05000341/LER-2015-001
| Fenni2 | |
| Event date: | |
|---|---|
| Report date: | |
| 3412015001R01 - NRC Website | |
Initial Plant Conditions:
Mode Reactor Power .100 percent
Description of the Event:
At 0303 EST on February 1.9, 2015, Fermi 2 experienced a trip of the non-safety related Reactor Building Ventilation (RBHVAC) VA during normal steady-state operations due to valid actuation of a freeze protection device. At the time of the trip, site area air temperature was -1 degree Fahrenheit. The plant Technical Specifications (TS) require that Secondary Containment NG vacuum be maintained greater than or equal to -0.125 inches of water gauge. As a result of the RBHVAC trip, this specification was not maintained for approximately 4 seconds until the safety-related Standby Gas Treatment (SGT) system VA restored Secondary Containment vacuum to an acceptable value.
At 0324 EST, the tripped "W Freeze-Stat was reset.
At 0335 EST, the West RBHVAC supply and exhaust fans were restored to operation.
At 0450 EST, the Division 1 SGTS was secured. As a result, Secondary Containment vacuum again degraded to less than -0.125 inches of water gauge for approximately 38 seconds until the SGTS was promptly re-actuated.
At 0458 EST, the East RBI-1VAC supply and exhaust fans were restored to operation.
At 0502 EST, the Division I SGTS was secured.
In both instances, the lowest observed vacuum was -0.11 inches of water gauge. There were no radiological releases associated with this event.
Safety Consequences and Implications:
There were none. At no time during this event was there a potential for endangering the public health and safety.
The specified safety function of the Secondary Containment is to contain, dilute, and hold up fission products that may leak from primary containment following a Design Basis Accident (DBA), In conjunction with operation of the Standby Gas Treatment System (SGTS) 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 and 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, or that take place outside primary containment. 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.). To prevent ground level exfiltration while allowing the secondary containment to be designed as a conventional structure, the secondary containment requires support systems to maintain the control volume pressure at less than the external pressure. For the secondary containment to be considered OPERABLE, it must have adequate leak tightness to ensure that the required vacuum can be established and maintained.
(continued on next page) (continued from previous page) During this particular event, the Secondary Containment vacuum degraded to a lowest value of -0.11 inches of water gauge for a maximum of 38 seconds. In Chapter 15 of the Fermi 2 UFSAR, RBHVAC is assumed lost at the onset of a LOCA concurrent with a Loss of Offsite Power. As a result, calculations show that the Secondary Containment would be pressurized until the SGTS restores vacuum. For this particular event, the Secondary Containment vacuum degraded when the non-safety related RBHIVAC system tripped due to freeze protection. The structural integrity (i.e., leak tightness) of the Secondary Containment was re-confirmed when the safety related Division 1 SGTS restored vacuum to greater than -0.125 inches of water gauge.
If the DBA LOCA for Secondary Containment concurrent with a Loss of Offsite Power had occurred during the time when Secondary Containment vacuum was between -0.11 and -0.125 inches of water gauge, the Secondary Containment was sufficiently leak tight such that the SGTS would still have established and maintained vacuum greater than the TS required value.
The radiological consequences of the DBA LOCA for Secondary Containment contained in Chapter 15 of the Fermi 2 UFSAR result in doses that are below 10CFR50.67, The Secondary Containment is assumed to be at a vacuum of -0.125 inches at the onset of the LOCA. For this particular event, had the DBA LOCA for Secondary Containment actually occurred, the increase in magnitude of radiological dose as a result of increased draw-down time from starting at -0.11 vice -0.125 inches of water gauge, would be minimal and negated by several very conservative assumptions in the existing analysis (e.g., 100% exfiitration from Secondary Containment during the first 15 minutes of drawdown with SGT in operation, 10% exfiltration from Secondary Containment with SGT in operation throughout the remaining 30 day duration of the accident, no holdup time in Secondary Containment throughout the 30 day duration of the accident, and all exfiltration and filtered releases are at ground level). These conservative assumptions are not reflective of actual plant conditions and configuration. This qualitative evaluation performed by the Fermi 2 Licensing and Engineering staff concludes that no actual loss of safety function occurred. This LER is required because the reporting threshold is "could have" prevented fulfillment of a safety function, which was valid at the time that Secondary Containment was declared INOPERABLE.
Cause of the Event:
The RBHVAC system has 14 steam-supplied heating coils HCL and 14 corresponding low temperature switches (Freeze-Stat) TS, The switches monitor the temperature of the air downstream of the heating coils to ensure the RBHVAC system does not freeze. If any single switch trips on low temperature, the RBHVAC system automatically trips. In this event, the "H" Freeze-Stat tripped on low temperature (35°F decreasing) and caused a trip of the RBHVAC system. A subsequent investigation concluded that the material properties of the "H" heating coil deviated from the original design and resulted in a lower rate of heat transfer. This particular "H" heating coil was installed in November 2011 and is made of aluminum. The original "H" heating coil was pure copper, as are the other 13 heating coils currently installed. The reduced thermal conductivity of the aluminum resulted in a reduced RBHVAC air temperature downstream of the "H" coil and actuated the "H" Freeze-State as designed. The Fermi 2 staff have documented the investigation and corrective actions in CARD 15-21350,
Corrective Actions:
A work order has been generated to replace the "H" heating coil with original design specification material. The work order is scheduled to be completed before the next winter season.
Previous Occurrences:
There have been 5 occurrences of Freeze-Stat actuations in the RBHVAC system between November 2014 and March 2015. This event is the only one that resulted in declaring the Secondary Containment INOPERABLE.
from a failed steam trap,