05000305/LER-2006-012, Regarding Reactor Trip from Loss of Instrument Bus
| ML070660045 | |
| Person / Time | |
|---|---|
| Site: | Kewaunee  |
| Issue date: | 12/29/2006 |
| From: | Hartz L Dominion Energy Kewaunee |
| To: | Document Control Desk, NRC/NRR/ADRO |
| References | |
| 06-1013 LER 06-012-00 | |
| Download: ML070660045 (6) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition 10 CFR 50.73(a)(2)(ix)(A) 10 CFR 50.73(a)(2)(x) 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2) 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(i) 10 CFR 50.73(a)(2)(viii)(B) |
| 3052006012R00 - NRC Website | |
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N-490 [ligIlw;~y 42' oauritce, \\VI 54112)16&9j5 II ojhijn DEC 2 9 2,05 U. S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, DC 20555 Serial No.
06-1013 KPS/LIC/RS: RO Docket No.
50-305 License No. DPR-43 DOMINION ENERGY KEWAUNEE, INC.
KEWAUNEE POWER STATION LICENSEE EVENT REPORT 2006-012-00
Dear Sirs:
Pursuant to 10 CFR 50.73, Dominion Energy Kewaunee, Inc., hereby submits the following Licensee Event Report applicable to Kewaunee Power Station.
Report No. 50-305/2006-012-00 This, report has been reviewed by the Plant Operating Review Committee and will be forwarded to the Management Safety Review Committee for its review.
If you have any further questions, please contact Mr. Richard Sattler at (920) 388-8121.
Very truly yours, X/'
Leslie N. Hartz Site Vice President, Kewaunee Power Station Attachment Commilments made by this letter: NONE fA~
/~I4 V
Serial No, 06-1013 Page 2 of 2 cc:
Regional Administrator, Region III U.S. Nuclear Regulatory Commission 2443 Warrenville Road Suite 210 Lisle, IL 60532-4352 Mr. L. Raghavan PrOject Manager U.S. Nuclear Regulatory Commission Mail Stop 8 H4A Washington, D. C. 20555 NRC Senior Resident Inspector Kewaunee Power Station
- &2004r LICEI U.S. NUCLEAR REGULATORY COMMISSION I APPROVED BY OMB NO. 3150-0104 tAI' IHLt b-5JU-ýdUUt NSEE EVENT REPORT (LER)
(See reverse forrequired number of digits!characters for each block)
Estimated burden per response to comply with this mandatory collection rtoquest: Z lours.
Reported lessons learned are incorporated into the Iicensig process and fed back to indulrs',
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FACILITY NAME (1,)
DOCKET NUMBEE(2)i PAGE 33)
F Kewaunee Power Station 050003 05 UE of 4
TITLE (4)
Reactor Trip from Loss of Instrument Bus EVENT DATE (')
LER NUMBER (6)
REPORT DATE (7)
OTHER FACILITIES INVOLVED (8)
SEoUENTIAL REV FACILITY NAME DOCKET NUMBER MO DAY YEAR YEAER I.,NO iMO DAY YEAR 10 30 2006 2006 -
012 00 12 29 2006 FACILITY NAME DOCKET NUMBER OPERATING THIS REPORT IS SUBMITTEo PURSUANTTO THE REOUIREMENTS OF 10 CFR.: (Check W*tI hal apply) (it)
MODE (9)
N 2022 1(b) 20.2203(a)(3)(ii) 50.73(a)(2)(ii)(B) 50.73(a)(2)(ix)(A).
POWER 20,2201 (d) 20.2203(a)(4) 50:73(a)(2)(iii) 50.73(a)(2)(x)
LEVEL (10) T 93
" 20.2203(a)(11) 50.36{c)(1)(i)(A)
X 50.73(a)(2)(iv)(A) 73.7 1(a)(4) 20.2203(a)(2)(i) 50.36(c)(1)(ii)(A)
.50I73(a)(2)(V)(A) 73.7 1 (a)(5) 20.2203(a)(2)(ii) 50.36(c)(2) 50.73(a)(2)(V()
__ OTHER 20.2203(aX2)(iii) 50.46(a)(3)(ii) 50,73(a)(2)(v)(C)
Specify in Abstract below or in 20,2203(a)(2)(iv) 50.73(a)(2)(i)(A) 50.73(a)(2)(v)(D) 20,2203(a)(2)(v) 50,73(a)(2)(i)(B) 50.73fa)(2)(vii) 20.2203(a)(2)(vi) 50.73(a)(2)(i)(C) 50,73(a)(2)(viii)(A) 20.2203(a)(3)(i) 50.73(a)(2y(ii)(A) 50.73(a)(2)(viii)(B)
LICENSEE CONTACT FOR THIS LER (12) lE TELEPHONE NUMBER (include. Area Code)
.,ch Sattlerj (920) 388-8121 COMPLETE ONE LINE FOR EACH COMPONENT FAILURE DESCRIBED IN THIS REPORT (13)
"M"J, U.
