05000251/LER-2016-001
| Turkey Point Unit 4 | |
| Event date: | 5-3-2016 |
|---|---|
| Report date: | 6-30-2016 |
| 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 |
| 2512016001R00 - NRC Website | |
comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by internet e-mail to used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection:
DESCRIPTION OF THE EVENT
On May 3, 2016 with Unit 4 in Mode 1 at approximately 100% rated thermal ppwer, Engineering personnel identified the potential past inoperability of Reactor Protection System Overtemperature Delta T (OTDT) and Overpressure Delta T (OPDT) Channel III [JC, CHA]. Corrected coefficients were entered into the Eagle 21 Process Protection Upgrade System for a spare Loop C resistance temperature detector (RTD) [AB, TE] that resulted in a significant change to the setpoint. The correct coefficients were input by instrumentation and control (I&C) personnel on May 2, 2016. Evaluation confirmed that the Channel III setpoint had exceeded the Technical Specification (TS) allowable value as specified in TS 2.2.1, Table 2.2-1, Functional Units (FU) 5 and 6, and was inoperable when required for approximately five days. Incorrect Eagle 21 RTD coefficients were entered for the spare RTD on April 20, 2016 during the Unit 4 refueling/maintenance outage. Unit 4 entered Mode 2 and the mode of applicability of TS 3.3.1, Table 3.3-1 for FU 5 and 6 on April 27, 2016 at approximately 2020 hours0.0234 days <br />0.561 hours <br />0.00334 weeks <br />7.6861e-4 months <br />.
Because the inoperable condition was not recognized at the time, TS Required Action to place the channel in the tripped condition within six hours was not taken. This is a condition prohibited by the TSs and reportable in accordance with 10 CFR 50.73(a)(2)(i)(B).
In addition, during the five day period that OTDT and OPDT Channel III was inoperable another of the three channels was inoperable at times during test/adjustment activities for a cumulative period of approximately four hours. With two channels inoperable, the specified safety function was not met because two operable or operating channels are required to achieve the safety function. This aspect is reportable in accordance with 10 CFR 50.73(a)(2)(v)(A) because of the impact on the reactor trip function to shut down the reactor.
CAUSES OF THE EVENT
The root cause is the absence of a controlled engineering document describing the derivation of RTD coefficient data.
A contributing cause is not implementing the temporary design change process when the Eagle 21 Redundant Sensor Algorithm (RSA) value was temporarily increased from 4 degrees F to 12 degrees F for the reactor startup.
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05000-251
3. LER NUMBER
2016 - 00
ANALYSIS OF THE EVENT
RCS hot leg temperature (Thot) is measured by three RTDs located 120 degrees circumferentially around the RCS pipe in each of the three loops and these indications are averaged. The Thot average is used to determine RCS Delta T and the RCS Ton. The RCS Ton and Delta T calculated values are used in the reactor trip system instrumentation safety functions associated with OTDT and OPDT. The Tavg calculated value is also used in the Engineered Safety Features (ESF) Actuation System associated with Safety Injection, Stearri Line Isolation and ESF interlocks. An evaluation concluded that there was no adverse impact on ESF functions.
The OTDT and OPDT reactor trip function consists of three independent channels with one channel in each of the three RCS Loops. Actuation of a trip signal in two of the three channels is required to trip the reactor. The two-out-of-three trip logic prevents a reactor trip from the spurious actuation of a trip signal from one channel while providing redundancy to accommodate the failure of one channel to generate the required trip signal.
During startup and power ascension from the refueling outage, Engineering personnel noted that the indication from the spare Channel III RTD deviated from the other two RTDs by up to 8 degrees F.
Engineering personnel realized that incorrect coefficients had been entered into Eagle 21 for the spare RTD. RTD cross-calibrations are performed when RCS temperature reaches 250, 350, 450 and 547 degrees F. The cross-calibrations at 250 and 350 degrees F were satisfactory.
When the RTD cross-calibration failed at 450 degrees F on April 25, 2016, it was concluded that it was due to the incorrect coefficients for the spare RTD. It was also assumed that this That input would be removed by the Eagle 21 RSA feature. The RCS hot leg RTDs for a particular channel are averaged and the output is compared to the average of the cold leg RTDs for the respective RCS loop. The signals from the RTDs are checked to see if they deviate from each other by more than a preset amount (DELTAH). The RSA takes the signal furthest from the average and removes it from the calculation before it is used for Delta 17. It was assumed that the DELTAH value was set at 4 degrees F as is normal for steady state power operation, but the setting was not verified. I&C personnel had adjusted the DELTAH value to 12 degrees F during the plant startup via work order instructions. The Eagle 21 RSA did not remove the input from the spare RTD. The change to DELTAH for startup is a temporary configuration change that was not performed in compliance with the temporary configuration change process.
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The correct coefficients for the spare RTD were entered into the Eagle 21 system for Loop C Channel III on May 2, 2016 returning the channel to operable status.
During the approximate five day period of OTDT/OPDT Channel III inoperability, the single failure criterion was not met. During two periods when more than one channel was inoperable (approximate four hours cumulative), the specified safety function was lost.
ANALYSIS OF SAFETY SIGNIFICANCE
During the period that OTDT and OPDT Channel III was inoperable, OTDT and OPDT Channels I and II were operable except for periods when testing of the channels occurred. During the testing periods each channel was placed in the tripped condition except during a few hour period when the channels were being returned to service in accordance with TS 3.0.6. During this short period, the TS safety function provided by OTDT and OPDT reactor trips was not available because two of the three channels were inoperable and the reactor trip logic requires two OTDT or OPDT signals. A bounding risk assessment concluded that for the approximate four hour period that the specified safety function was not available, the core damage probability was well below the NRC acceptance criteria for minimal risk impact. As a result, the safety significance of the event is considered low.
CORRECTIVE ACTIONS
Corrective action is in accordance with condition report AR 2129632 and includes:
1. Revise the RCS RTD replacement procedure to require validation of the correct methodology for deriving RCS RTD coefficients.
2. Establish a controlled calculation that contains the basis and methodology for deriving RCS RTD coefficients.
3. Update the work order process to ensure any changes to the Eagle 21 RSA complies with the temporary configuration change process.
comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by intemet e-mail to used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
ADDITIONAL INFORMATION
component function identifier (if appropriate)].
FAILED COMPONENTS IDENTIFIED: None.
PREVIOUS SIMILAR EVENTS: None.