|Joseph M Farley Nuclear Plant, Unit 2|
|Reporting criterion:||10 CFR 50.73(a)(2)()|
10 CFR 50.73(a)(2)(v), Loss of Safety Function
10 CFR 50.73(a)(2)(i)
|3642017005R00 - NRC Website|
|Person / Time|
|Site:||Joseph M Farley Nuclear Plant, Unit 2, Farley|
|From:||Madison D R|
Southern Nuclear Operating Co
Document Control Desk, Office of Nuclear Reactor Regulation
|Download: ML18011A702 (5)|
During reactor startup on November 13, 2017 at 0136, while at approximately 1.5% power (MODE 2), an Excore Power Range Nuclear Instrument (N-42) [EIIS:IG] was declared inoperable due to lower than expected detector amps and indicated power. N-42 was reading approximately 0.4% lower power than the other three Power Range instruments. The malfunction was determined to be the result of a failed High Voltage (HV) cable center pin connector to N-42. The HV cable connector was installed during N-42 resealing on November 10, 2017 in preparation for startup physics testing and entry into MODE 2. Entry into MODE 2 occurred on November 12, 2017 at 1138. Technical Specification (TS) 3.3.1, Function 6, requires three operable channels of Over Temperature Delta Temperature (OTDT) during MODES 1 and 2.
N-42 provides an input signal to Channel 2 of the OTDT Reactor Trip Signal [ElIS:JC], while N-41 (Channel 1) and N-43 (Channel 3) supply the remaining input signals. On November 11, 2017 at 0522 (prior to MODE 2 entry), Channel 3 of OTDT had also been declared inoperable due to a failed Pressurizer Pressure Transmitter (PT-457) [EllS:PIT]. The N-42 failure was identified and declared inoperable as Power Range Nuclear Instruments came on scale on November 13, 2017 at 0136 following completion of startup physics testing. This condition existed until Channel 3 of OTDT was restored on November 13, 2017 at 0115. Therefore, it was determined that when MODE 2 was entered, N-42 was inoperable and that two channels of OTDT were inoperable longer than allowed by TS 3.0.3. This condition is reportable per 10CFR50.73(a)(2)()(8).
EVENT CAUSAL ANALYSIS:
The N-42 instrument failure was caused by a bent center conductor that prevented the instrument from making full contact with the N-42 circuit and providing HV power to the instrument. To rescale N-42, the HV connectors on the back of the drawer are disconnected such that the instrument is isolated. During this evolution the center pin on one of the connectors became bent or damaged during reinstallation by the Maintenance journeyman. These HV cable connectors are assembled in place and are not prefabricated. Due to repetitive removal and installation of these connectors over a cycle, it is not uncommon for one to fail or become damaged. Since there is no design test circuit, this condition was not detectable until neutron flux increased to a point where the detectors displayed actual values.
This event was classified as a Maintenance fundamental failure due to the HV power supply center pin being either damaged before or during installation. Verifying the proper alignment and ensuring that the connectors are not damaged prior to installation is a fundamental part of manipulating electrical connections on plant equipment.
REPORTABILITY AND SAFETY ASSESSMENT:
N-42 is part of the Nuclear Instrumentation System (NIS) which consists of two Source Range instruments (N-31, N-32), two Intermediate Range instruments (N-35, N-36), and four Power Range instruments (N-41, N-42, N-43, N-44). Per TS 3.3.1, Reactor Trip System (RTS) Instrumentation, three channels of OTDT are required in MODES 1 and 2. OTDT consists of three channels fed by N-41, N-42, and N-43 respectively. Thus, Farley unknowingly operated in a condition prohibited by TS as a result of two channels (Channel 2 and Channel 3) of OTDT being inoperable upon entry into MODE 2. This condition is reportable per 10CFR50.73(a)(2)(iXB).
The OTDT trip function is provided in a 2 out of 3 trip logic to ensure that the design departure from nucleate boiling ratio (DNBR) is met. During this event Channel 1 of OTDT was fully OPERABLE. Prior to entering MODE 2 the bistables for PT-457 (Channel 3 of OTDT) had been tripped by placing the channel in "Test" which is considered an actuated condition.
Even though Channel 2 of OTDT was inoperable, the safety function was met with Channel 1 fully OPERABLE and with Channel 3 bistables tripped. Additionally, the design of the Intermediate Range instruments (2 channels) provide redundant protection to the low setpoint of the Power Range instruments. Therefore, this is not reportable as a loss of safety function under 10CFR50.73(a)(2)(v).
N-42 HV cable connector was repaired on November 14, 2017.
Revise rescaling procedures for disconnecting and reconnecting signal cables to reinforce the need to take care of the center conductor to preclude damage; include required Inspections be performed after removal and prior to reinstallation; and to require caution during reinstallation to ensure connectors thread on smoothly and are fully engaged in the jack.
Communicate lessons learned through the Maintenance organization.
Training related to electrical connection fundamentals to be included in initial and continuing Maintenance training.
No similar events were identified.
OTHER SYSTEMS AFFECTED:
All systems affected were listed by EllS codes.