05000346/LER-2003-009

LER-2003-009, Loss Of Offsite Power due to Degraded Regional Grid Voltage

Davis-Besse Unit Number 1
Event date:	08-14-2003
Report date:	10-13-2003
Reporting criterion:	10 CFR 50.73(a)(2)(iv), System Actuation 10 CFR 50.73(a)(2)(iii) 10 CFR 50.73(a)(2)(v), Loss of Safety Function
Initial Reporting
ENS 40070	10 CFR 50.72(a)(1)(i), Emergency Class Declaration
3462003009R00 - NRC Website

DESCRIPTION OF OCCURRENCE:

On August 14, 2003, the Davis-Besse Nuclear Power Station (DBNPS) was in Mode 5 with the equipment hatch installed. Train 2 of the Decay Heat System 1BP) was providing core cooling■ and a planned Reactor Coolant System fABYdrain was in progress to perform maintenance activities.

At approximately 1516 hours0.0175 days <br />0.421 hours <br />0.00251 weeks <br />5.76838e-4 months <br /> (EDT), a low voltage alarm of 329 kV was received on the DBNPS Switchyard 345 kV Bus 41 (PK). This and other low voltage alarms on 345 kV Bus K and 4160 V Essential Buses Cl and D1 (EA] actuated and cleared several times until 1606 hours0.0186 days <br />0.446 hours <br />0.00266 weeks <br />6.11083e-4 months <br />, when the 345 kV Bus voltages experienced a step drop to 308 kV. The 4160 V Essential Bus voltages dropped to 3870 V, and low voltage alarms were received on the following buses:

• 13.8 kV Buses A and B

• 4160 V Essential Buses Cl and D1

• 4160 V Non-Essential Buses C2 and D2 MB),

• 480 V Essential Buses El and Fl (EC]

• 480 V Non-Essential Buses E2, E3, E6, F2, and F7 (ED].

The 345 kV Bus voltages continued to slowly lower from 308 kV to 300 kV with a corresponding lowering of 4160 V Bus voltage until at 1608 hours0.0186 days <br />0.447 hours <br />0.00266 weeks <br />6.11844e-4 months <br /> the 90 percent under voltage relay (EB-27) of the Safety Features Actuation System (SPAS) (

• 7E] on 4160 V Essential Bus D1 actuated. This relay disconnected essential Bus D1 from non-essential Bus D2, and the 59 percent loss of voltage relay actuated to automatically fast start Emergency Diesel Generator (EDC)) 2 (EK-DG). EDC) 2 re-energized Bus D1, and subsequently Service Water Pump 2 (BI-P] and Component Cooling Water (CCW) Pump 2 (CC-P] automatically restarted to supply cooling to EDO 2. Approximately 4G seconds later, the 90 percent under voltage relay on 4160 V Essential Bus Cl similarly actuated on the lowering voltage, which resulted in a start of EDC; 1 along with Service Water Pump 1 and CCW Pump 1.

All DBNPS Switchyard breakers remained closed during the event since no fault was sensed that would have initiated a trip/open signal. However, the low- side breakers LEA-521 for the two Startup Transformers (EA-XFMR] opened on under frequency to isolate the 13.8 kV Buses A and B from the 345 kV distribution network.

As a result of the loss of offsite power, an Unusual Event was declared at 1621 hours0.0188 days <br />0.45 hours <br />0.00268 weeks <br />6.167905e-4 months <br /> based on Emergency Action Level (EAL) 4.A.1. The Emergency Response Organization was activated to assist with plant recovery. Outage- related work was halted during the Unusual Event including the ongoing drain of the Reactor Coolant System. Electrical bus restoration was performed in an orderly and controlled manner, and the majority of the on-site electrical distribution system was re-energized by 0300 hours0.00347 days <br />0.0833 hours <br />4.960317e-4 weeks <br />1.1415e-4 months <br /> on August 15, 2003.

Following the restoration, EDO 1 was unloaded and shutdown at 0617 hours0.00714 days <br />0.171 hours <br />0.00102 weeks <br />2.347685e-4 months <br />, and EDC) 2 was unloaded and shutdown at 1425 hours0.0165 days <br />0.396 hours <br />0.00236 weeks <br />5.422125e-4 months <br /> on August 15, 2003.

