05000305/LER-2009-003
| Docket Number | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3052009003R00 - NRC Website | |
CONTINUATION SHEET
Kewaunee Power Station 05000305 � 2� OF� 4
Event Description:
On January 31, 2009, a Dominion Energy Kewaunee (DEK) engineering evaluation determined that at the allowed setpoint of the degraded voltage protection relay [27], the 1A containment spray [BE] pump [P] could potentially trip. A postulated starting of the motor [MO] during the lowest voltage conditions possible at the relay setting could result in starting currents that are sufficiently high and last long enough to trip the motor's protective device [FU].
The evaluation was based on accident load sequencing, which starts the containment spray pump coincident with one containment fan coil unit [BK, FCU]. Due to the postulated initial low voltage and timing of motor starts, which temporarily depress voltage, individual acceleration of motor loads is lengthened. Spray pump 1A acceleration time would have been lengthened enough such that the large current drawn during acceleration could result in a time-current combination that challenged the protective device settings.
Consequently, automatic containment spray pump start to mitigate a design basis accident in accordance with the plant's safety analyses is not assured under all postulated conditions. Based on the nonconservative settings, the pump was declared inoperable.
Kewaunee Power Station (KPS) Technical Specifications (TS) 3.3, "Engineered Safety Features and Auxiliary Systems", provide operability requirements for the containment spray system.
TS 3.3.c.1.A.1 states:
Two containment spray trains are OPERABLE with each train comprised of:
(i) ONE containment spray pump.
TS 3.3.c.1.A.3, which allows conditions of inoperability to exist during the time intervals specified, states:
One containment spray train may be out of service for 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> provided the opposite containment spray train remains OPERABLE.
Upon discovery that the settings were nonconservative, the containment spray pump breaker settings were adjusted and pump operability was restored within the time required by TS. Electrical bus [BU] distribution characteristics associated with the opposite train spray pump were sufficiently more conservative (with respect to degraded voltage) such that the existing setpoints were adequate to provide motor protection for the 1B pump and allow for pump start during the accident load sequencing. Therefore, the 1B spray pump was not affected by this condition.
The settings were nonconservative since initial installation of the second level undervoltage relays; therefore, the 1A containment spray pump was inoperable for longer than the time allowed by TS. Consequently, this condition is being reported pursuant to 10 CFR 50.73(a)(2)(i)(B), as any operation or condition which was prohibited by the plant's TS.
� The containment cooling function is provided by two systems: containment fan coil units and containment spray system. The containment fan coil units and containment spray system protect containment [NH] integrity by limiting the temperature and pressure that could be experienced following a Design Basis Accident. The Limiting Design Basis accidents relative to containment integrity are the loss-of-coolant accident and steam line break. During normal operation, the fan coil units are required to remove heat lost from equipment and piping within the containment. In the event of the Design Basis Accident, either of the following combinations will provide sufficient cooling to limit containment pressure to less than design values:
four fan coil units or two fan coil units plus one containment spray pump.
The degraded voltage relays are provided for detecting a sustained degraded voltage condition on the electrical bus. The time delay associated with the degraded voltage relays ensures that engineered safeguards equipment operates within the time delay assumptions of the accident analyses. Degraded voltage conditions can result in motors failing to start due to blown control power fuses and/or protective device actuation caused by the large starting surge currents that develop when starting a motor under low voltage conditions.
The condition in which the containment spray pump's acceleration time could have challenged the protective device settings on accident sequencing existed only if the containment spray pump were to start coincidently with the start of a containment fan coil unit. Therefore, during the times when both containment fan coil units were already running, the containment spray pump would have started appropriately without challenging its protective device settings. Additionally, neither the opposite train containment spray pump nor the containment fan coil units were impacted by this condition. Therefore, this condition did not constitute a safety system functional failure.
Additionally, the probability of a containment spray pump start being required to mitigate a design basis accident, coincident with the conditions needed to sufficiently depress voltage to actuate the pump motor's protective device, is very low. Therefore, there was minimal safety significance associated with this event.
Cause:
This condition appears to have been created by incompatible breaker settings following initial installation of the second level undervoltage protection relays. The cause of this condition was due to the difference in electrical distribution system installation attributes (e.g., length of interconnecting electrical cabling) for train A electrical distribution than for train B. These attributes allowed for a sufficiently lower voltage to be delivered to the A containment spray pump, such that the originally calculated settings were nonconservative.
Corrective Actions:
As immediate corrective action, the containment spray pump motor protective relay settings were adjusted to setpoints that were determined to provide appropriate protection. This restored pump operability.
As part of a broader corrective action for degraded grid voltage protection concerns in general, station electrical calculations were initiated to determine more accurate electrical distribution system performance characteristics and verify protective feature settings.
A review of Licensee Event Reports covering the past three years identified the following similar events.
Auxiliary Transformer �