag modules at BVPS Unit No. 1 contain incorrect test methods and scaling values. This error renders the OPDT T-Average dynamic compensation non-functional for lower T-Average rates of changes (less than approximately 2.5F per second). The BVPS Unit No. 1 Technical Specifications require that the OPDT channel be FACILITY NAME (1) DOCKET (2) LER NUMBER (6) PAGE (3) operated with values as shown in the BVPS Unit 1 COLR. The COLR specified that the Tau 3 time constant used in the OPDT T-Average rate/lag compensator be greater than or equal to 10 seconds.
The incorrect scaling values caused the Tau 3 to not meet the COLR criterion. Therefore, the previously implemented Tau 3 time constant did not comply with BVPS Unit 1 Technical Specification requirements.
The calibration and test during initial plant startup in 1975 of the lead/lag modules in the three OPDT channels at BVPS Unit No. 1 specified a module Bias value of 0.00 VDC. This Bias value was still being utilized in current surveillance procedures when this extent of condition evaluation determined that the Bias value should have been set at 1.111 VDC. This discrepancy did not apply to BVPS Unit 2.
As a result of the continuing extent of condition review on July 23, 2002, with BVPS Unit 2 at 100% power, it was determined that the gain setting for the Tau 7 and K5 constants used in the OPDT reactor protection system channels at BVPS Unit 2 were incorrectly set on all three channels. The Tau 7 used as a rate lag function in the OPDT channels and the K5 used as a gain in the OPDT channels were found to be set at a value slightly less than required by the Licensing Requirements Manual for BVPS Unit 2, which was not in compliance with BVPS Unit 2 Technical Specification requirements. This condition had existed since at least 1998.
REPORTABILITY
BVPS Unit 2 Technical Specification 3.3.1.1 states that the reactor trip system instrumentation channels must be operable as shown in Table 3.3-1. The allowable value for OTDT in Table 3.3-1 is provided in Item 7 and listed in Notation A. Notation A states that Tau 4 is a time constant utilized in the lead-lag compensator for the T-Average circuit and shall be as specified in the COLR. The COLR in the Licensing Requirements Manual (LRM) lists Tau 4 as greater than or equal to 30 seconds. The Tau 4 (lead) time constant that was previously implemented is inconsistent with Technical Specification requirements (in a non-conservative direction). This condition has existed since this value was implemented in August 2001. Because the implemented value of Tau 4 does not meet Technical Specifications requirements, this is a condition prohibited by plant Tech Specs and is reportable pursuant to 10 CFR 50.73 (a)(2)(i)(B). Similarly, BVPS Unit 2 Technical Specification 3.3.1.1 allowable value for OPDT in Table 3.3-1 is provided in Item 8 and listed in Notation B. Notation B states that Tau 7 and K5 are time constants which shall be as specified in the COLR. The values that was previously implemented were not in compliance with the COLR (in a non-conservative direction) and hence not in compliance with Technical Specification requirements.
Because the implemented values of Tau 7 and K5 did not meet Technical Specifications requirements, this was a condition prohibited by plant Tech Specs and is reportable pursuant to 10 CFR 50.73 (a)(2)(i)(B).
BVPS Unit 1 Technical Specification 3.3.1.1 states that the reactor trip system instrumentation channels shall be operable as shown in Table 3.3-1. The allowable value for OPDT in Table 3.3-1 is provided in Item 8 and listed in Notation B. Notation B states that Tau 3 is the time constant utilized FACILITY NAME (1) DOCKET (2) LER NUMBER (6) PAGE (3) in the rate lag compensator for T-Average and shall be as specified in the COLR. The COLR in the LRM listed Tau 3 as greater than or equal to 10 seconds. The Tau 3 time constant that was previously implemented was not in compliance with Technical Specification requirements (in a non- conservative direction). This condition existed since initial plant startup in 1975. Because the implemented value of Tau 3 did not meet Technical Specifications requirements, this was a condition prohibited by plant Tech Specs and is reportable pursuant to 10 CFR 50.73 (a)(2)(i)(B).
