05000440/LER-2012-001
| Perry Nuclear Power Plant | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 47710 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation |
| 4402012001R00 - NRC Website | |
Energy Industry Identification System (EllS) codes are identified in the text as [XX].
INTRODUCTION
On March 1, 2012, at 0224 hours0.00259 days <br />0.0622 hours <br />3.703704e-4 weeks <br />8.5232e-5 months <br />, with the plant operating at 100 percent rated thermal power, a turbine generator [TG] runback occurred while restoring from the calibration of the generator stator water cooling [TJ] inlet pressure gauge. A low pressure condition was created in a sensing line common to the gauge and associated pressure switch when a technician opened the gauge isolation valve.
Operators manually initiated a reactor protection system (RPS) [JC] actuation in response to the turbine generator runback. This event was reportable in accordance with 10 CFR 50.72(b)(2)(iv)(B) as any event or condition that results in actuation of the reactor protection system when the reactor is critical.
At 0551 hours0.00638 days <br />0.153 hours <br />9.11045e-4 weeks <br />2.096555e-4 months <br />, the required notification was made to the NRC Operations Center (ENS Number 47710).
This event is being reported in accordance with 10 CFR 50.73(a)(2)(iv)(A) as any event or condition that resulted in a manual or automatic actuation of the RPS.
EVENT DESCRIPTION
On March 1, 2012, at 0100 hours0.00116 days <br />0.0278 hours <br />1.653439e-4 weeks <br />3.805e-5 months <br />, technicians commenced calibration of the generator stator water cooling inlet pressure gauge. The technician closed the isolation valve to the pressure gauge and then performed the gauge calibration. At 0221 hours0.00256 days <br />0.0614 hours <br />3.654101e-4 weeks <br />8.40905e-5 months <br />, as the technicians were opening the gauge isolation valve, the low pressure and generator protection circuit actuation alarm was observed on a local panel and a turbine generator runback occurred. Turbine bypass valves began to open and Operators manually initiated the RPS in accordance with plant procedures.
The result of this event was an unplanned manual RPS actuation with all rods in verified. No Emergency Core Cooling systems (ECCS) were required to maintain level. The manual RPS actuation did not result in any significant equipment failures or issues.
CAUSE OF EVENT
Two root causes were identified for this event:
1. Inadequate updating of the system operating instruction (S01) resulted in incorrect technical information in the procedure. A design change was implemented during refuel outage 13 that affected the normal pressure indication at the generator stator water cooling inlet pressure gauge. The SOI was not revised to reflect the new value. The pressure gauge was indicating the correct pressure but plant personnel validated the pressure reading via the SOI and believed it was incorrect based on the outdated information in the SOI. The calibration that resulted in the manual RPS actuation was actually not required to be performed.
2. Less than adequate site implementation of the risk management process resulted in the execution of a maintenance activity online with an unacceptable nuclear safety and generation risk.
EVENT ANALYSIS
On May 31, 2011, a generator stator rewind modification was implemented. As a result, new system setpoints were determined and the new normal reading for the value indicated at the generator stator water cooling inlet pressure gauge was changed. The SOI was not revised to reflect this change.
On July 4, 2011, a low pressure reading was identified at the generator stator water cooling inlet pressure gauge. The condition was validated by comparison with the incorrect value referenced in the SOI. Documents were generated to facilitate calibration of the gauge in accordance with the work management process. The gauge calibration was determined to be a green risk activity to be worked online.
On March 1, 2012, at 0100 hours0.00116 days <br />0.0278 hours <br />1.653439e-4 weeks <br />3.805e-5 months <br />, technicians commenced calibration of the generator stator water cooling inlet pressure gauge. A low pressure condition was created in a sensing line common to the gauge and associated pressure switch when a technician opened the gauge isolation valve. The low pressure condition resulted in pressure switch actuation on low pressure and caused a turbine generator runback. Turbine bypass valves began to open and a manual RPS actuation was initiated at 0224 hours0.00259 days <br />0.0622 hours <br />3.703704e-4 weeks <br />8.5232e-5 months <br /> in accordance with plant procedures.
The manual RPS actuation was uncomplicated in that all control rods fully inserted, reactor coolant level and pressure were maintained within expected parameters, and no ECCS systems actuated in response. The RPS functioned as designed.
A Probabilistic Risk Assessment (PRA) evaluation was performed for the March 1, 2012, manual RPS actuation. The analysis indicates that the manual RPS actuation event delta core damage frequency (CDF) of 4.3E-08/yr is well below the acceptable threshold of 1.0E-06/yr as discussed in Regulatory Guide 1.174. Since core damage is required to obtain a large release, the Large Early Release Frequency (LERF) must be below CDF and is therefore below the threshold of 1.0E-07/yr in Regulatory Guide 1.174. The risk of this event is therefore considered to be small in accordance with the regulatory guidance.
CORRECTIVE ACTIONS
An interim order review process has been implemented to ensure work being performed online has an acceptable level of risk. This process will remain in place until training on risk assessment is complete.
Troubleshooting was completed to recreate the low pressure condition and validated the direct cause of the turbine generator runback.
The SOI has been revised to reflect the correct generator stator water cooling inlet pressure value.
The required operating condition for future gauge calibrations will be changed to outage.
An inspection of the gauge snubber will be performed to determine if wear or fouling is present and may have contributed to the pressure response during the gauge calibration.
Additional procedure changes will be implemented and training will be provided to applicable plant personnel with regard to the lessons learned from this event, including risk assessment awareness.
PREVIOUS SIMILAR EVENTS
A review of Licensee Event Reports and the corrective action database for the past three years determined that one similar event had occurred.
documents an event where operators manually initiated an RPS actuation. The RPS actuation was required due to a failed master trip unit causing an invalid division 2 loss of coolant accident initiation signal.
This event was a result of equipment failure, not a maintenance activity. Therefore, the corrective actions associated with this event would not have reasonably been expected to have prevented the event documented in LER 2012-001.
COMMITMENTS
There are no regulatory commitments contained in this report. Actions described in this document represent intended or planned actions, are described for the NRC's information, and are not regulatory commitments.