I � DESCRIPTION OF OCCURRENCE: (Continued) The loss of offsite power resulted in loss of power to the operating Decay Heat Pump (Pump 2). At 1641 hours0.019 days <br />0.456 hours <br />0.00271 weeks <br />6.244005e-4 months <br />, Decay Heat Pump 2 was manually restarted in accordance with the Loss of Decay Heat Removal Abnormal Operating Procedure.

During the loss of offsite power, a Service Water System leak of approximately 4-5 gallons per minute was discovered on the end bell of CCW Heat Exchanger 3 [CC-HX]. The Service Water inlet and outlet isolation valves for the heat exchanger were closed to isolate the leak.

The loss of offsite power also resulted in loss of power to the non-essential Spent Fuel Pool Cooling System [DA]. At 0253 hours0.00293 days <br />0.0703 hours <br />4.183201e-4 weeks <br />9.62665e-5 months <br /> on August 15, 2003, following offsite power restoration, Spent Fuel Pool Cooling was returned to service.

The Unusual Event was terminated at approximately 1920 hours0.0222 days <br />0.533 hours <br />0.00317 weeks <br />7.3056e-4 months <br /> on August 15, 2003.

Following the event, a walkdown of Containment identified that the expansion bellows [BK-EXJ] in the Containment Air Cooler (CAC) Service Water return piping were distorted. These expansion bellows had been installed dufing the current plant outage as part of the re-design of the CAC piping to reduce the thermal loading of the cooling coil nozzles. Refer to DENPS LER 2002-008 for further information on the replacement of the CACs.

The declaration of an Emergency Classification (Unusual Event) is reportable per 10CFR50.72(a)(1)(i) and 10CFR50.72(a)(3) within one hour of the declaration. Notification to the NRC was completed at 1654 hours0.0191 days <br />0.459 hours <br />0.00273 weeks <br />6.29347e-4 months <br /> on August 14, 2003 (Event Notification Number 40070).

This event resulted in a valid actuation of the Emergency AC electrical power system; therefore, it is being reported in accordance with 10CFR50.73(a)(2)(iv) as an actuation of a specified system.

In accordance with the guidance contained in NUREG-1022, Event Reporting Guidelines, the offsite power and the onsite emergency power systems are considered to have separate safety functions by 10CFR50 Appendix A Criterion 17 (General Design Criteria or GDC 17). The safety function of each of these systems is to provide sufficient capacity and capability, etc., assuming that the other system is not available. A loss of either system results in a loss of the safety function of that system. Therefore, this event involving the loss of offsite power is also being reported in accordance with 10CFR50.73(a)(2)00 as an event or condition that could have prevented the fulfillment of a safety function. Because the loss of safety function was due to an external condition, this event is also being reported in accordance with 10CFR50.73(a)(2)(iii) as an external condition that posed a threat to the safety of the nuclear power plant.

APPARENT CAUSE OF OCCURRENCE:

The events that led to the degraded grid voltage conditions in several Eastern and Midwestern states as well as Canada were outside the control and responsibility of the DBNPS. An international commission is currently investigating the cause for the degraded grid voltage conditions.

The distortion of the CAC Service Water piping expansion bellows was the result of an inadequate hydrodynamic transient analysis. The analysis used to design the CAC Service Water piping under-predicted the peak pressures that would occur for a loss of offsite power condition, when the Service Water Pump Motors would be de-energized and would subsequently restart in approximately 30 to 50 seconds, in conjunction with low Service Water System flow/high Service Water System pressure that would exist with the plant in a shutdown condition. Non-conservative hydrodynamic transient model parameters used in the analysis included Service Water System pressure, lake level, pump start times, valve alignments, and residual void volume.

The Service Water System leakage from the end bell of CCW Heat Exchanger 3 was caused by inadequate thickness dimension of the gasket material installed during the current outage. The gasket material was unable to accommodate the irregularities that existed in the gasket sealing surfaces. The inadequate gasket thickness had been identified prior to the loss of offsite power event but had not yet been repaired. The leak manifested itself because of the anticipated hydrodynamic transient that occurred as a result of the loss of power to the operating Service Water Pumps.

