05000443/LER-2002-001
| Seabrook Station | |
| Event date: | 05-28-2002 |
|---|---|
| Report date: | 07-26-2002 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 4432002001R00 - NRC Website | |
I. Description of Event
On May 27, 2002, at 9:31 PM, North Atlantic Energy Service Corporation (North Atlantic) personnel commenced performance of the 18 month Digital Rod Position Indication (DRPI) (M) operability surveillance 0X1410.07, "Rod Position Indication System 18 Month Operability Test." The surveillance consisted of starting the rod drive motor generator sets (AA) and closing the reactor trip breakers (AA). Each of the 5 shutdown banks (AA) and each of the 4 control banks (AA) are withdrawn and then reinserted one bank at a time while verifying that DRPI rod position for each rod and demand rod position agree over the full travel of the bank.
The shutdown bank surveillance had been completed satisfactorily. During the withdrawal of control bank A, at the transition to the 72-step position, a DRPI urgent alarm was received for rod L5 (AA). The other control and shutdown banks were fully inserted in the core at the time of the alarm. Pursuant to Technical Specification (TS) 3.1.3.3, the Control Room Operator, using the reactor trip switch (JC), manually opened the reactor trip breakers at 2:31 AM on May 28, 2002. All rods fully inserted. All systems functioned as required.
At the time of the event, the plant was in Mode 4, with reactor coolant temperature approximately 276 degrees Fahrenheit and reactor coolant pressure approximately 552 psig.
An eight-hour event notification to the Nuclear Regulatory Commission (NRC) was made pursuant to the requirements of 10CFR50.72(b)(3)(iv)(A). The event notification was 38947.
II. Cause of Event
The cause of this event was the failure of DRPI cabinet Train B detector encoder card (AA) for rod L5, card position CBA-4.
III. Analysis of Event
During rod withdrawal, the DRPI system examines the data and generates alarm signals if errors are detected.
The Detector Encoder card is designed to detect control rod position by sensing changes in the magnetic flux of a stack of electrical coils concentrically mounted around the rod extension housing. During rod withdrawal as the control rod moves upward into the housing penetrating the coil stack, the impedance of the coils penetrated increases, which reduces the associated coils' current. The Detector Encoder card senses the change in coil current and converts the analog signal to a digital signal assigning a unique gray code that corresponds to the detected rod height. The Detector Encoder card malfunction resulted in the assignment of an incorrect gray code for the rod position detected. The symptoms of the failure indicated that both the A and the B data codes for L5 were valid codes but disagreed by more than 1 bit in gray code. This condition is recognized by the DRPI system as a malfunction where rod position becomes indeterminate. As a result, DRPI generated the urgent alarm for the rod, forcing it to read rod bottom. The symptoms were indicative of a failure of the detector encoder card in the B data cabinet (AA) for rod L5, since the failure occurred on the transition to an indicated position, which was a multiple of 12 ('B' coil penetrated). The DRPI monitoring system functioned as designed to detect a failure in the output of the detector encoder card.
As a result of the failure detection, an urgent alarm (AA), general warning for rod L5 (AA), and rod bottom light (AA) for rod L5 were received while transitioning to the 72 step position. The Control Room Operator recognized the failure, and pursuant to TS 3.1.3.3, manually opened the reactor trip breakers using the reactor trip switches.
All rods fully inserted, and the rod control system performed as designed.
III. Analysis of Event (continued) A manual reactor trip due to a failed rod position indication is classified as a Category II, Fault of Moderate Frequency, plant operating condition. The UFSAR, section 15.4.3, "Rod Control Cluster Assembly Misoperation (System Malfunction or Operator Error) states that "If one or more rod position indicator channels should be out of service, detailed operating instructions shall be followed to assure the alignment of the nonindicated [Rod Cluster Control Assembly] RCCA (AA). The operator is also required to take action as outlined by the Technical Specifications." During this event, the Control Room Operator followed the direction contained within the action statement for TS 3.1.3.3, which states "With less than the above required position indicator(s) OPERABLE, immediately open the Reactor Trip System breakers.
The significance of this event is minimal due to the reactor plant being in Mode 4 with only one control bank partially withdrawn.
IV. Corrective Action The faulty card was replaced and the new card was tested satisfactorily. The reactor trip breakers were closed and procedure OX1410.07 was repeated for control bank A satisfactorily. The surveillance was then completed satisfactorily.
V. Additional Information
An operating experience search was performed using the EPIX database. No examples of detector/encoder card failures were identified.
Similar Events One previous control bank detector/encoder board failure has occurred at Seabrook Station on January 26,2001.
Plant Engineering routinely monitors printed circuit board failures. An adverse trend regarding printed circuit board failures has not been identified for the DRPI system.