05000456/LER-2012-005
| Telephone Number (Include Area Code) | |
| Event date: | 12-13-2012 |
|---|---|
| Report date: | 02-11-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| Initial Reporting | |
| 4562012005R00 - NRC Website | |
A. Plant Operating Conditions Before the Event:
Event Date: December 13, 2012 Unit: 1 MODE: 1 Reactor Power: 100 percent Unit 1 Reactor Coolant System [AB]: Normal operating temperature and pressure No structures, systems or components were inoperable at the start of this event that contributed to the event.
On December 13, 2012, a question was asked regarding the pre-evaluated compensatory action note in the Plant Barrier Impairment Program procedure for the diesel oil storage tank (DOST) room water tight doors. One of the options for an impaired DOST room water tight door was to declare the affected emergency diesel generator (DG) [EK] inoperable and enter the applicable Technical Specification (TS) Limiting Condition for Operation Action Requirement. Because the access door between a unit's two DOST rooms is not water tight, it was questioned whether both DG trains on that unit would be inoperable due to an impaired water tight door.
A review determined that both DOST rooms for the affected unit would be vulnerable to flooding with the water tight door for one DOST room impaired and that the option in the compensatory note should have considered both DG trains to be inoperable. An evaluation of completed plant barrier impairments (PBIs) during the previous three years was performed to determine if the option for declaring one DG inoperable while the corresponding water tight door was impaired was used. One instance was identified where the option may have been used while the DGs were required to be operable. On September 11, 2012 from 0804 hours0.00931 days <br />0.223 hours <br />0.00133 weeks <br />3.05922e-4 months <br /> to 1300 hours0.015 days <br />0.361 hours <br />0.00215 weeks <br />4.9465e-4 months <br />, a PBI was in effect for the water tight door for the Unit 1 Train B DOST (i.e., door SD-192) while the Unit 1 Train B DG was out-of-service for maintenance.
If both the 1B and 1A DGs were considered inoperable during the time the PBI was in effect, on September 11, 2012, at 0804 hours0.00931 days <br />0.223 hours <br />0.00133 weeks <br />3.05922e-4 months <br />, TS 3.8.1, "AC Sources — Operating," Condition F should have been entered, with the associated Required Action to restore one DG to operable status in two hours. At 1004 hours0.0116 days <br />0.279 hours <br />0.00166 weeks <br />3.82022e-4 months <br />, with the Condition F Required Action not met, Condition G should have been entered, with the associated Required Action to be in Mode 3 within six hours (i.e., 1604 hours0.0186 days <br />0.446 hours <br />0.00265 weeks <br />6.10322e-4 months <br />) and in Mode 5 within 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br /> (i.e., 2204 hours0.0255 days <br />0.612 hours <br />0.00364 weeks <br />8.38622e-4 months <br /> on September 12, 2012).
On September 11, 2012, at 1300 hours0.015 days <br />0.361 hours <br />0.00215 weeks <br />4.9465e-4 months <br />, when DOST door SD-192 was restored to a normal condition, TS 3.8.1 Conditions F and G would have been exited. Since TS 3.8.1 Condition G allows six hours to shut down Unit 1 and the unit was in Condition G for less than six hours, there was not a condition prohibited by the plant's TS.
Because using the PBI option on the DOST water tight doors would have rendered both the 1A and 1B DGs inoperable, this event is reportable under 10 CFR 50.73(a)(2)(v)(D), any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident.
A historical review of the procedure guidance for compensatory actions determined that the compensatory actions developed between 1986 and 2012 did not adequately recognize and address the risk of having a single DOST room watertight door impaired. Additionally, there were multiple historical missed opportunities to recognize the risk and develop the appropriate compensatory actions.
B. Description of Event:
C. Cause of Event
NHO FORM 366A (10-2010) Therefore, the most probable cause of the event was determined to be inadequate technical rigor, which led to an improper compensatory measure being included in the Plant Barrier Impairment Program procedure.
This condition had no actual safety consequences impacting plant or public safety.
Because using the P61 option on the DOST water tight doors would have rendered both the 1A and 18 DGs inoperable, this event is considered a condition that could have prevented the fulfillment of a safety function.
From a risk perspective, the loss of function would only occur if the turbine building was flooded to the DOST level through a major circulating water system [KE] line break coincident with other accidents or failures that require emergency diesel generator availability. The probability of a scenario with multiple concurrent initiators or failures is assumed to be low. Furthermore, the risk impact of an impaired DOST water tight door affecting both the 1A and 1B DGs is minimized given the short amount of time spent in the impaired configuration. The potential impact of an impaired flood barrier for either the Unit 1 Train A or Train B DOST room is considered to be of low safety significance due to the low probability associated with multiple concurrent accidents or failures and a limited amount of time in the impaired configuration.
The worst case scenario, (i.e., a complete flooding of the turbine building with a loss of both Unit 1 DGs combined with a Loss of Offsite Power resulting from the trip of the units), represents the equivalent of a station blackout.
Braidwood Station is able to withstand and recover from a station blackout of four hours in accordance with the requirements of Regulatory Guide 1.155, "Station Blackout." In the event of a station blackout, either one of the two Unit 2 DGs can serve as an alternate a-c power source for Unit 1. The alternate a-c power source is available within 10 minutes of the onset of the station blackout event and has sufficient capacity and capability to operate equipment necessary to safely shutdown both Unit 1 and Unit 2 and maintain the units in a safe shutdown condition. During the subject event, both Unit 2 DGs were operational and available to provide an alternate a-c power source to Unit 1.
Corrective actions include revising the option in the compensatory action note for the Plant Barrier Impairment Program procedure for the DOST water tight doors to consider both trains of the affected unit's DGs inoperable (this action is complete).
No previous, similar Licensee Event Reports were identified at the Braidwood Station.
Manufacturer Nomenclature Model Mfg. Part Number N/A N/A N/A N/A
E. Corrective Actions:
F. Previous Occurrences:
G. Component Failure Data:
NHt, FO HNI 366A (10-2010)