05000483/LER-2002-005
| Callaway Plant Unit 1 | |
| Event date: | 2-28-2002 |
|---|---|
| Report date: | 4-12-2002 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 4832002005R00 - NRC Website | |
This event is reportable under: (1) 10CFR50.73(a)(2)(i)(B), a condition prohibited by Technical Specifications, (2) 10CFR50.73(a)(2)(ii)(B), an unanalyzed condition, and (3) I OCFR50.73(a)(2)(v)(A), a condition that could have prevented fulfillment of a Safety Function to shutdown the reactor and maintain it in a safe shutdown condition.
B. PLANT OPERATING CONDITIONS PRIOR TO THE EVENT
On 2/28/02, Callaway Plant was in Mode 1 at 100 percent reactor power.
C. STATUS OF STRUCTURES, SYSTEMS OR COMPONENTS THAT WERE INOPERABLE AT THE START
OF THE EVENT AND THAT CONTRIBUTED TO THE EVENT
After it was determined that the Westinghouse NSAL letter was applicable to Callaway, this rendered the portion of the Reactor Protection System responsible for a S/G Low-Low Level Trip, Inoperable.
D. NARATIVE SUMMARY OF THE EVENT, INCLUDING DATES AND APPROXIMATE TIMES
On 2/18/02, Callaway Plant received Westinghouse Letter SCP-02-14, which transmitted Nuclear Safety Advisory Letter (NSAL) 02-03. This NSAL addressed an error in the Westinghouse Steam Generator (S/G) water level setpoint analysis in which the pressure drop across a mid-deck plate internal to the S/G separator assembly was not accounted for in analysis calculations. This pressure drop adversely affected S/G low-low setpoint uncertainty calculations. Based upon a review of information provided by Westinghouse and Operating Event 13294 from Diablo Canyon regarding S/G Narrow Range Low-Low Level trips not responding as expected, an assessment was performed of the S/G Narrow Range (NR) Low-Low Level trip setpoints at Callaway.
On 2/28/02, this assessment determined that the Low-Low Level trip setpoint of 14.8 percent did not account for uncertainties associated with the differential pressure (dP) created by the steam flow past the mid-deck plate in the moisture separator section of the S/G. This dP phenomena would cause the S/G NR level channels to read higher than actual water level at high steam flow rates. Callaway staff engineers determined that this situation rendered the S/G low-low setpoints nonconservative. A decision was made to reduce reactor power to approximately 30 percent where engineering calculations indicated that the S/G mid-deck plate dP condition no longer resulted in non- conservative setpoints and then to adjust the S/G Low-Low Trip setpoints. The setpoints were reduced to 21.6 percent narrow range level for normal containment environment and 27 percent narrow range level for adverse containment environment, which would satisfy the safety analysis requirements. After the new S/G Narrow Range Low-Low Level Trip setpoints values were installed, reactor power was returned to 100 percent.
Additional corrective actions included revision of plant procedures utilizing the S/G Low-Low Trip values.
E. METHOD OF DISCOVERY OF EACH COMPONENT, SYSTEM FAILURE, OR PROCEDURAL ERROR
This problem was discovered after receiving Westinghouse NSAL 02-03.
EVENT DRIVEN INFORMATION
A. SAFETY SYSTEMS THAT RESPONDED
Not applicable.
B. DURATION OF SAFETY SYSTEM INOPERABILITY
The RPS S/G Narrow Range Low-Low-Level Trip function was declared inoperable from1625, 2/28/02, until 1354, 3/1/02 when all setpoints had been revised. The total inoperable time span was 21 hours2.430556e-4 days <br />0.00583 hours <br />3.472222e-5 weeks <br />7.9905e-6 months <br />, 29 minutes.
C. SAFETY CONSEQUENCES AND IMPLICATIONS OF THE EVENT.
The increase in Core Damage Frequency (delta CDF) determined for this issue, using values from the Callaway Probabilistic Risk Assessment (PRA) and conservative assumptions, is 4.4 E-7 per year.
III. CAUSE OF THE EVENT
The cause of the event was an error in design calculations as documented in Westinghouse NSAL 02-03.
IV. CORRECTIVE ACTIONS
Corrective actions consist of adjusting the S/G Narrow Range Low-Low Level Trip setpoints to new corrected values, revision of plant procedures that reference or utilize this setpoint, and revising plant specific calculations as necessary.
Additionally, a revision to Callaway Plant Technical Specifications and Bases is being pursued.
V. PREVIOUS SIMILAR EVENTS
A search of Callaway Action Request System documents from 1999 until present, yeilded 5 separate documents addressing NSAL issues involving S/G level accuracy concerns. They were CARs 200200503, 200201103, 200201157, 200201170, and 200201328. A similar review was conducted of LERs written at Callaway for the time frame of 1999 until present and there were no LERs involving NSAL S/G level accuracy issues.
VI. ADDITIONAL INFORMATION
The system and component codes listed below are from the IEEE Standard 805-1984 and IEEE Standard 803A-1984 respectively.
System: JG Component: 94 RLY