05000458/LER-2004-005-01, Regarding Unplanned Automatic Scram Due to Loss of Non-vital 120 Volt Instrument Bus: Difference between revisions

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#REDIRECT [[05000458/LER-2004-005, Regarding Unplanned Automatic Scram Due to Loss of Non-vital 120 Volt Instrument Bus]]
| number = ML051790232
| issue date = 06/22/2005
| title = Regarding Unplanned Automatic Scram Due to Loss of Non-vital 120 Volt Instrument Bus
| author name = Lorfing D
| author affiliation = Entergy Operations, Inc
| addressee name =
| addressee affiliation = NRC/Document Control Desk, NRC/NRR
| docket = 05000458
| license number = NPF-047
| contact person =
| case reference number = RBF1-05-0096, RBG-46448
| document report number = LER 04-005-01
| document type = Licensee Event Report (LER)
| page count = 7
}}
{{LER
| Title = Regarding Unplanned Automatic Scram Due to Loss of Non-vital 120 Volt Instrument Bus
| Plant =
| Reporting criterion = 10 CFR 50.73(a)(2)(i), 10 CFR 50.73(a)(2)(vii), 10 CFR 50.73(a)(2)(ii)(A), 10 CFR 50.73(a)(2)(viii)(A), 10 CFR 50.73(a)(2)(ii)(B), 10 CFR 50.73(a)(2)(viii)(B), 10 CFR 50.73(a)(2)(iii), 10 CFR 50.73(a)(2)(ix)(A), 10 CFR 50.73(a)(2)(iv)(A), 10 CFR 50.73(a)(2)(x), 10 CFR 50.73(a)(2)(v)(A), 10 CFR 50.73(a)(2)(v)(B), 10 CFR 50.73(a)(2)(i)(A), 10 CFR 50.73(a)(2)(v), 10 CFR 50.73(a)(2)(i)(B), 10 CFR 50.73(a)(2)
| Power level =
| Mode =
| Docket = 05000458
| LER year = 2004
| LER number = 5
| LER revision = 1
| Event date =
| Report date =
| ENS =
| abstract =
}}
 
=text=
{{#Wiki_filter:A= Entergy Entergy Operations, Inc.
River Bend Station 5485 U.S. Highway 61 N St. Francisville, LA 70775 Tel 225381 4157 Fax 225 635 5068 dlorfin@entergy.com David N. Lorfing Manager-Licensing June 22, 2005 U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555
 
==Subject:==
Licensee Event Report 50-458 / 04-005-01 River Bend Station - Unit 1 Docket No. 50-458 License No. NPF-47 File Nos.
G9.5, G9.25.1.3 RBG-46448 RBF1 0096 Ladies and Gentlemen:
In accordance with 10CFR50.73, enclosed is the subject Licensee Event Report.
This report is a supplement to LER 50-458/04-005-00 submitted on February 8, 2005.
This document contains no commitments.
Sincerely, David N. Lorfing Manager - Licensing DNL/dhw Enclosure C!'_01--
 
Licensee Event Report 50-458 / 04-005-01 June 22, 2005 RBG-46448 RBF1 0096 Page 2 of 2 cc:
U. S. Nuclear Regulatory Commission Region IV 611 Ryan Plaza Drive, Suite 400 Arlington, TX 76011 NRC Sr. Resident Inspector P. 0. Box 1050 St. Francisville, LA 70775 INPO Records Center E-Mail Mr. Jim Calloway Public Utility Commission of Texas 1701 N. Congress Ave.
Austin, TX 78711-3326 Louisiana Department of Environmental Quality Office of Environmental Compliance Attn: Mr. Ronnie Wascom Emergency and Radiological Services Division P.O. Box 4312 Baton Rouge, LA 70821-4312
 
NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 06130/2007 6-2004)
, the NRC may digits/characters for each block) not conduct or sponsor, and a person Is not required to respond to. the information collection.
: 3. PAGE River Bend Station - Unit 1 05000458 1 of 5
: 4. TITLE Unplanned Automatic Scram Due to Loss of Non-vital 120 Volt Instrument Bus
: 5. EVENT DATE
: 6. LER NUMBER
: 7. REPORT DATE
: 8. OTHER FACILITIES INVOLVED SEUNILRVFACILITY NAME DOCKET NUMBER MONTH DAY YEAR YEAR SENUMENIAL NO.
MONTH DAY YEAR I 05000 FACILITY NAME DOCKET NUMBER 12 10 2004 2004 - 005 -
01 06 22 2005 05000
: 9. OPERATING MODE
: 11. THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR§: (Check al that apply) o 20.2201(b)
El 20.2203(a)(3)(i)
D3 50.73(a)(2)(i)(C)
El 50.73(a)(2)(vii) 1 C 20.2201(d) l 20.2203(a)(3)(ii)
El 50.73(a)(2)(ii)(A)
E 50.73(a)(2)(viii)(A)
C3 20.2203(a)(1) a 20.2203(a)(4) 0 50.73(a)(2)(ii)(B)
E 50.73(a)(2)(viii)(B) o 20.2203(a)(2)(i)
El 50.36(c)(1)(i)(A)
El 50.73(a)(2)(iii) al 50.73(a)(2)(ix)(A)
: 10. POWER LEVEL El 20.2203(a)(2)(ii)
[I 50.36(c)(1)(ii)(A)
ED 50.73(a)(2)(iv)(A)
D 50.73(a)(2)(x) o 20.2203(a)(2)(iii) a 50.36(c)(2)
El 50.73(a)(2)(v)(A)
El 73.71(a)(4)
El 20.2203(a)(2)(iv)
Dl 50.46(a)(3)(ii) al 50.73(a)(2)(v)(B)
[I 73.71(a)(5) 100 E 20.2203(a)(2)(v)
El 50.73(a)(2)(i)(A)
El 50.73(a)(2)(v)(C) 0 OTHER El 20.2203(a)(2)(vi)
[I 50.73(a)(2)(i)(B)
El 50.73(a)(2)(v)(D)
Specify in Abstract below or In NLRC Form 366A
: 12. LICENSEE CONTACT FOR THIS LER FACILITY NAME David N. Lorfing, Manager - Licensing TELEPHONE NUMBER (Include Area Code) 225-381-4157
: 14. SUPPLEMENTAL REPORT EXPECTED El YES (If yes, complete 15. EXPECTED SUBMISSION DATE)
ABSTRACT (Limit to 1400 spaces, ie., approximately 15 single-spaced typewritten lines)
On December 10, 2004, at 1:17 p.m. CST, an automatic reactor scram occurred while the plant was operating at 100 percent power. The scram was the indirect result of the failure of a 120 volt AC uninterruptible power supply (UPS) on a non-safety related instrument bus. The UPS failed due to a shorted capacitor on an internal circuit board. The loss of the instrument bus resulted in the downshift of the reactor recirculation pumps to slow speed and the lockup of the main feedwater regulating valves. The decrease in coolant flow caused a flow-biased simulated thermal power signal to be sensed in the average power range monitoring system, which actuated the reactor protection system (RPS). The reactor core isolation cooling (RCIC) system was manually started in response to the failure of the feedwater regulating system. RCIC subsequently shut down as designed due to a high reactor water level signal. While it was idle, an alarm actuated indicating the presence of water in the RCIC turbine exhaust drain trap. A conservative decision was made to leave RCIC out of service and to start the high pressure core spray (HPCS) system as needed for reactor water level control. The reactor scram occurred as designed, and reactor water level was maintained above the low-low alarm setpoint. This event is being reported in accordance with 10CFR50.73(a)(2)(IV)(A) as a condition that resulted in the actuation of the RPS, RCIC, and HPCS systems. This event was of minimal safety significance.
NRC FORM 366(6-2004)
PRINTED ON RECYCLED PAPER NRC FORM 366 (6-2004)
PRINTED ON RECYCLED PAPERU.S. NUCLEAR REGULATORY COMMISSION (1-2001)
LICENSEE EVENT REPORT (LER)
FAILURE CONTINUATION
: 1. FACILITY NAME
: 2. DOCKET
: 6. LER NUMBER
: 3. PAGE YEAR lSEQUENTIAL I REV(ISION Y
NUMBER NUMBER River Bend Station-Unit 1 05000-458 2004 005 01 2
OF 5
REPORTED CONDITION On December 10, 2004, at 1:17 p.m. CST, an automatic reactor scram occurred while the plant was operating at 100 percent power. The scram was the indirect result of the failure of a 120 volt AC uninterruptible power supply (**INVT**) on a non-safety related instrument bus. The loss of the instrument bus resulted in the downshift of the reactor recirculation pumps to slow speed and the lockup of the main feedwater regulating valves. The decrease in coolant flow caused a flow-biased simulated thermal power signal to be sensed in the average power range monitoring system, which actuated the reactor protection system (RPS).
The reactor core isolation cooling (RCIC) system was manually started in response to the failure of the feedwater regulating system. RCIC subsequently shut down as designed due to a high reactor water level signal. While it was idle, an alarm indicating the presence of water in the RCIC turbine exhaust drain trap actuated. A conservative decision was made to leave RCIC out of service and to start the high pressure core spray (HPCS) system as needed for reactor water level control.