REPORTABLE MANrU.
O ýOti'AE
CAUSE
SYSTEM COMRPONENT FACTURER TO EPIX
CAUSE
SYSTEM COMPONEtjr FA CTURER TO EPIX SUPPLEMENTAL REPORT EXPECTED (14)
EXPECTED MONTH DAY YEAR Y ES Iys, complete EXPECTED SUBMISSION DATE).
DAT (1O SUM1SO ABSTRACT At 8:48 CST on 10/30/2006, Kewaunee Power Station tripped from approximately 92% full power due to steam generator B steam flow/feedwater flow mismatch coincident with low water level on steam generator B. The cause of the plant transient that led to the reactor trip was a loss of the red instrument bus. The bus deenergized due to component failure during the performance of maintenance on its inverter.
Following the reactor trip; o The auxiliary feedwater pumps automatically started, as designed, due to low-low level in the steam generators. No other safeguards systems actuated during the transient.
Non-safety related valve MS-201 81, steam supply to reheater 1 B reheater inlet valve, did not fully close.
This resulted in pressurizing high pressure feedwater heater (158) causing relief valve HD-130B, 15B feedwater heater relief valve, to lift.
o After the turbine trip, during bus transfer, nonm-safety related 4160V reserve auxiliary transformer breaker failed to close resulting in bus 4 losing power. This resulted in a loss of circulating water flow, which, in turn, caused a loss of condenser vacuum and condenser steam dump valves.
s event is being reported under 10 CFR 50.73(a)(2)(iv)(A) as an event or condition that resulted in automatic actuation of the reactor protection system and the auxiliary feedwater system.
IU.S. NUCLEAR REGULATORY COMMISSION 1 (12001)
LICENSEE EVENT REPORT (LER)
TEXT CONTINUATION FACILITY NAME (1)
DOCKET NUMBER (2)
LER NUMBER (6 PAGE (3)
K SEQUENTIAL REVISION Kewaunee Power Station 05000305 NUMBER NUMBER 2 of 4 2006 012 00 TEXT (11 mwe space is required, use additional copies of NRC Form 366A) (17)
EVENT DESCRIPTION
At 8:48 CST on 10/30/2006, shortly after a scheduled increase in reactor power from 88% to 92%, and during preventative maintenance to adjust the free running frequency of inverter BRA-i 11 [INVTj, the inverter output failed. This deenergized instrument bus 1 tEE] which resulted in the steam generator (SG)[SG] B feedwater regulating valve [SJJ[LCVJ going closed. This caused a steam flow to feed flow mismatch which resulted in a reactor trip on "SG B Steam Flow Greater Than Feedwater Flow" coincident with low water level in SG B [JB].
Following the reactor trip;
- The auxiliary feedwater (AFW) pumps [BA][P] automatically started, as designed, due to low level in the SGs.
Non-safety related valve MS-201 El, steam supply to reheater 1B reheater inlet valve IV], did not fully close: This resulted in pressurizing high pressure feedwater heater 15B [HX] causing HD-1 30B, 15B feedwater heater relief valve [RV], to lift.
After the turbine, trip, during bus transfer, non-safety related 4160V reserve auxiliary transformer (RAT)
[XFMR) breaker [BKR] 1-407 failed to close resulting in bus 4 losing power. This resulted in a loss ol circulating water (KE] flow, which, in turn, caused a loss of condenser vacuum [SH] and condenser steam dumps [JII[FCV].
.er quarantine and a preliminary investigation, 4160 volt circuit reserve aux transformer breaker 1-407 was
,oplaced with a spare breaker and power was restored to bus 4.
By 11:00 CST, the plant status was; at normal operating temperature and pressure with the unaffected circulating water pump [P] running and condenser vacuum restored.
EVENT ANALYSIS anrd SAFETY SIGNIFICANCE The plant trip was caused by the inverter failure which caused the SG B feedwater regulating valve to fail closed resulting in "SG B Steam Flow Greater Than Feedwater Flow" coincident with low water level in SG B.
Thle inverter failure was determined to.be due to failure of the, static switch silicon controlled rectifiers (SCRs)[SCR], which caused an over current condition on the SCR gating circuit, damaging the gating resistors. The vendor was contacted and recommended against replacement 'of the same SCRs in the other station inverters (since they have no expected in-service life expectancy and no recommended preventative maintenance(PM)). Based on this and the operating and PM history of these inverters, the decision was made to limit the extent of condition to the failed inverler only.
