05000263/LER-2008-006

LER-2008-006,

Docket Number
Event date:	09-17-2008
Report date:	11-14-2008
2632008006R00 - NRC Website

Event Description

At the Monticello Nuclear Generating Plant (MNGP), three transformers are provided to supply the plant with offsite power from the substation. All three sources can independently provide adequate power for the plant's safety-related loads. These transformers and their interconnections to the substation are as follows: The primary station auxiliary transformer (2R) is fed from a 345 KV Bus and underground cabling. The 2R transformer is of adequate size to provide the plant's full auxiliary load requirements. The reserve transformer (1R) is fed from a 115 KV substation via an overhead line. The 1R transformer is of adequate size to provide the plant's full auxiliary load requirements. The reserve auxiliary transformer (1AR) may be fed from two separate 13.8 KV sources. The 1AR transformer is sized to provide only the plant's essential 4160 Vac buses and connected loads.

On September 17, 2008, the plant was shutdown in a forced outage with the 2R transformer [XFMR] isolated and tagged out for repair. Off-site power was supplied via transformer 1R with 1AR and on-site diesel generators [DG] as backup power sources. At approximately 0934 a man-lift being serviced by a vendor came into contact with a 115 kV line [FK] line, de­ energizing the 1R transformer and causing a Loss of Normal Off-site Power (LONOP). All equipment and systems performed as required. The site lost normal shutdown cooling for approximately 90 minutes as a result of the LONOP.

Event Analysis

The event was reported under 10 CFR 50.72(b)(3)(iv)(A) ESF Actuation. Therefore, the event is reportable under 50.73(a)(2)(iv) (A) "System Actuation," and a Licensee Event Report is required for this event. The event is also reportable under 50.73(a)(2)(v)(B) "Event or Condition that could have Prevented the Fulfillment of a Safety Function.

The event is considered a safety system functional failure since there was a loss or inability of a safety system (RHR) to remove decay heat.

Safety Significance

The station Probabilistic Risk Analysis (PRA) group reviewed the event and provided the following safety significance.

At the time of the transient the reactor had been shutdown for approximately five days, and all systems with the exception of the 2R transformer were available and capable of performing their intended function. Without any cooling available to remove decay heat, it is estimated that over 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> were available to restore cooling or makeup injection before water level would have boiled down to the top of active fuel. This time-to-uncover fuel is conservatively calculated based on initial conditions of 70 inch reactor water level, 95°F reactor water temperature, and decay heat levels consistent with 130.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> following the reactor scram. A risk estimate was performed assuming no potential recovery for the 2R Transformer and a 90% recovery probability for the 1R Transformer. This approach is conservative in that recovery of any failed equipment would have the benefit of additional available time (more than twelve hours versus the normal -0.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> to boil down to TAF) due to the relatively low decay heat level at the time of the transient. Results of the assessment indicated a Conditional Core Damage Probability (CCDP) of 3.76 E-07, and a Conditional Large Early Release Probability (CLERP) of 9.76 E-08.

In conclusion, based on the above information, the safety significance of this event was low.

Cause

The cause of the Loss of Normal Off-site Power was the portable man-lift equipment coming into contact with the 115 kV line resulting in the trip of the 1R transformer.

Corrective Action The following corrective actions have been completed or are planned:

• There were no required station corrective actions from the standpoint of equipment performance.

• Due to the on-going OSHA and Station investigation into the associated vendor fatality, there have been no formal corrective actions taken to date. In the interim, the following actions have been taken:

o All rental equipment will be delivered to the outage parking lot or inside the OCA with station personnel in attendance prior to unloading of any equipment. No allowances to unload equipment under the 115 kV lines.

o Vendors were contacted regarding the equipment unloading restrictions.

Failed Component Identification None

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