05000335/LER-2010-001
| St. Lucie Unit 1 | |
| Event date: | 02-04-2010 |
|---|---|
| Report date: | 12-16-2010 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 3352010001R01 - NRC Website | |
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) St. Lucie Unit 1 05000335
Description of the Event
On October 16, 2008, while Unit 1 was in Mode 1, the CCW system [EIIS:CCL] experienced air intrusion from a containment instrument air compressor [EIIS:CMP].
At the time, operators detected and eliminated the source of the problem with no significant impact during normal plant operation. During this period the CCW system was in an indeterminate operability condition and both trains of CCW should have been declared inoperable and T.S. 3.0.3 entered. Further, Engineering concluded that the amount of air intrusion into the CCW system was an unanalyzed condition and a review of operating alignments indicated that had the design basis accident of a loss of coolant accident (LOCA) concurrent with a loss of offsite power (LOOP) occurred, both trains of CCW could have become inoperable.
Cause of the Event
There were six (,6) root causes associated with the event: 1) The organization failed to demonstrate a commitment to achieving a high level of human performance with nuclear safety as the overriding priority; 2) The organization missed several opportunities to promptly identify, fully analyze and resolve in a timely manner the Unit 1 CCW air intrusion events; 3) Inadequate fleet/site procedures resulted in the failure to recognize the condition and the significance of CCW air intrusion in a timely manner: 4) Management did not effectively implement site and fleet policies and procedures; 5) Less than adequate design of the Containment Instrument Air Compressor System resulted in recurrent CCW air intrusion events; and 6) Less than CCW system air intrusion event in November 2009.
The original CCW design was vulnerable to gas intrusion that could have resulted in a common mode system failure. Gas intrusion was not typically considered with respect to CCW design at the time that St. Lucie Unit 1 was designed as evidenced from its absence from licensing bases documents. Consequently, Unit 1 operating procedures did not provide instructions to detect and mitigate gas intrusion occurrences. The vulnerability to gas intrusion from the Unit 1 containment instrument air compressors was not recognized since the leakage path required the failure of an unloader valve on an idle instrument air compressor. In addition, design deficiencies associated with check and isolation valves in the leak path were not recognized.
Analysis of the Event
The Engineering evaluation of the CCW system indicated that operators detected and eliminated the source of the air ingress prior to the CCW system becoming incapable of supporting normal power operation. Although the conclusion indicates that during this air intrusion event, the CCW system was capable of supporting normal operation, the degree of air ingress resulted in an unanalyzed condition. If a design basis accident LOCA occurred, CCW system realignment would occur automatically. On receipt of the safety injection actuation signal (SIAS) the non-essential header would be isolated from the two separate and redundant essential headers. This would temporarily stop any further air ingress until Operators realigned the non-essential header to the one available CCW essential header to supply cooling to the reactor coolant pump (RCP) seals. The realignment is (assuming a single failure of one essential header) performed early in the emergency operating procedures and would re- initiate air ingress into the CCW system. Assuming operators did not isolate the air FACILITY NAME (1)
DOCKET
ingress source after realignment, this continuous air ingress into the CCW system would at some point result in the inoperability of this train of the CCW system.
During the actual event, CCW main header indicated flow became erratic with both high and low flow instrument swings around a small (-5%) base shift in flow and low flow alarms for the radiation monitors located immediately downstream of the CCW heat exchangers. While there are numerous points within the CCW system for which low flow alarms might have been received, no other flow alarms were logged from any other CCW system location. Evaluation of the CCW system indicated that operators detected and eliminated the source of the air ingress prior to the CCW system becoming incapable of supporting normal power operation. However; as stated above, subsequent realignment of the non-essential header would reintroduce air into the system and would have led to eventual CCW system failure.
Further evaluation by Engineering concluded that the CCW system had already ingested enough air and would have been inoperable during a postulated LOOP/LOCA event, even without the realignment of the non-essential header. Therefore, air intrusion resulting from this event resulted in an unanalyzed condition which could have prevented the fulfillment of a safety function. Consequently 10 CFR 50.73 (a) (2) (ii) (B), 10 CFR 50.73(a) (2) (v) (D), 10 CFR 50.73(a) (2) (vii), and 50.73 (a)(2)(i)(B) require notification of the NRC via a License Event Report (LER).
Analysis of Safety Significance Air intrusion of the amount which occurred during the October 2008 event into the CCW system is an unanalyzed condition. Operators detected and eliminated the source of the air ingress prior to the CCW system becoming incapable of supporting normal power operation. A subsequent engineering evaluation concluded that this air ingress into the CCW system resulted in the inoperability of the Safety-Related function of both trains of CCW. Based upon a significance determination performed for this event, the dominant accident sequence is operators failing to stop the air intrusion prior to CCW failure followed by operators failing to trip the reactor coolant pumps (RCPs) upon a loss of CCW resulting in a RCP seal LOCA. Accordingly, the CCW air intrusion event is considered to have significant safety implications.
Corrective Actions
The corrective actions are tracked by the Site Correction Action Program (CAP). The corrective actions to prevent recurrence are as follows:
1) Implement training for licensed operators on the immediate operability determination (IODs) process; 2) Implement training on the Corrective Action Program; 3) Create St. Lucie site specific procedure for the Condition Identification and Screening Process, which includes qualification, performance measures and expectations for Corrective Action Program Coordinators (CAPC0s), Initial Screening Team (IST) and Management Review Committee (MRC)members; 4) Revise station Correc'tive Action Program Expectations Handbook procedure to include training, qualification, performance measures and expectations for corrective action coordinators (CAPC0s), Initial Screening Team (IST) and management review committee (MRC) nembers.
FACILITY NAME (1)
DOCKET
5) Implement training for licensed operators and engineering personnel on Operational Decision Making process, impact of gas intrusion into safety- significant systems, information and actions on gas intrusion into systems from the NRC 95002 Inspection, impact of gas intrusion into safety-significant systems, and procedure/process changes made as a result of the root cause analysis.
6) Create St. Lucie site specific procedure for Condition Identification and Screening Process and revise Corrective Action Program Expectations Handbook to screen and classify CRs based on risk significance, and specify that station management is notified of high risk issues.
7) Revise the Fleet Operability Determination procedure to include procedural guidance for immediate operability determinations (IODs), significance of operability, and characterization of Risk to improve overall quality of CR Operability/Reportability Screenings.
8) Revise Unit 1 & 2 Component Cooling Water System Off-Normal Operating Procedures to add guidance for air intrusion events.
9) Revise High CCW Surge Tank Level Unit 1 & 2 Annunciator Response Procedures to add guidance for air intrusion events.
10) Implement the appropriate initial and continuing training on Corrective Action Program management alignment.
11) Implement a design change to abandon the containment instrument air compressor system, install a containment mini purge and use outside instrument air supply for containment.
12) Develop and implement a program to address potential failures of safety related (SR) and non-nuclear safety (NNS) systems, structures and component that could affect the design basis functions of safety related (SR) and risk significant (RS) systems.
13) Clarify configuration control expectations in the configuration management procedure for maintenance work activities to assure all alterations within a clearance are approved and documented.
Similar Events This event is not considered a repeat event, however, a similar event subsequently occurred on September 9, 2009, and appropriate actions were taken to address operability. Numerous opportunities to learn from internal and external operating experience (OE) were missed so that the 2008 CCW air intrusion event was not prevented. Missed opportunities resulting from Industry operating experience have been added to lesson plans for Licensed Operator and Non-Licensed Operator Initial and Continuing Training.
Failed Component SE1814A, solenoid Valve, Asco/Auto Switch Co. 8211C13