The breaker failure was determined to be due to a failed clos ing coil interlock switch [331. A misalignment of the roll pin for the interlock switch actuating arm allowed excessive free movement to occur during breaker racking, closing, opening, and spring charging. The free movement allowed a misalignment to occur while actuating the interlock switch. This misalignment impacted the switch by the application of forces not parallel to the plunger normal travel. The forces, over time, damaged the switch enclosure. The damaged enclosure impeded the contacts of the switch internals to correctly make up (normally closed) during the event.
investigation, revealed that there -were other similar non-safety related breakers (load and supply) busceptible to this failure mechanism. The safety related breakers are of a different design and thus not NRC FORM, 366A (1-2001)U.S. NUCLEAR REGULATORY COMMtSSIONi
(.t-20D31)
LICENSEE EVENT REPORT (LER)
TEXT CONTINUATION FACILITY NAME (1')
OOCRETf NUMBER'4213 LER NUMBER 6PAG (3)
I Kewaunee.Power Station 05000305 YEAR SEQUENTrAL R3SON of 4 I
2006 012 00 TEXT (U *mfore space is required, use additional coies of NRC Foon 3GWA) (17) subject to. the same failure mechanism. For the non-safety breakers, the damage is on the back side of the switch enclosure and is not visible without some disassembly. The non-safety breakers will be inspected as plant conditions permit, but the breakers of highest concern are the other three supply breakers with an automatic closure function (supply from the RAT). These supply breakers closed properly-following the plant trip, and are required to be closed to maintain their buses energized with the plant off-fine. All affected breakers have been scheduled for inspection in accordance with the Work Management Process.
The sticking of the steam supply to reheater 1 B reheater inlet valve was determined to be due to a galled valve plug (showing that the bottom bonnet was not installed squarely). The valve stem was replaced, the valve plug was polished, the packing was replaced, and the valve was tested satisfactorily. No other valves exhibited sticking during the trip, so the extent of condition was limited to this valve only. Subsequent to the recovery from this event, another reactor trip occurred and this valve stuck again. For that event, an inspection of the valve was performed and stem bushings were replaced.
With the exceptions noted above, the plant responded as designed. All safety-related equipment performed its safety-related functions as designed. There was a loss of normal heat sink, but even though it was consciously not placed into immediate service, the alternate circulating water pump was available throughout the eventand could have been used to restore. the normal heat sink. There were no plant or public health and qafety consequences. Therefore, this event is considered to have negligible safety significance.
. post-trip event review was performed and, following troubleshooting, and repairs, the plant was restarted at 04:27 CST on.11/1/06.
This event is being reported under 10 CFR 50.73(a)(2)(iv)(A) as an event or condition that resulted in an automatic actuation of the Reactor Protection System and the AFW System.
CAUSE and CORRECTIVE ACTIONS
Inverter Failure The cause oftthe inverter failure was determined to be failure of the static switch SCRs. which caused an over current condition on the SCR gating circuit, damaging the gating resistors.
By 21:22 CSTon 10/31/06, the inverter had been repaired and restored to normal operation.
Reserve Aux Transformer 4160V Breaker The cause of the breaker failure was determined to be a failed closing, coil interlock switch [33]. The failed supply breaker was replaced with a spare breaker and power was 'restored, to Bus 4. The failed supply breaker was subsequently repaired. The other three supply breakers were scheduled for inspection when the main auxiliary transfo-rmer is' supplying house loads. The load, breakers were scheduled for inspection as plant conditions permit.
NRC FORM.366A (1 -2001)U.S. NUCLEAR REGULATORY COMMISSION (1,2o01)
LICENSEE EVENT REPORT (LER)
TEXT CONTINUATION FACILITY NAME (1)
DOCKET NUM2ER 2
LER NUMBER (6 PAGE (3)
Kewaunee Power Station YEALEuVSiON L4 4
S2006 012 00 TEXT (1! more space is required, use adc;iionra copies of NRC Fotm 366A) (17)
Steam Supply To Reheater 1 B Reheater Inlet Valve The cause of the valve sticking was determined to b.e a galled valve, plug (showing that-the bottom bonnet was not installed squarely). This valve had previously failed to close( during the shutdown for the outage on 9/2/06 and was repaired during that outage. Following this occurrence, the valve stem was replaced, the valve plug was polished, the-packing was. replaced, and the valve was tested satisfactorily. Subsequent to the recovery from this event, another trip occurred and this valve stuck again. For that event, an inspection of the valve was performed and stem bushings were replaced.
PREVIOUS SIMILAR EVENTS
LER.84-014, Reactor Trip Due To A Loss Of Power On Instrument Bus IV