05000272/LER-2011-005
| Salem Generating Station - Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2722011005R00 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
Westinghouse — Pressurized Water Reactor (PWR/4) Reactor. Monitoring System {IG/-} * Energy Industry Identification System {EIIS} codes and component function identifier codes appear as {SS/CCC}
IDENTIFICATION OF OCCURRENCE
Event Date: November 24, 2011 Discovery Date: December 11, 2011
CONDITIONS PRIOR TO OCCURRENCE
Salem Unit 1 was in Operational Mode 1 at 55% Reactor Power prior to the November 24, 2011 occurrence and at 100% reactor power on December 11, 2011 at the time of discovery.
No additional structures, systems or components were inoperable at the time of the discovery that contributed to the event.
DESCRIPTION OF OCCURRENCE
On November 23, 2011 full power intermediate range trip setpoints {IG/-} were calculated in accordance with the Intermediate Range (IR) Nuclear Instrumentation System (NIS) Trip Setpoint Evaluation and Determination procedure. This activity is completed with the reactor at approximately 47% Rated Thermal Power (RTP).
Power ascension was placed on hold for data collection and for reactor engineering to perform a flux map.
On November 24, 2011, MC technicians installed the IR NIS trip setpoints. Prior to the installation of the new trip setpoints, the IR High Level Trip and Rod Stop bistables were in the tripped condition, as expected. Once the new setpoints were installed, the IR High Level Trips and Rod Stop bistables were not tripped on both 1N35 and 1N36 channels. This was not expected for the current reactor power of 55% since the nominal High Level trip should correspond to 25% RTP. The fact that the bistables were not in the tripped condition after the new setpoints were installed was not immediately identified by the plant staff.
On December 11, 2011, the Unit 1 reactor operator observed that the IR NIS trip bistables were not in the tripped condition, as expected, and questioned the current configuration. Technical Specification (TS) 3.0.3 was entered for failure to meet TS 3.3.1.1 requirements for the minimum channels operable. IR trip setpoints were recalculated and installed for 1N35 and 1N36. The 1N35 channel was declared operable at 1628 hours0.0188 days <br />0.452 hours <br />0.00269 weeks <br />6.19454e-4 months <br /> and TS 3.0.3 was exited. The channel 1N36 was declared operable at 1733 hours0.0201 days <br />0.481 hours <br />0.00287 weeks <br />6.594065e-4 months <br />.
CAUSE OF OCCURRENCE
The cause was the failure to accurately record the input data (IR currents) used to calculate the IR trip setpoints.
This resulted in the miscalculation of the IR trip setpoints and subsequent installation of incorrect setpoints on both of the IR NIS channels.
PREVIOUS OCCURRENCES
A review of LERs for the previous three years did not identify any previous similar events.
SAFETY CONSEQUENCES AND IMPLICATIONS
There was no actual safety consequence associated with this event.
The intermediate range (IR) NIS detectors 1N35 and 1N36 are designed for multiple decade use and have a compensated ion chamber. The detector current is a function of the detector characteristics and the neutron and gamma flux leakage from the core, which is proportional to reactor power. The primary function of the IR NIS detectors is to provide a relative indication of reactor power when transitioning from the source range through the power range. The IR NIS channels provide a High Flux Reactor Trip at an IR Detector current equivalent to 25% trip is required by Technical Specifications, but no credit is taken for this trip in the safety analysis.
Technical Specification (TS) Table 3.3-1, Reactor Trip System Instrumentation, requires that both trains of IR NIS detectors be operable while in Modes 1 and 2 and when the reactor trip system breakers are in the closed position and the control rod drive system is capable of rod withdrawal.
The intermediate range trip at 25% power is intended to mitigate the consequences of a rod cluster control assembly (RCCA) bank withdrawal accident from sub-critical conditions, a boron dilution event, or a rod ejection accident.
Chapter 15, Accident Analysis, of the UFSAR credits the NIS Power Range (PR) low setting trip for protection in the event of a RCCA bank rod withdrawal. The boron dilution event analysis credits Over Temperature Delta Temperature (OTDT) for protection. Rod Ejection event analysis credits the PR high and low range trips.
Though the IR NIS detectors provide backup protection for the accident scenarios and are currently required by TS, no credit is taken for their use in ensuring shutdown of the reactor and maintaining it in a safe condition. Inoperability of both IR NIS detectors would not be safety significant as they serve as backup to the PR NIS and OTDT systems.
CORRECTIVE ACTIONS
1. New IR NIS trip setpoints were calculated and installed for 1N35 and 1N36.
2. Procedures were enhanced to require a Concurrent Verification
COMMITMENTS
No commitments are made in this LER.