ANALYSIS OF OCCURRENCE:

Because the DBNPS has been in an extended shutdown, the decay heat output of the Reactor Core is very low. As a result, during the approximately 30 minutes Decay Heat Pump 2 was not operating due to the loss of offsite power, the temperature of the Reactor Coolant System only increased approximately one degree Fahrenheit.

The Spent Fuel Pool Cooling System was not operating for approximately 11 hours1.273148e-4 days <br />0.00306 hours <br />1.818783e-5 weeks <br />4.1855e-6 months <br /> as a result of the loss of offsite power. As a result of the system's unavailability, the temperature of the Spent Fuel Pool only increased approximately 9 degrees Fahrenheit (from an average of approximately 92.7 degrees prior to the event to a maximum of 101.7 degrees). Decay Heat Pump 1 remained available to provide cooling to the Spent Fuel Pool during the time the Spent Fuel Pool Cooling System was not available.

Because of the loss of offsite power, the Service Water Pump Motors were de- energized and subsequently restarted when the EDGs restored power to the 4160V Essential Busses. This temporary loss of Service Water resulted in an anticipated hydrodynamic transient (water hammer). However, because the analysis performed to design the CAC Service Water piping underestimated the peak pressures and resultant piping forces from a loss and restoration of the

• � ANALYSIS OF OCCURRENCE: (Continued) Service Water System with the plant in shutdown conditions, distortion and misalignment of the CAC expansion bellows occurred. Because the discharge isolation valve for CAC I was closed at the time of the loss of offsite power, the peak pressure experienced was significantly higher as compared to CAC 2 and CAC 3, and the expansion bellows for CAC 1 experienced a greater degree of distortion. The discharge isolation valve for CAC 3 was also closed at the time of the event, but with the supply to CAC 3 aligned to the supply of CAC 2, the open discharge isolation valve for CAC 2 reduced the magnitude of the hydrodynamic transient.

None of the expansion bellows on any of the three CACs leaked as a result of the hydrodynamic transient. Though not required to be operable with the plant in a shutdown condition, CAC 2 was declared inoperable as a result of the expansion bellows distortion. The distortion of the expansion bellows of CAC 2 and 3 did not render them incapable of performing their designated safety functions. Visual inspections were performed for the remainder of the Service Water System addressed by the Technical Specifications to ensure other equipment had not been damaged by the hydrodynamic transient, and no other damage or distortion was noted.

The leakage from the end bell of CCW Beat Exchanger 3 was not sufficient to have an adverse effect on the operation of either the Service Water or CCW Systems.

All other safety systems performed as designed, including the starting of the EDGs and the loading of the Essential 4160 V Busses. Reactor Coolant System inventory was maintained at approximately 28 feet on the medium-range indication during the loss of offsite power. Based on the equipment response and operator actions during this event, the loss of offsite power had minimal safety significance.

CORRECTIVE ACTIONS:

Immediate actions taken in response to the loss of offsite power are described in the Description of Occurrence section of this report. No onsite actions to prevent recurrence of the loss of offsite power are identified due to the external nature of the event.

In accordance with the DBNPS Corrective Action Process, the expansion bellows for all three CACs are scheduled to be replaced prior to reactor startup.

Additional design and configuration changes will also be implemented to assure the CACs remain operable following anticipated hydrodynamic transients.

An appropriately sized gasket was installed on CCW Heat Exchanger 3 on August 21, 2003.

FAILURE DATA:

offsite power was lost while conducting bus transfer testing during a refueling outage. The corrective actions from this 2000 event, which was a result of personnel error, could not have prevented this current event, which was caused by external events outside the control and responsibility of the DBNPS.

complete loss of offsite power occurred due to the DBNPS switchyard being damaged from a tornado and accompanying straight-line winds, rain and lightning. Corrective actions were not feasible to prevent a loss of offsite power due to external events of this nature.

Energy Industry Identification System (EII8) codes are identified in the text as MX).

NP-33-03-009-00 � CRs 03-06590, 03-06597, 03-06651