05000266/LER-2003-002

LER-2003-002,

Docket Number
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(iv), System Actuation
Initial Reporting
2662003002R00 - NRC Website

NRC CORM WA

P.m) U.S. NUCLEAR REGULATORY COMMISSION FACILITY NAME (I) ' � DOCKET (2) ' LERNUMBER(d) PAGE OL Point Beach Nuclear Plant Unit 1 05000 266

Event Description:

On July 15, 2003, Point Beach Nuclear Plant (PBNP) Unit 1 was In full power operation. At approximately 1339 (all times are Central Daylight Time) Operators received a "0-07 Battery Charger Trouble" alarm' and shortly thereafter Unit 1 'Rod Control Urgent" and "Rod Control Non- Urgent" Alarms2. Operators immediately referenced the alarm response procedures, however, at 1340, a Unit 1 automatic reactor trip occurred followed by a loss of Feedwater/Turbine Trip due to the reactor trip and Main Feedwater Regulating Valve3 closure. Control Room personnel entered Emergency Operating Procedure EOP-0, "Reactor Trip or Safety Injection" and transitioned to EOP Rod Drive Room4, reported that there was smoke coming from the Rod Drive Motor Generator (MG) Sets. At 1347, both the 1G-06 and 1G-07 Rod Drive MG Sets were de-energized by opening the supply breakers° per procedure 01-31. At 1355, it was reported to the control room, that the 1G-06 Voltage Regulator showed signs of overheating on the fuses and wiring and at 1412, the DC feed breaker to the Voltage Regulator was opened which cleared the "D-07 Trouble" and "DC Ground Fault" alarms.

At 1429, an event notification (EN# 39996) was made to the NRC Operations Center via the Emergency Notification System. At 1755, a shutdown margin calculation was performed and verified satisfactorily that the unit was stable in Mode 3. Systems and equipment necessary to mitigate the consequences of this equipment failure and subsequent reactor trip functioned as designed and the plant was maintained in a stable hot shutdown condition during the troubleshooting and repair of 1G06.

Following completion of the Initial review of the event and completion of Interim corrective actions, Unit 1 restart was authorized. On July 24, a Unit 1 reactor startup commenced and the reactor was critical at 1110. On July 25, 2003 at 1110, full power operation was achieved.

Event Analysis:

A root cause evaluation team was appointed to determine the cause of the failure of 1G-06, and to assess the vulnerability of 1G-07 and the PBNP Unit 2 rod drive motor generators to the same failure mechanisms. Investigation revealed that the 1G-06 Voltage Regulator failed due to a failure of an internal component denoted on plant drawings as VOLTRAP SP1, which acts as a surge suppressor in the voltage regulator power and feedback circuit. This failure caused the voltage regulator to draw high current from transformer 2T overloading the transformer. Overloading a 3 4 5 7 EIIS System EIIS System EIIS System EIIS System EIIS System Identifier:

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90 transformer 2T caused It to bum, melt and eventually fault the primary windings causing a fault on the 260VAC bus supplying the Control Rod Drive Mechanisms (CRDMs).

The failure of the 1G-06 voltage regulator also caused a loss of excitation to the 1G-06 generator.

The 1G-06 output breaker did not open and 1G-06 continued to operate in parallel with 1G-07. As a result, 1G-07 began to feed large circulating currents to the 10-06 generator (i.e. 1G-06 became a large inductive load on 10-07). In addition, the real loading on I G-07 increased rapidly due to the CRDM load previously shared with 1G-06. It is also believed that 1G-07 was still feeding the faulted transformer 2T. The result of each of these loads on 1G-07 caused the bus voltage to drop significantly, eventually causing control rods to fall and CRDM power supplies to fail.

The failure associated with the 1G-06 MG set voltage regulator was reviewed to determine if 10- 07, 2G-06 or 2G-07 were susceptible to a similar failure. It was determined that they are susceptible, and the same actions taken to modify 10-06 were taken on 1G-07 via modification MR 03-031. The Unit 2 Rod Drive MG set voltage regulators will be inspected during the fall 2003 refueling outage. There are no other voltage regulators of this type in use at PBNP thus limiting the extent of condition review to the Rod Drive MG set components.

This event is being reported in accordance with 10 CFR 50.73(a)(2)(iv) System Actuation; "Any event or condition that resulted in manual or automatic actuation of any of the systems listed in paragraph (a)(2)(iv)(B) of this section".

Cause:

As discussed in the Event Analysis, the root cause of this event was the failure of the VOLTRAP SP1 component internal to the 1G-06 Voltage Regulator. This caused a high current from transformer 2T to overload the transformer. Overloading transformer 2T caused it to bum, melt and eventually fault the primary windings causing a fault on the 260VAC bus supplying the CRDMs. A contributing cause was the fact that the voltmeters and ammeters on the cabinet doors used to balance loads between MG sets were out of tolerance. As a result , when loads were balanced in accordance with plant procedures, circulating currents between the MG sets were created which imposed additional stress on the components.

Corrective Actions:

Work orders were implemented to trouble shoot and repair 1G-06 and 1G-07. These actions were completed before returning Unit 1 to power operations. Repairs included installation of an equipment modification (MR 03-031) to provide more reliable voltage regulation of the MG sets.

The modifications included relocating the 2T transformer and installing fuses on the primary side of the transformer that provides input into the generator field regulating circuit.

The Unit 2 Rod Drive MG sets will be Inspected and evaluated during the Fall 2003 refueling outage to determine whether modifications similar to those completed on Unit I are necessary.

NRC FORM 1,66A 0-2001) .

P1RC FORM 366A � U.S. NUCLEAR REGULATORY COMLISSION (7.2IX)1) FACILITY NAME (I) DOCKET (2) LER NUMBER (6) PAGE (3) Point Beach Nuclear Plant Unit 1 05000 266

Safety Significance:

With the exception of the failure of the 10-06 MG set, which initiated this event, the plant response during and following this reactor trip and AFW actuation was as expected. Systems and equipment necessary to mitigate the consequences of this transient performed as designed and maintained the plant in a stable hot shutdown condition. Although this was an actuation of the reactor protection system and other plant equipment necessary to remove shutdown decay heat and maintain the plant in a stable configuration, the safety significance of this event was minimal.

The safety and welfare of the public and the plant staff was not impacted by this event.

During this event and the subsequent recovery actions there was at no time a loss of a system, structure, or component related safety functions; therefore, this event did not Involve a Safety System Functional Failure.

Similar Occurrences:

A review of LERs submitted in the past three years identified the following events which involved a reactor trip due to equipment fault or failure:

LER NUMBER � Mg1 301/2003-004-00 � Reactor Trip Due to Failure of *Er Main Feedwater Pump 301/2001-001-00 � Ground Fault Relay Actuation Causes Generator Lockout and Reactor Trip 30112000-007-00 � Fault Associated with *C" Phase Main Step-up Transformer Results in Reactor Scram 301/2000-006-00 � Failed Fuse in Intermediate Range Nuclear Detector Results in Reactor Scram razz FoRM 3.141A (7-2060