05000263/LER-2008-004

LER-2008-004,

Docket Number
Event date:
Report date:
2632008004R00 - NRC Website

Event Description

On April 29, 2008, during a review of the calculation of record for a Feedwater HELB break at the Feedwater pumps, personnel discovered that the calculation assumes that the fire water flow should be isolated in 20 minutes and that the service water flow should be isolated within ten minutes. It was determined during this review there was no procedural guidance to ensure these isolations occur within the specified time frames.

Event Analysis

The station determined there was no current operability concern due to the installed flood barrier and therefore the event was not reportable under 10 CFR 50.72. However, due to past operability concerns (the condition existed from November 29, 2001 to present), the event is reportable under 50.73(a)(2)(ii) (B) "Any event or condition that resulted in the nuclear power plant being in an unanalyzed condition that significantly degrades plant safety," and a Licensee Event Report is required for this event.

The event is not considered a safety system functional failure.

Safety Significance

The Probabilistic Risk Assessment (PRA) group performed an evaluation of the risk of core damage attributable to floodwater resulting from actuation of the sprinkler system upon a HELB event that exceeds temperatures at which the sprinkler heads activate. This assessment is intended to address past risk associated with the postulated HELB events, therefore the flood barrier erected outside of the lower 4KV switchgear room is assumed not to exist.

It was determined that a HELB frequency of 2.60 E-02/yr (one HELB event every 39 years) would be required to result in a CDF increase of 1.00 E-06/yr for the scenario of interest. Since an annual increase in core damage probability of 1.00 E-06 is considered to be very small as reflected in guidance provided by RG 1.174, the frequency of the HELB break(s) of concern must therefore be greater than 2.60 E-02/yr to be considered more than a very small risk impact. Since the Monticello plant and the commercial nuclear industry as a whole do not observe HELB events in general at or near this frequency, a reasonable conclusion can be drawn, that the additional CDF risk attributable to the vulnerability of division I 4KV switchgear to HELB events described above, is very small.

Combined flow resulting from the HELB and fire sprinkler activation was determined to be bounded by a large fire protection system break modeled in the PRA. Review of quantification results related to the postulated HELB scenario show that the flooding, unless detected and suppressed early, generally fails the lower 4KV equipment as well as offsite power, leaving #12 EDG [DG] as the only remaining major power source to supply division II equipment.

Failure of #12 EDG due to any of a wide array of causes will result in a station blackout (SBO), and limited capability to provide long term core cooling to prevent melting of the core. Even with a SBO, however, adequate core cooling can be accomplished through the use of HPCI [BJ] and/or RCIC [BN] for short term (several hours) high pressure injection followed by either manual operation of RCIC or depressurization with low pressure injection of fire water (recovered from the sprinkler activation diversion).

In conclusion, the risk of core damage attributable to floodwater resulting from actuation of the sprinkler system upon a HELB event that exceeds temperatures at which the sprinkler heads would actuate is considered to be very small.

Cause

The cause of the event was a failure by station personnel to transfer the timeframe requirements from the calculation to the station response procedures.

Corrective Action The station procedure was revised to address required time frames to mitigate the effects of flooding.

Failed Component Identification None

Previous Similar Events

MNGP LER 263-2000-004: An analysis of a high energy line break (HELB) on the 911ft elevation of the Turbine Building indicated flooding of the Division I 4kV switchgear room and possible loss of the Division I 4kV switchgear. With an assumed loss of offsite power, Division II Emergency Diesel Generator was considered the worst case single active failure.

Therefore, this event could potentially result in loss of the station AC power from both divisions of the 4kV distribution system. Modifications were installed to prevent water from entering the Division I 4kV switchgear room.

MNGP LER 263-2008-001: On January 31, 2008 during a review of the High Energy Line Break (HELB) calculations for the plant's Extended Power Uprate (EPU) project, it was determined that the existing HELB calculations failed to consider the actuation of the fire sprinklers in the condenser bay and the resultant flooding impact on the lower Division 1 4kV equipment. The station had previously installed a flood barrier near the 4kV Switchgear room door therefore present operability was not impacted. The station determined that prior to the installation of the barrier, there was a potential for the loss of the lower Division 1 4kV equipment. The cause of the event was a failure to consider the impact of the fire sprinklers.

Corrective actions taken or planned are: the flood barrier will remain in place and a revision of the affected HELB calculations will be performed.