05000423/LER-2002-003
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| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 4232002003R01 - NRC Website | |
1. Event Description On April 26, 2002, with the Unit in Mode 1 at 100% power, it was determined that emergency operating procedure (EOP) directed actions did not provide adequate assurance that power operated relief valve (PORV) [PCV] function would be disabled within a timeframe that would prevent a fire-induced spurious PORV actuation from depressurizing the reactor coolant system (RCS) [AB]. During subsequent review, on August 22, 2002, with the unit in Mode 1 at 95% power, it was determined that the assumed timeframe for mitigation of fire-induced spurious opening of a main steam [SB] atmospheric dump valve (ADV) bypass valve would not maintain pressurizer [PZR] within acceptable levels as required by fire safe shutdown design basis. A plant response to spurious operation of a PORV is similar to a small break loss of coolant accident and spurious operation of the ADV bypass valve is similar to an excess steam demand event. The -ability to effectively diagnose and manage events like these coincident with a design basis fire was not consistent with the fire safe shutdown design basis. These circumstances represent an unanalyzed condition that could significantly degrade plant safety which is reportable pursuant to 10CFR50.73(a)(2)(ii)(B).
In 1985, it was assumed that 15 minutes would be available to isolate the PORVs in order to establish control of the reactor coolant system pressure boundary as part of a sequence of actions required to establish stable hot standby conditions following a fire in the control room, cable spreading room, or instrument rack room. Local manual operations necessary to achieve hot standby were considered acceptable for the alternate fire safe shutdown design and EOP, provided that the operating staff could support those actions in the prescribed timeframe. Note that no question exists as to the ability of the operating staff to meet the 15 minute PORV isolation criteria. Recent results from a Unit 2 fire safe shutdown analysis prompted review of the Unit 3 control room fire EOP operator actions and response times. The conclusions of this preliminary review determined that the application of the 15 minute timeframe in the EOP would not assure isolation of the PORV prior to RCS depressurization. The subsequent loss of cooling would challenge performance of a safe shutdown at the alternate plant location.
As a result of on-going efforts to validate assumptions used in the fire safe shutdown analysis, it was determined that the early analysis did not model the pressurizer level response to a fire-induced operation of an ADV bypass valve. For this event, 30 minutes was assumed to be sufficient to manually close the valve, however, the pressurizer level would actually decrease below acceptable levels required by the licensing basis in less than 30 minutes.
2. Cause This condition is historical in nature dating to the original development of the assumptions used to support the fire safe shutdown analysis. Consequently a root cause evaluation was not performed. The apparent cause is attributed to a failure to properly validate fire safe shutdown analysis assumptions. Specifically, the original licensing basis assumption of 15 minutes to disable the PORVs and the 30 minute assumption to close the ADV bypass valve following a reactor trip are invalid.
3. Assessment of Safety Consequences At the onset of a fire in the control room, cable spreading area, or instrument rack room, the potential exists for hot shorts to cause a PORV to open. Fires in any of the areas listed also have the potential to degrade shutdown capability from the control room. If control room functions became substantially degraded, the control room would be abandoned and plant shutdown would be accomplished at various alternate shutdown locations. The limited set of plant indications that are available at these alternate locations may not support timely diagnosis and mitigation of a PORV actuation. Additionally, procedures for shutdown from these alternate control locations do not currently anticipate and address the significant challenges associated with maintaining adequate core cooling I.
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with a depressurized RCS. Similarly, a hot short could cause the motor operated ADV bypass valve to open and create a plant response that is not in accordance with the licensing basis which requires pressurizer level to be maintained within the indicating range. In this event, the allowed time to manually close the ADV bypass valve is too long, and significant RCS shrinkage (decreasing pressurizer level) would occur before the valve could be fully closed.
The safety significance of this condition is considered low. It should be noted that the original assumptions (15 minutes, 30 minutes) are judged to be acceptable for slowly developing fires where the event continues to be managed from the control room for an extended period of time. The fire scenario of concern in this case is a rapidly developing fire of significant magnitude which forces an evacuation of the control room shortly after detection and commencement of safe shutdown from alternate plant locations. Based on the very low probability of occurrence of this type of fire and based on the availability of fire detection and suppression systems in the specified areas, the safety significance of this condition is considered low.
4. Corrective Action Upon discovery of this condition, compensatory actions to minimize risk of fire in the areas of concern were implemented including increased surveillances to verify operability of detection and suppression systems, and to confirm control of transient combustibles and ignition sources in affected areas traversed by PORV and ADV bypass valve control circuits.
Investigation and resolution of the issues presented in this LER are being addressed in accordance with the Millstone Corrective Action Program. Review of additional fire safe shutdown analysis assumptions is ongoing.
5. Previous Occurrences No previous similar events/conditions were identified.
Energy Industry Identification System (EllS) codes are identified in the text as [XX].