05000366/LER-2012-001
| Edwin I. Hatch Nuclear Plant | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| Initial Reporting | |
| 3662012001R00 - NRC Website | |
On December 10, 2012, at approximately 1431 EST, with Unit 2 operating at approximately Standby Gas Treatment (SGT) filter train. The 2B SGT train had recently been operating near its upper limit of 4000 cfm. The combination of operating so close to its upper flow limit and the increase in flow due to the removal of the refueling floor hatch, the 2B SGT train exceeded its upper flow limit by approximately 30-50 cfm during the performance of the secondary containment test on November 15, 2012.
When the secondary containment surveillance was completed on November 15, 2012, a timeout was called while the operating crew and engineers reviewed the results of the test.
Since drawdown (dP greater than 0.20 in WG within 120 seconds) was achieved and maintained for one hour with considerable margin, secondary containment was determined to be operable throughout the test. While the flow rate of SGT is required to be between 3000 and 4000 cfm by the Ventilation Testing Program in Technical Specifications 5.5.7, the upper and lower limits were derived from calculations prepared for each of the dual safety functions of SGT. The minimum flow rate ensures drawdown of secondary containment while the upper flow limit ensures holdup and residence time in the treatment train are effective for removal of activated gaseous elemental iodine and methyl iodide in the carbon bed.
The SGT train exceeded its technical specifications upper limit at approximately 20 minutes into the surveillance and fluctuated between 4020 and 4030 cfm during the rest of the surveillance. No release had occurred, but the 2B train of SGT was determined to be inoperable for its residence time requirement and the crew entered a required action statement on November 15, 2012 in accordance with Technical Specification 3.6.4.3. A subsequent apparent cause determination found that HNP currently has no trending or monitoring programs that are capable of determining the point at which normal wear and tear of mechanical seals in the reactor building is likely to cause the SGT train to operate at increased flow rates. In response, an administrative limit of 3900 cfm flow was set on SGT 28 while trending and monitoring were updated. This flow rate bounds secondary containment configurations and ensures the SGT flow remains below the upper Technical Specification flow limit, while ensuring that drawdown can be achieved.
The question of whether or not the "as found" condition of the 2B SGT filter train was a condition that existed prior to its discovery on 11/15/2012 was pursued in order to determine if the condition met NRC reporting requirements. This information was provided 12/10/2012, at which time the determination was made that there was firm evidence that the "as found" condition of the 2B SGT filter train would have automatically operated at greater than the 4,000 cfm Technical Specifications limit. This response established the starting time frame for the 60 day report to the NRC.
REPORTABILITY ANALYSIS AND SAFETY ASSESSMENT
This event is reportable in accordance with Title 10 of the Code of Federal Regulations (CFR), Part 50.73(a)(2)(i)(B) because an event occurred which is prohibited by Technical Specification 3.6.4.3.
review of relevant information (e.g., the equipment history and the cause of failure) to indicate that the discrepancy occurred earlier. Based on this guidance and the discovery that the refueling floor hatch had been removed for 11 days, the determination was made that this "as found" condition is reportable under the requirements of 10CFR50.73(a)(2)(i)(B).
The function of the secondary containment is to contain, dilute, and detain fission products that may leak from primary containment following a Design Basis Accident (DBA). In conjunction with operation of the Standby Gas Treatment (SGT) System and closure of certain valves whose lines penetrate the secondary containment, the secondary containment is designed to reduce the activity level of the fission products prior to release to the environment.
This is necessary to isolate and contain fission products that are released during certain operations that take place inside primary containment, when primary containment is not required to be OPERABLE. It is also necessary for events that take place outside primary containment.
The secondary containment is a structure that completely encloses the primary containment and those components that may be postulated to contain primary system fluid. This structure forms a control volume that serves to detain and dilute the fission products. It is possible for the pressure in the control volume to rise relative to the environmental pressure (e.g., due to pump and motor heat load additions). The secondary containment encompasses three separate zones: the Unit 1 reactor building (Zone I), the Unit 2 reactor building (Zone II), and the common refueling floor (Zone III).
For the secondary containment to be considered OPERABLE, it must have adequate leak tightness to ensure that the required vacuum (0.20 inch of vacuum WG) can be established within 120 seconds and be maintained. The secondary containment boundary required to be OPERABLE is dependent on the operating status of both units, as well as the configuration of doors, hatches, refueling floor plugs, SCIVs, and available flow paths to SGT Systems. The required boundary encompasses the zones which can be postulated to contain fission products from accidents required to be considered for the condition of each unit, and furthermore, must include zones not isolated from the SGT subsystems being credited for meeting LCO 3.6.4.3.
The Unit 1 and Unit 2 SGT Systems automatically start and operate in response to actuation signals indicative of conditions or an accident that could require operation of the system.
Following initiation, all required charcoal filter train fans start. Upon verification that the required subsystems are operating, the redundant required subsystem is normally shut down.
There are two principal accidents for which credit is taken for secondary containment operability. These are a loss of coolant accident (LOCA) and a fuel handling accident inside secondary containment. The secondary containment performs no active function in response to either of these limiting events; however, its leak tightness is required to ensure that the release of radioactive materials from the primary containment is restricted to those leakage paths and associated leakage rates assumed in the accident analysis and that fission products SGT Systems prior to discharge to the environment. Postulated LOCA leakage paths from the primary containment into secondary containment include those into both the reactor building indication that the increase in flow of the 28 SGT filter train did not affect the presumption of operability of secondary containment. The test revealed the degraded condition of the 2B SGT filter train flow, which affected its operability. This "as found" condition had no effect on the ability to maintain the secondary containment structure as the required vacuum was maintained and the secondary containment safety function was preserved. Based on this information this event had no adverse impact on nuclear safety.
CORRECTIVE ACTIONS
The volume damper on the discharge of the 2B SGT filter train fan (2746-0001B) was adjusted to achieve the administrative limit of 3900 cfm. Engineers were directed through the Corrective Action Program to utilize the results from the Operations surveillance procedure to determine the optimal flow rate at which actions are taken'to repair seals and other Conservation measures well before the 2B SGT technical specification upper flow rate limit is approached. These limits were incorporated into plant procedure(s).
ADDITIONAL INFORMATION
Other Systems Affected: None Failed Components Information:
Master Parts List Number: 2T46-D001B EIIS System Code: BH Manufacturer: Farr Company Reportable to Epix: Yes Model Number: N-208-2 Root Cause Code: D Type: Filter Train EIIS Component Code: BLO Manufacturer Code: F030 Commitment Information: This report does not create any new permanent licensing commitments.
PREVIOUS SIMILAR EVENTS
None were identified.
NRC FORM 3436A 004010)