05000530/LER-2002-001
| Palo Verde Nuclear Generating Station Unit 3 | |
| Event date: | 01-16-2002 |
|---|---|
| Report date: | 03-13-2002 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 5302002001R00 - NRC Website | |
1. REPORTING REQUIREMENT(S):
This LER (50-530/2002-001-00) is being submitted pursuant to 10 CFR 50.73(a)(2)(i)(B), to report a condition that was prohibited by the plant's Technical Specifications (TS).
Specifically, Technical Specification (TS) Limiting Condition for Operation (LCO) 3.2.4, Departure From Nucleate Boiling Ratio (DNBR), requirements for operation without the core operating limit supervisory system (COLSS) (EllS:ID) inservice were not complied with.
2. DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):
The core operating limit supervisory system is a digital computer-based on-line monitoring program used to provide information to aid the operator in complying with the technical specification operating limits on rated thermal power (total core power), linear heat rate (LHR), departure from nucleate boiling ratio (DNBR), axial shape index (ASI), and azimuthal power tilt. To do so, COLSS uses measurements of incore detector (EIIS:IG) signals, control element assembly (EIIS:AA) positions and plant thermal/hydraulic properties to determine the core power distribution and thermal performance. The COLSS indicates continuously to the operator, via the power operating limit (POL), how far the core is from the operating limits and provides an audible alarm when an operating limit is exceeded. Such a condition signifies a reduction in the capability of the plant to withstand an anticipated transient, but does not necessarily imply an immediate violation of fuel design limits. No trip functions are provided by this system.
The core protection calculators (CPC) (EIIS:JC) also are capable of verifying that the LHR and DNBR do not exceed their limits. The CPCs perform this function by continuously calculating an actual value of DNBR and local power density for comparison with the respective trip setpoints. If power distribution or other parameters are perturbed, the high local power density or low DNBR trips in the reactor protection system (EIIS:JC) initiate a reactor trip consistent with the safety analyses and UFSAR.
3. INITIAL PLANT CONDITIONS:
On January 16, 2002 Unit 3 was operating in Mode 1, Power Operation, at approximately 99 percent rated thermal power when a problem with the COLSS constants was identified to control room personnel. There were no structures, systems, or components that were inoperable that contributed to the condition.
4. EVENT DESCRIPTION:
On January 16, 2002, during the development of Unit 2 cycle 11 reload products, reload analysis personnel determined that there was an non-conservative error in COLSS DNBR Power Operating Limit (POL) calculation. Calculation RA-03-C10-2001-020 rev 0, Unit 3 Cycle 10 Master Setpoint Overall Uncertainty Analysis, incorrectly established the value for COLSS constants: EPOL2 = 0.03 and EPOL4 = 0.03. This error had been in place since the unit completed its 9th refueling outage on November 5, 2001.
Upon discovery of the error, reactor engineering personnel were notified who in turn notified Unit 3 control room personnel. On January 16, 2002 at 1316 MST, control room personnel declared COLSS inoperable. At 1325 MST control room personnel determined that linear heat rate (LHR) and DNBR were not within specified limits for COLSS out of service and Condition B of LCO 3.2.1 and 3.2.4 was entered. The constants were subsequently updated in COLSS and LCO 3.2.1 and 3.2.4 Condition B was exited at 1352 MST.
5. ASSESSMENT OF SAFETY CONSEQUENCES:
The error resulted in a COLSS DNBR calculated POL that was 4% higher (non-conservative) than it should have been. A review of the Unit 3 Cycle 10 core data determined that the minimum COLSS calculated POL was approximately 109 percent rated thermal power in early November of 2001. Reducing this value by 4% still yields an acceptable margin in COLSS. Therefore, no safety limits were exceeded. It should also be noted that this error did not impact the CPCs ability to generate a reactor trip signal when needed.
No automatically or manually initiated safety system actuations occurred and none were required. The condition would not have prevented the fulfillment of a safety function and the condition did not result in a safety system functional failure as defined by 10C F R50.73(a)(2) (v).
6. CAUSE:
An investigation of this condition is being conducted in accordance with the corrective action program. The direct cause of the error in the EPOL2 and EPOL4 values was an error by the responsible engineer (utility, non-licensed) for the analysis in applying an incorrect correction adjustment in the hand calculation for these parameters in the U3C10 Master Setpoint Overall Uncertainty Analysis (RA-03-C10-2001-020, revision 0). This hand calculation was required because when the automated computer runs were performed the engineer used an incorrect COLSS database file. An experienced engineer applied corrections by hand rather than re-running the computer program, since the calculations to be done were simple. Our human performance error prevention tools failed to prevent an error in choosing an incorrect input constant in this case. The corrective action program has captured this error, and a thorough evaluation will be completed to identify where and why the error prevention barriers failed and to identify corrective actions needed to prevent recurrence.
7. CORRECTIVE ACTIONS:
As a transportability action, Units 1 and 2 were verified to have the correct values installed in COLSS for the EPOL 2 and EPOL4 constants.
In addition, the calculation for Unit 3 Cycle 10 Master Setpoints Overall Uncertainty Analysis (RA-03-C10-2001-020) was revised to correct the values for the EPOL2 and EPOL4 constants.
Upon completion of the investigation, other corrective action(s) will be implemented.
8. PREVIOUS SIMILAR EVENTS:
PVNGS has not reported any similar conditions in the last three years in which an incorrect value for a constant was installed in the COLSS or the CPCs that resulted in a TS violation.
9. ADDITIONAL INFORMATION:
None