05000315/LER-2015-004
| Donald C. Cook Nuclear Plant Unit 1 | |
| Event date: | 08-26-2015 |
|---|---|
| Report date: | 10-23-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| LER closed by | |
| IR 05000315/2017001 (1 May 2017) | |
| 3152015004R00 - NRC Website | |
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INTRODUCTION
On August 26, 2015, with both Unit 1 and Unit 2 operating in Mode 1 at 100% power, a self-assessment was conducted identifying that during a dual unit Loss of Offsite Power (LOOP), one of three Pressurizer Power Operated Relief Valves (PORVs)[AB][RV] would be rendered inoperable with the loss of power to the associated Control Air Compressor (CAC)[LD][CMP]. Two of three PORVs are equipped with back up air and thus would not be rendered inoperable during a LOOP. Subsequently it was recognized that when the CAC is rendered unavailable for any reason, the associated PORV without back up air would also be rendered inoperable.
An evaluation concluded that the Unit 1 and Unit 2 CACs were rendered unavailable when removed from service for maintenance on multiple occasions over the last three years. For each occasion identified, Unit 1 or Unit 2 Limiting Condition for Operation (LCO) 3.4.11 requirements were not met and is being reported pursuant to 10 CFR 50.73(a)(2)(i)(B), Operation or Condition Prohibited by Technical Specifications.
EVENT DESCRIPTION
It was determined that on three occasions in Unit 1 and four occasions in Unit 2, the associated unit's CAC was made unavailable and LCO 3.4.11 requirements were not met. Event dates and durations were obtained for the CACs for the last three years.
Events Occurrences with Unit 1 CAC rendered unavailable for greater than allowable action time for LCO 3.4.11.
February 27, 2013 — Unit 1 CAC was removed from service for scheduled maintenance, duration 15.87 hours0.00101 days <br />0.0242 hours <br />1.438492e-4 weeks <br />3.31035e-5 months <br />.
November 8, 2014 — Unit 1 CAC was rendered unavailable for emergent maintenance, duration 24.95 hours0.0011 days <br />0.0264 hours <br />1.570767e-4 weeks <br />3.61475e-5 months <br />.
May 6, 2015 — Unit 1 CAC was removed from service for scheduled maintenance, duration 38.67 hours7.75463e-4 days <br />0.0186 hours <br />1.107804e-4 weeks <br />2.54935e-5 months <br />.
Occurrences with Unit 2 CAC rendered unavailable for greater than allowable action time for LCO 3.4.11.
May 14, 2013 — Unit 2 CAC was removed from service for scheduled maintenance, duration 12.37 hours4.282407e-4 days <br />0.0103 hours <br />6.117725e-5 weeks <br />1.40785e-5 months <br />.
January 16, 2014 — Unit 2 CAC was removed from service for scheduled maintenance, duration 13.63 hours7.291667e-4 days <br />0.0175 hours <br />1.041667e-4 weeks <br />2.39715e-5 months <br />.
March 24, 2015 — Unit 2 CAC was removed from service for scheduled maintenance, duration 9.45 hours5.208333e-4 days <br />0.0125 hours <br />7.440476e-5 weeks <br />1.71225e-5 months <br />.
May 4, 2015.— Unit 2 CAC was removed from service for scheduled maintenance, duration 9.00 hours0 days <br />0 hours <br />0 weeks <br />0 months <br />.
EVENT ANALYSIS
The Pressurizer PORVs are credited in the mitigation of a Steam Generator Tube Rupture event. Two of three Pressurizer PORVs are equipped with backup air systems; therefore one of the Pressurizer PORVs is not. The normal supply of air to the PORVs is the Plant Air Compressor (PAC)[LD][CMP]. When the PAC is not available, the CAC supplies air to the PORVs. For the CAC to be considered operable/functional, power shall be available from its normal reserve feed supply [EK] or from its emergency CD emergency diesel generator [EK][DG] supply. Therefore, during a dual unit LOOP concurrent with unavailability of the CAC, the PORV without a back-up air supply should be considered inoperable. This applies when the associated unit's CAC is out-of-service for any reason.
For LCO 3.4.11, each PORV and associated block valve shall be operable in modes 1, 2, and 3 with one or more PORVs inoperable and not capable of being manually cycled. Condition B requires closure of the associated block valve and for power to be removed from the associated block valve within one hour. If condition B is not met then condition H requires the unit to be in mode 3 within six hours. Therefore, each occasion when the CAC was unavailable and condition B and H were not met, is an "Operation or Condition Prohibited by Technical Specifications.
ASSESSMENT OF SAFETY CONSEQUENCES
NUCLEAR SAFETY
The Safety Significance of the CAC resulting in inoperability of a PORV is low, because this dependency is already accounted for in the baseline probabilistic risk assessment (PRA) model, including the consequences of a dual unit LOOP. Average unavailability of the CAC is also accounted for in the baseline PRA model. The risk associated with work on the CAC is managed under the Configuration Risk Management program and is considered a small risk for the durations identified. Thus, there is no significant change in risk associated with the identified condition.
INDUSTRIAL SAFETY
There was no actual or potential industrial safety hazard resulting from the inoperable CACs.
RADIOLOGICAL SAFETY
There was no actual or potential radiological safety hazard resulting from the inoperable CACs.
CAUSE
This scenario was not previously recognized as a condition that would affect the operability of the Pressurizer PORV (the PORV without a source of back up air), thus conditions for LCO 3.4.11 were never entered when the associated CAC was rendered unavailable.
CORRECTIVE ACTIONS
Immediate Corrective Actions Taken Temporary compensatory actions were established to declare the associated PORV inoperable when the CAC is taken out of service for any reason.
Corrective Actions to Preclude Repetition Revise operating and on-line risk management procedures to add a precaution to declare associated PORV inoperable when the CAC is made unavailable.
Evaluate the necessity for a plant modification to add back up air to the remaining PORV.
There are no further planned corrective actions.
PREVIOUS SIMILAR EVENTS
A review of the past three years Licensee Event Reports identified no similar events.