05000263/LER-2006-002

LER-2006-002,

Docket Number
Event date:	05-27-2006
Report date:	05-11-2007
2632006002R01 - NRC Website

Description At approximately 1600 Central Daylight Time on May 27, 2006, the control room received computer alarm "TB 931' 4KV AREA HI TEMP", and dispatched a Non-Licensed Operator (NLO) to the Division II 4KV switchgear [EA] room to investigate. The operator reported the room temperature was 104F. The Balance of Plant (BOP) operator was dispatched from the control room with a calibrated temperature instrument to assist the NLO. The highest temperature monitored with the calibrated instrument was 103F. An additional NLO was dispatched to the roof to check the condition of the non safety-related chiller [CHU] which provides room cooling (V-CH-27). This operator found the chiller tripped on high pressure and reset it. Temperatures in the switchgear room immediately began to lower.

After the chiller was reset and 4KV room temperatures began to lower, the following actions occurred.

• The crew reviewed the Ventilation System Failure procedure for guidance should the ventilation system fail again. At this time, the crew identified that per the procedure, if the temperature in the room reached 104F, the affected switchgear would be considered inoperable.

• The crew briefed the Turbine Building NLO on the need to monitor room temperature every 15 minutes.

At 1714, the Turbine Building Operator notified the control room of a rising trend in the Division II 4KV switchgear room temperature. Ambient temperature in the Division II 4KV switchgear room was determined to be 105F. The Ventilation System Failure procedure was entered and the Division H 4KV switchgear was declared inoperable. A 24-hour shutdown LCO was declared. Investigation by Operators found that the chiller had tripped again. This time, per Control Room Supervisor (CRS) direction, the chiller hand switch was placed to the off position to reposition dampers and draw cooler outside air (81F) into the room. The CRS also directed, as allowed by the procedure, opening room doors and placing portable fans in the switchgear area to aid in cooling the room. As described in the procedure, opening the doors also directed Division I 4KV switchgear to be declared inoperable. This created a condition where both electrical divisions were potentially susceptible to failure from a single HELB. This was not initially recognized by the shift operating crew, but was later recognized by the next shift operating crew.

At approximately 1714, doors into the Division II 4KV room were opened and a continuous fire watch was established.

At 1745, the temperature in the Division II 4KV room was reported as 101F and stable. At approximately 1803 the doors were closed, the LCO exited, and the continuous fire watch secured.

An event notification to the NRC was made at 0055 on May 28, 2006 for a condition that existed that could have "Prevented the Fulfillment of the Safety Function of Systems that are needed to Remove Residual Heat" and "Mitigate the Consequences of an Accident" because both divisions of 4KV switchgear had been declared simultaneously inoperable.

Event Analysis

In accordance with 10 CFR 50.72 (b)(3)(v)(B and D), "Event or Condition that could have Prevented Fulfillment of a Safety Function," an eight-hour event notification was made to the USNRC. The event notification was completed because both divisions of 4KV switchgear were declared inoperable and the potential existed to lose residual heat removal and accident mitigation capability. Per 10 CFR 50.73 (a)(2)(v)(B and D), a Licensee Event report is required for this event. This event is also reportable pursuant to 10 CFR 50.73 (a)(2)(ii)(B), "The nuclear power plant being in an unanalyzed condition that significantly degraded plant safety.

The event is classified as a safety system functional failure.

Safety Significance

As a result of the procedurally prescribed high temperature inoperability in the Division II 4KV room, the directed actions taken to address the high temperature in the Division II 4KV room, rendered the plant vulnerable to the effects of a single postulated HELB which could have affected the ability safely shut down and remove residual heat.

For Monticello, a HELB is considered an initiating event with a less than 1 per 105 year frequency. In addition, the 4KV switchgear was exposed to the additional vulnerability from a HELB event for a limited time (less than one hour). Based on the limited time the Division II switchgear room doors were opened the additional exposure to a postulated HELB event was minimal and no significant increase in plant risk occurred.

Cause

The cause of this event was a failure of a chiller fan [FAN] motor [MO]. The chiller failure resulted in implementation of an abnormal operating procedure that directed operator actions which rendered the plant susceptible to a postulated HELB that could have caused the loss of portions of both safety related electrical divisions. The operators initially failed to declare both safety related electrical systems inoperable as directed by the abnormal procedure and as required by Technical Specifications and enter the appropriate action statement.

Corrective Action The following actions have been completed:

1. Temperature in the Upper (Division II) 4KV Room was returned to within specification, and the HELB doors were closed. All affected 4KV equipment was declared operable. The shutdown LCO was exited.

2. The compressor fan motor on the chiller was replaced and the condensing coils were cleaned.

3. An analysis was completed which determined that the temperature limit in the 4KV switchgear rooms could be raised to 111F. The Abnormal Operating procedure, "Ventilation System Failure," has been revised to reflect the modified temperature limit.

4. The Abnormal Operating procedure was revised to provide clear guidance on the effects of opening HELB boundary doors.

Failed Component Identification V-CH-27 4KV Switchgear Room Chiller Manufacturer: McQuay, Model # - ALP027C

Previous Similar Events

No previous similar events were identified.