05000289/LER-2006-002
| Docket Number | |
| Event date: | 11-02-2006 |
|---|---|
| Report date: | 01-02-2007 |
| Initial Reporting | |
| ENS 42957 | 10 CFR 50.72(b)(2)(xi), Notification to Government Agency or News Release, 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation |
| 2892006002R00 - NRC Website | |
EVENT DESCRIPTION
Plant Conditions before the event:
Babcock & Wilcox — Pressurized Water Reactor — 2568 MWth Core Power Date/Time: November 2, 2006/1334 hours Power Level: 100% steady state power prior to the event Mode: Power Operations There were no structures, systems, or components out of service that contributed to this event.
On November 2, 2006, at 1334 hours0.0154 days <br />0.371 hours <br />0.00221 weeks <br />5.07587e-4 months <br />, the Three Mile Island, Unit 1 (TMI-1) reactor tripped from 100% power during a calibration check of the main turbine Digital Turbine Control System (DTCS) *(JJ] condenser pressure instruments. The reactor trip was initiated from Reactor Protection System (RPS) ARTS (Anticipatory Reactor Trip System) due to a main turbine trip initiated from greater than 45% power. The turbine trip resulted from an invalid DTCS protective system actuation. The turbine trip initiated from the two in-service DTCS protective channels not being tested at the time of the event. Both channels received an invalid trip signal. The channel under test at the time of the event was properly bypassed from the trip logic and did not input a trip signal to the DTCS. Plant systems responded properly to the reactor trip transient and the plant was stabilized at hot shutdown conditions.
The TMI-1 DTCS provides both control and protection functions for the main turbine. Each of the three low pressure (LP) turbines (A LP Turbine, B LP Turbine, and C LP Turbine) is protected from high backpressure by an automatic turbine trip function.
In 2001, this protective function was modified to increase trip margin during summer months via a variable trip setpoint for loss of condenser vacuum. The turbine trip is now initiated based on a load dependent variable, instead of a fixed pressure setpoint. Absolute pressure transmitters were installed for pressure monitoring in the exhaust hoods of each LP turbine instead of pressure switches. The DTCS inputs were changed from contact inputs to milli-amp inputs.
The initial condenser backpressure milli-amp transmitter calibrations were performed during the 2001 modification, coincident with a refueling outage. The transmitters were next calibrated with the plant on line in June 2004, with no impact on the main turbine trip logic. No other corrective or preventive maintenance had been performed on any of the condenser backpressure transmitters since they were installed during the 2001 modification.
On November 1, 2006, maintenance technicians performed an event-free calibration of the three A LP Turbine exhaust hood pressure transmitters. On November 2, 2006, technicians began calibration of the B LP Turbine exhaust hood pressure transmitters. As the technicians connected the test lead to one of the three B LP Turbine backpressure transmitters to be calibrated, the DTCS trip logic circuit received invalid trip signals from the exhaust hood high pressure alarm circuits for the two LP turbines not being tested, followed closely by a main turbine trip. The main turbine trip in turn actuated an automatic RPS reactor trip.
The actuation of the Reactor Protection System was reported to the NRC in accordance with 10CFR 50.72 (b)(2)(iv)(B), reference EN# 42957; and submittal of this LER constitutes reporting to the NRC in accordance with 10 CFR 50.73 (a)(2)(iv)(A).
DOCKET (21 LER NUMBER (61 PAGE (31
CAUSE OF EVENT
The root cause of the event is a latent design weakness within the DTCS. Subsequent testing demonstrated that a short circuit of a single milli-amp input to the DTCS signal processing board will affect all of the processor inputs associated with that board and initiate a main turbine trip signal from the in-service trip channels. The connection of the test lead during the transmitter calibration activities most likely resulted in a momentary short circuit on the DTCS input, which initiated the turbine trip from the two in-service trip channels.
ANALYSIS / SAFETY SIGNIFICANCE
The reactor protection system functioned as designed to initiate the automatic reactor trip in response to the trip of the main turbine. There were no engineered safeguard system actuations. The post-trip equipment response was within the expected range, operator response was appropriate, and stable hot shutdown conditions were established. The existing plant risk assessment (PRA) assumptions for turbine trip probability bound this event. The PRA assumes a probability of 2.65E-01/ yr for turbine trip frequency. This is based on generic industry and plant specific historical turbine trip data. Therefore, this event had minimal safety significance.
This event does not involve a safety system functional failure, which would be reported in accordance with NEI 99-02. All safety-related equipment performed in accordance with design in response to the event.
CORRECTIVE ACTIONS
Immediate and Short Term Actions:
All online transmitter preventative maintenance activities that provide inputs to the DTCS trip logic circuit have been suspended until the cause for the turbine trip is eliminated.
Long Term Corrective Actions:
A DTCS modification is being evaluated to prevent a single short circuit on a milli-amp input from causing a main turbine trip.
PREVIOUS OCCURENCES
There are no previous TMI Unit 1 reactor trips related to problems associated with the turbine digital control system.
- Energy Industry Identification System (ENS), System Identification (SI) and Component Function Identification (CFI) Codes are included in brackets, [SI/CFI] where applicable, as required by 10 CFR 50.73 (b)(2)(ii)(F).