CAUSE OF EVENT
In June 2001, due to pending changes in BVPS Unit 2 technical specifications, a procedure engineer drafted a revision to the maintenance surveillance procedures (MSP) involving Reactor Coolant Temperature Loop Delta-T T-Average protection calibrations at BVPS Unit 2. Engineering organizations reviewed these MSPS. Comments were provided and discussed with the originators, which included changes to the OTDT lead/lag test setup. A calculation was developed by the procedure engineer documenting the scaling and number changes to procedure data sheets in accordance with previously discussed comments. In August 2001, during the implementation of the technical specification amendment, Maintenance Engineering performed a final review and approval of the procedures and the Instrumentation and Control personnel performed these procedures.
During the performance of these MSPS, incorrect Tau 4 time constants were installed in all three OTDT loops at BVPS Unit 2 due to an incorrect test setup.
The cause for the Tau 4 discrepancy in the OTDT channels and for the Tau 7/K5 discrepancy in the OPDT channels at BVPS Unit 2 was determined to be a less than adequate method of presentation of written communication. It was determined that the existing program, which controls and documents procedure scaling, is inadequate in that it does not include a basis for every procedure value. The fact that the test setup in obtaining the lag time is not included in a documented procedure basis resulted in a wrong assumed value being used in the procedure time constant evaluation. A contributing cause was less than adequate content of written communications. This was due to the failure of maintenance surveillance procedures to contain the proper value on curves resulting in a wrong assumed value being used in the procedure time constant evaluation.
The cause for the Tau 3 discrepancy at BVPS Unit 1 was determined to be non-existent/inadequate process to translate, verify and review that design requirements are correctly implemented in calibration procedures. The BVPS Unit 1 scaling manual, as issued in 1979, contained the required Bias value for the OPDT dynamic compensation module. However, the scaling manual was considered a scaling guidance document and not a design document. Prior to 1999, no process existed to translate design requirements into procedure scaling values; thereby no process existed to ensure design requirements were installed in the plant. Although a process is in place today, it is inadequate. The present program consists of documents that translate design requirements into procedure values only for design requirement changes that have occurred since the process began in 1999. These documents do not provide a basis or check for every value used in the procedure and did not exist at all prior to 1999. Therefore, no process exists to check that all values in the FACILITY NAME (1) PAGE (3) LER NUMBER (6) DOCKET (2) procedure properly reflect required design values. The non-existent/inadequate process resulted in an incorrect module setting going undetected for many years.
SAFETY IMPLICATIONS
An evaluation was performed on the incorrectly set lead/lag time constants at BVPS Unit 2. The areas that were considered relative to this evaluation are as follows: (1) Non-Loss of Coolant Accident (LOCA) events, (2) Control Systems, (3) LOCA and LOCA-related events, (4) Containment analyses, and (5) Steam Generator Tube Rupture (SGTR) events.
As a result of this review, the only impacted areas of concern relative to the incorrect settings of the lead/lag function were those non-LOCA events that rely on OTDT for protection. Of the current licensing basis analyses for Beaver Valley Unit 2, only the Rod Withdrawal at Power and Loss of Load events credit OTDT for protection. The other remaining non-LOCA events are not impacted by the incorrectly set OTDT Lead/Lag time constants because OTDT is not credited in these analyses.
The Rod Withdrawal at Power and Loss of Load cases are the only events that credit OTDT for Updated Final Safety Analysis Report (UFSAR) remain valid. The erroneous Lead/Lag time constants cause a slightly later reactor trip; but in all cases, the applicable acceptance criteria continue to be met.
Other events such as LOCA, and containment analysis are not impacted, since they do not model the Overtemperature Delta T reactor trip function. The SGTR event models Overtemperature Delta T function, but does not take credit for Reactor Protection System operation. Therefore, there is no impact.
The Overpower Delta-T (OPDT) gain, designated as K5, and a rate lag function, Tau 7, less than Technical Specification requirements are primarily a concern for safety analyses involved with non- LOCA events. K5 and Tau 7 are both part of the Beaver Valley Unit 2 OPDT setpoint equation. In the current non-LOCA safety analyses for Beaver Valley Unit 2, the OPDT trip function is only credited statically, along with the steam system safety valves, such that it defines boundaries for the range of Delta-Temperature versus Tavg conditions. The OTDT function is required to provide protection over this range of conditions. Neither K5 or Tau 7 which are associated only with dynamic effects have any impact on this static calculation. No non-LOCA transient explicitly credits the OPDT trip function for primary protection. Thus, the lower K5 and lower Tau 7 values do not affect the non- LOCA analyses. Non-conservative values of K5 and Tau 7 relative to Technical Specifications would not render the OPDT trip functionally unavailable and has no effect on PRA analyzed core damage frequency.
