05000289/LER-2006-002, Re Automatic Reactor Trip Due to an Invalid Turbine Protective System Actuation
| ML070100307 | |
| Person / Time | |
|---|---|
| Site: | Crane  |
| Issue date: | 01/02/2007 |
| From: | Dougherty T AmerGen Energy Co |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| 5928-06-20613 LER 06-002-00 | |
| Download: ML070100307 (5) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(1) 10 CFR 50.73(a)(2) 10 CFR 50.73(a)(2)(iii) 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 2892006002R00 - NRC Website | |
text
AmerGenm AmerGen Energy Company, LIC Telephone: 717-948-8000 An Exelon Company Three Mile Island Unit 1 Route 441 South, P.O. Box 480 Middletown, PA 17057 January 2, 2007 5928-06-20613 10 CFR 50.73 U.S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, D.C. 20555 THREE MILE ISLAND NUCLEAR STATION, UNIT 1 (TMI-1)
OPERATING LICENSE NO. DPR-50 DOCKET NO. 50-289 SUBJECT: LICENSEE EVENT REPORT (LER) NO. 2006-002-00 "Automatic Reactor Trip Due to an Invalid Turbine Protective System Actuation" This report is being submitted in accordance with 10 CER 50.73 (a)(2)(iv)(A). For additional information regarding this LER contact Adam Miller of TMI Unit 1 Regulatory Assurance at (717) 948-8128.
Plant Manager TJD/awm ATTACHMENT: List of Regulatory Commitments cc:
TMI Senior Resident Inspector Administrator, Region I TMI-1 Senior Project Manager File No. 06058
SUMMARY OF AMERGEN ENERGY CO. L.L.C. COMMITMENTS The following table identifies commitments made in this document byAmerGen Energy Co. L.L.C.
(AmerGen). Any other actions discussed in the submittal represent intended or planned actions by AmerGen. They are described to the NRC for the NRC's information and are not regulatory
commitments
COMMITMENT
COMMITTED DATE OR "OUTAGE" No regulatory commitments are being made in this submittal.
N/A
NRIC FORqM 366 U.S. NUCLEAR REGULATORY COMMISSION
, the N4RC may not conductor sponsor, and a person is not required to respond to. the information collection.
FACILITY NAME (1)
~f DOCKET NUMBER (2)
J PAGE (3)
Three Mile Island, Unit 1 ii 05000289 1 OF 3 TITI F: IAI Automatic Reactor Trip Due to an Invalid Turbine Protective System Actuation________
EENT DATE (5)
LER NUMBER (6)1 REPORT DATE (7)
OTHER FACILITIES INVOLVED (8)
SEQUENTIAL REVISION FACILITY NAME [DOCKET NUMBER MNH DAY YEAR YEAR F
NUMBER NUMBER MONTH DAY YEAR N/A N/A
~Z
~ Z FACILITY NAME j DOCKET NUMBER 11 0 006.12006
,,002 00 01 02 2007 N/A I
N/A OPERATING 1
.... THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIR~EMENTS OF 10 CFR §:(Check neo or)(1 MODE (9)
N J20.2201 (b) 20.2203(a)(2)(v) 1
- - 50.73(a)(2)(1) 150.73(a)(2)viii)
POWER I
]-20.2203(a)(1) 20.2203( )(3)(1) 1
- - 50.73(a)(2)(11) 50.73(a)2)(4 LEVEL (10) 1100 j-20.2203(a)(2)(i) 20.2203(a)(3)(Ii) 50.73(a)(2)(iii)
I 73.71
__20.2203(a)(2)(li) 20.2203(a)(4)
X -50.73(a)(2)(Iv)
OTHER
__20.2203(a)(2)(lii)
__50.36(c)(1)
- - 50.73(a)(2)(v)
ISpecify in Abstract below or J20.2203(a)(gg(iv) 1_ 150.36(c)(2) 50.73(a)(2)(vil) in NRC Form 366A LICENSEE CONTACT FOR THIS LER t12)
NAME ITELEPHONE NUMBER (include Area Code)
Adam W.Miller of TMI-1 Regulatory Assurance 1(717) 948-8128 COMPLETE ONE LINE FOR EACH COMPONENT FAILURE DESCRIBED IN THIS REPORT (3)~
REPORTABLE RETIR PORTA LE CAUS SYTEM COMPONENT MANUFACTURER TO EPIX CAUSE jSYSTEM COMPONENT.fMANUFACTURER TO EPIX NANAN/A N/A N/A N/A I
NI N/A I
N/A N/A SUPPLEMENTAL REPORT EXPECTED (14)
[
EXPECTED IMONTH DAY YEAR YES I
SUBMISSION (If yes. como~lete EXPECTED SUBMISSION DATE).
FXI NO
[
DATE (115)
ABSTRACT (Limit to 1400 spaces, i.e., approximately 15 single-spaced typewritten lines) (16)
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 Isl and, Unit 1 (TMI-1) reactor tripped from 100%
power during a calibration check of the main turbine Digital Turbine Control System (DTCS) 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 root cause of the turbine trip 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. 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.
The actuation of the Reactor Protection System was reported to the NRIC in accordance with 1 OCER 50.72 (b)(2)(iv)(B), reference EN 42957. Submittal of this LER constitutes reporting to the NRC in accordance with 10 CER 50.73 (a)(2)(iv)(A).
- - 'I..
DOCKET (21 LER NUMBER (6)I PAGE (31 ISEQUENTIAL I REVISION YEAR I
NUMBER I NUMBER 050289i 2006 002 00 2
OF 3
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 (TM I-i) reactor tripped f rom 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 TM I-i 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 1 OCER 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 (13 ISEQUENTIAL REVISION YEAR INUMBER INUMBER 050289i 2006 002 00 3S OF 3
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 TMVI Unit 1 reactor trips related to problems associated with the turbine digital control system.
- Energy Industry Identification System (EIIS), System Identification (SI) and Component Function Identification (CFI) Codes are included in brackets, [SI/CEI] where applicable, as required by 10 CFR 50.73