05000529/LER-2002-002
| Palo Verde Nuclear Generating Station Unit 2 | |
| Event date: | 04-18-2002 |
|---|---|
| Report date: | 06-17-2002 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 5292002002R00 - NRC Website | |
1. REPORTING REQUIREMENT(S):
This voluntary LER 529/2002-002-00 is being submitted to report a condition related to human performance that does not meet the reporting threshold of 10CFR50.73 (a) for submitting a LER, but may prove useful and be of generic interest to the nuclear industry.
Less conservative post-testing (versus pre-testing) addressable constant values had been inserted in all four Core Protection Calculators (CPCs) for the uncertainty multipliers for departure from nucleate boiling ratio (DNBR) and local power density (LPD).
2. DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):
There were no other structures, systems, or components that were inoperable that contributed to this condition. There were no failures that rendered a train of a safety system inoperable and no failures of components with multiple functions were involved.
The CPCs consist of four separate, redundant channels. Each channel of CPCs is a computer that continuously calculates thermal conditions and thermal limits. The CPC system is an integral part of the plant protection system (JC), in that it provides two trips to the reactor protection system (JC): DNBR and LPD. The CPCs initiate protective action to ensure specified acceptable fuel design limits (SAFDL) on DNBR and LPD are not exceeded during anticipated operational occurrences. The BERR3 addressable constant may be considered an attendant control that is required for the specified safety function (in this case, LPD — High). Similarly, the BERRI addressable constant may be considered an attendant control that is required for the specified safety function (in this case, DNBR — Low). Although the post-test addressable constant values were not as conservative as the pre-testing values, the CPCs were at all times capable of performing their design function.
3. INITIAL PLANT CONDITIONS:
On April 18, 2002, at approximately 17:49 Mountain Standard Time (MST), Palo Verde Unit 2 was in Mode 1 (POWER OPERATION), at approximately 48 percent rated thermal power, conducting power ascension testing (PAT) following its tenth refueling outage.
There were no major structures, systems, or components that were inoperable at the start of the event that contributed to the event.
4. EVENT DESCRIPTION:
On April 18, 2002, at approximately 17:49 Mountain Standard Time (MST), Palo Verde Unit 2 was in Mode 1, conducting power ascension testing following its tenth refueling outage. It was discovered that the less conservative post-testing (versus pre-testing) addressable constant values had been inserted in all four CPCs for the uncertainty multipliers for DNBR and LPD. At the time of discovery, the control room personnel did not have reasonable assurance that the four CPCs could perform their safety functions with the less conservative values inserted. At 17:59 MST, the control room declared all four channels of CPC INOPERABLE and entered TS LCO 3.0.3 Condition 1. At 18:31 the control room exited LCO 3.0.3 Condition 1. At 18:33 MST, with the correct addressable constant values installed and verified in the fourth CPC, the control room exited LCO 3.3.1 Condition A.
5. ASSESSMENT OF SAFETY CONSEQUENCES:
Technical Specification Limiting Condition for Operation (LCO) 3.3.1, Reactor Protective System (RPS) Instrumentation - Operating, states, in part:
Four RPS trip and bypass removal channels for each Function in Table 3.3.1-1 shall be OPERABLE.
BERRI (uncertainty multiplier for DNBR) impacts DNBR — Low and BERR3 (uncertainty multiplier for LPD) impacts LPD — High. Per TS Table 3.3.1-1, these trip functions are applicable in Modes 1 and 2.
The BERRI and BERR3 values installed in the four Unit 2 CPCs for Unit 2 Cycle 11 (U2C11) did not agree with those specified in the U2C11 Master Setpoints Overall Uncertainty Analysis.
Cycle 11 Master Setpoints Overall Uncertainty Analysis contains two sets of values for BERRI and BERR3. A more conservative "pre-test" value is specified to provide additional margin prior to validation of the uncertainties associated with the U2C11 reload. A less conservative "post-test" value is specified to recover the additional pre- test margin while still ensuring the CPCs are capable of performing their design function. Reload Power Ascension Test (PAT) performs testing during the initial power ascension following a refueling operation to verify core design parameters pursuant to Surveillance Requirements (SR) 3.2.2.1 and 3.3.1.11.
The function of the post-test values is to support the CPC design function of protecting the plant. Installation of the post-test values of BERRI and BERR3 prior to validation of the uncertainties per PAT did not prevent the CPCs from performing their design function.
The specified safety function of the CPCs is to shut down the reactor and maintain it in a safe shutdown condition. The CPCs were always capable of performing their specified safety function during the Cycle 11 PAT.
The event did not result in any challenges to the fission product barriers or result in any release of radioactive materials. Therefore, there were no adverse safety consequences or implications as a result of this event. This event did not adversely affect the safe operation of the plant or health and safety of the public.
The condition would not have prevented the fulfillment of the safety function and did not result in a safety system functional failure as defined by 10CFR50.73(a)(2)(v).
6. CAUSE OF THE EVENT:
Unit 2 Cycle 11 Master Setpoints Overall Uncertainty Analysis Table 4.1 contains two sets of values for BERRI and BERR3. A more conservative pre-test value is specified to provide additional margin prior to validation of the uncertainties associated with the U2C11 reload.
Reactor Engineering personnel selected and inserted the post-test values instead of the intended pre-test values. These less conservative values were independently verified, but the second party verification failed to identify the wrong set of values had been selected.
7. CORRECTIVE ACTIONS:
At the time of discovery that the post-test values were installed in all four CPC channels, Operations declared the CPCs INOPERABLE and suspended the power ascension testing at 48 percent power.
The correct pre-test values for BERRI and BERR3 were installed and verified to be correct in all four CPC channels. Additionally, Reactor Engineering checked and independently verified other addressable CPC/COLSS (Core Operating Limit Supervisory System; EMS - ID) constants were correct in Unit 2. No errors were found. Operations declared the CPCs OPERABLE and recommenced power ascension.
The CPC/COLSS constant were also reviewed and verified to be correct in Units 1 and 3.
8. PREVIOUS SIMILAR EVENTS:
A previous similar event was reported in Unit 3, LER 50-530/2002-001. In this event, the Unit 3 Cycle 10 (U3C10) Master Setpoint Overall Uncertainty Analysis was revised when it was determined that a manual calculation to the U3C10 analysis for two COLSS constants used an incorrect adjustment.
The corrective actions of the manual calculation error reported in Unit 3, LER 50- 530/2002-001 would not have prevented engineering personnel from selecting the data from the incorrect location in the uncertainty analysis table.
9. ADDITIONAL INFORMATION:
None.