Based on the information discussed above, it can be concluded that at no time was Beaver Valley Unit 2 operating in an unsafe condition. Therefore, the conclusions presented in the Beaver Valley Unit 2 UFSAR remained valid.
U.S. NUCLEAR REGULATORY COMMISSION The incorrect settings from the OTDT trip setpoints would not render the OTDT trip function unavailable and had no effect on Probabilistic Risk Analysis (PRA) analyzed core damage frequency for BVPS Unit 2.
An evaluation determined that the BVPS Unit No. 1 safety analysis results as described in the current BVPS Unit No. 1 UFSAR would remain valid for a Tau 3 rate/lag value of greater than or equal to zero seconds at least back through 1998. The review evaluated all design basis events including LOCA and non-LOCA design basis events; though the effect on Tau 3 is primarily a concern for the non-LOCA events. Thus, although the maintenance surveillance procedures did not comply with the BVPS Unit 1 Technical Specifications requirements for Tau 3, the plant conditions would have provided equivalent or conservative operation as compared to a Tau 3 setting of zero seconds. Therefore, the BVPS Unit 1 OPDT function would have operated conservatively with respect to current plant safety analyses.
A non-conservative value of Tau 3 for BVPS Unit 1 relative to its Technical Specifications would not render the OPDT trip functionally unavailable and had no effect on PRA analyzed core damage frequency for BVPS Unit 1.
Based on the above, the safety significance of this condition was low.
CORRECTIVE ACTIONS
1. The affected BVPS Unit 2 OTDT maintenance surveillance procedures were revised and the instrument channels were re-calibrated using the correct time constant values prior to returning to Mode 2 from the Unit 2 refueling outage.
2. The Tau 3 time constant used in the BVPS Unit 1 OPDT T-Average rate/lag compensator was revised to a value greater than or equal to 0 seconds in the COLR. An evaluation performed pursuant to 10 CFR 50.59 determined that reducing the Tau 3 required rate/lag value from 10 seconds to greater than or equal to zero seconds would still produce acceptable BVPS Unit No. 1 safety analysis results as described in the BVPS Unit No. 1 Updated Final Safety Analysis Report (UFSAR).
3. The minimum required BVPS Unit 2 Tau 7 and K5 values used in OPDT were immediately revised to a value greater than or equal to zero in the Licensing Requirement Manual when the control room was initially notified of the concern by Engineering. It was determined that reducing the Tau 7 and K5 required values to greater than or equal to zero would still produce acceptable BVPS Unit 2 safety analysis results as described in the BVPS Unit 2 UFSAR since safety analyses do not credit this dynamic function. The existing set points for Tau 7 and K5 then became acceptable.
FACILITY NAME (1) PAGE (3) LER NUMBER (6) DOCKET (2) 4. A review is being performed of the scaling manual of both Units to ensure that they reflect current design requirements and on the maintenance procedure review process to ensure that the values used in the calibration procedures are correct with respect to required design values.
5. An independent team was assembled to review the BVPS OTDT and OPDT calculation/review process. As a result of that team's review, process improvements are being evaluated for the Delta-T calibration procedure, procedure review process, cross discipline review, and development of procedural guidance on the scaling process. The associated calculations for BVPS Unit 2 OPDT were revised.
6. A training needs analysis was performed on process scaling training involving the OTDT and OPDT RPS trip functions for designated engineers, technicians, supervisors and procedure developers to determine if additional training is warranted. Additional corrective actions were initiated based upon this needs analysis.
7. Additional actions have been evaluated which would enhance the RPS surveillance testing program. Background bases documents for RPS OTDT and OPDT values used in maintenance surveillance procedures have been developed. Various enhancements have also been identified for the testing procedures.
8. The extent of condition review on these issues is now complete.
9. A latent issues review will be conducted on the BVPS Unit 1 Reactor Protection System.
Completion of the above and other corrective actions are being tracked through the corrective action program.
PREVIOUS SIMILAR EVENTS
A review of past Beaver Valley Power Station Licensee Event Reports for the last five years found one event involving the Delta-Temperature Reactor Protection System function at BVPS Unit 1 or Unit 2.
Resulting Noncompliance with Tech Spec 3.3.1.1, Table 3.3-1, Action 7, Item a.