05000263/LER-2008-001
| Docket Number | |
| Event date: | 01-31-2008 |
|---|---|
| Report date: | 03-31-2008 |
| 2632008001R00 - NRC Website | |
Event Description
On January 31, 2008, Monticello Nuclear Generating Plant (MNGP) personnel discovered during a review of the Extended Power Uprate (EPU) High Energy Line Break (HELB) calculations that the station had not considered the impact of the actuation of fire sprinklers [SPRNK] in the existing HELB calculations. Since the station had a flood barrier installed outside the lower Division 1 4kV [EC] switchgear room, current operability of the plant was not affected. However, for the plant conditions which existed prior to installation of the HELB barrier outside the 4KV switchgear room, the issue 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.
The station conducted a review of information related to NRC requirements and guidance as well as correspondence applicable to High Energy Line Breaks (HELB). Based on this review, it was determined that without the HELB barrier in place to protect the Division I 4KV room, the plant would be in an unanalyzed condition for the following HELB events in the condenser room: a feedwater [SJ] pump [PMP] discharge line break, a condensate [SD] (feedwater suction line) break, or a Main Steam [SB] line break in the condenser room. Without the barrier in place the additional volume of water from the fire suppression system causes the flood level within the Division I 4KV room to reach a level which renders the equipment within that room inoperable.
Event Analysis
The station determined there was no current operability concern 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 condenser bay 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). Both long term RCIC operation and fire water injection are dependent on long term decay heat removal capability for success.
In conclusion, the risk of core damage attributable to floodwater resulting from actuation of the condenser bay sprinkler system upon a HELB event that exceeds temperatures at which the sprinkler heads would actuate is considered to be very small.
Cause
MNGP calculations of record for HELB did not model the actuation of the fire water sprinklers in the condenser room when the condenser room exceeded 165 deg F during a postulated HELB. The sprinkler actuation adds to the liquid water volume emptied into the condenser room following a HELB. This extra liquid volume in the condenser room is enough to have exceeded the maximum allowable postulated water level of 3.75" in the lower 4 kV switchgear room.
Corrective Action The applicable HELB calculations will be revised to reflect the new data.
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. The analysis indicated that the peak flood level on the 911ft elevation of the Turbine Building Division I 4kV switchgear room would cause a loss of Division I 4kV power. 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.