This event is being reported in accordance with 10CFR50.73(a)(2)(IV)(A) as a condition that resulted in the actuation of the RPS, RCIC, and HPCS systems.
There were no safety systems out of service at the beginning of the event.
INVESTIGATION AND IMMEDIATE CORRECTIVE ACTIONS Troubleshooting of the UPS discovered a failed circuit board. This caused the shutdown of the UPS and loss of power to the instrument bus supplying logic circuits in the reactor recirculation system and power for the feedwater control system. The loss of the instrument power supply caused the reactor recirculation pumps to shift to slow speed, and caused the main feedwater regulating valves to lock up at the position corresponding to full reactor power, as well. Both of these are expected responses for this malfunction.
When the RCIC system was manually started, it operated for 11 seconds and then the turbine steam supply valve closed as designed due to a high reactor water level signal. While RCIC was idle, an alarm actuated to indicate the presence of water in the RCIC turbine exhaust line drain trap. This was conservatively assessed by the operators as a condition that could potentially cause damage to the turbine if it was restarted. Subsequent evaluations have determined that the RCIC system could have been restarted with the alarm NRC FORM 3668 (1-2001)U.S. NUCLEAR REGULATORY COMMISSION (1-2001)
LICENSEE EVENT REPORT (LER)
FAILURE CONTINUATION
: 1. FACILITY NAME
: 2. DOCKET
: 6. LER NUMBER
: 3. PAGE I ER SEQUENTIALI REVISION YEAR NUMBER NUMBER River Bend Station - Unit 1 05000-458 2004 005 01 3
OF 5
actuated. No damage would have occurred, and the system would have performed its design function.
The instrument bus was shifted to an alternate power source by placing the UPS in the "manual bypass" mode. The feedwater regulating system was restored to service at approximately 4:57 p.m. on the same day, and the HPCS system was returned to its standby configuration.
The five reactor water level instrument (**LI**) ranges include narrow range, wide range, and upset range indicators. The individual channels may be monitored on the Emergency Response Information System (ERIS) computer, as well as on recorders installed in the operator's control panels. The maximum reading on the narrow and wide range control panel recorders is 60 inches, however due to input signal and programming configuration the digital readout on the recorders can indicate levels above the 60 inch maximum. The maximum indication on the upset range recorder is 180 inches. The ERIS graphic displays have a greater scale, but in the training simulator, the narrow and wide range levels are clamped at 60 inches maximum. The wide range recorders in the simulator indicate "Over Range" when level is above 60 inches. Following the scram, the control room ERIS graphic displays for the narrow and wide range instruments increased to 88 inches and 144 inches, respectively, and the wide range recorder digital readout increased to 141 inches. These indications correspond to water levels which are above the reference leg taps on the reactor vessel, such that the readings could not be considered valid. The operators were confused by these level indications as they were not consistent with responses seen in the simulator for similar events. The upset range indication did not function properly following the scram, as it is powered by the instrument bus that was de-energized by the UPS failure.
CAUSAL ANALYSIS The logic power for controlling the static switch is fed through a single filter, creating a single-point vulnerability. Failure at this point caused a loss of UPS output. The UPS is an Elgar Model UPS-503-1-102. The failure of the UPS was caused by the short circuit of a tantalum capacitor (**CAP**) on the static switch silicon-controlled rectifier (**SCR**) drive board. Elgar UPS's are original to the plant and were manufactured in the early 1980's. Most circuit boards in the UPS's have not been replaced. Static switch SCR drive boards have not failed historically at River Bend. A review of external operating experience found no evidence to suggest that SCR drive boards are a significant problem. The NRC FORM 3668 (1-2001)U.S. NUCLEAR REGULATORY COMMISSION (1-2001)
LICENSEE EVENT REPORT (LER)
FAILURE CONTINUATION
: 1. FACILITY NAME
: 2. DOCKET
: 6. LER NUMBER
: 3. PAGE YEAR SEQUENTIAL REVISION YEA NUMBER NUMBER River Bend Station - Unit 1 05000-458 2004 005 01 4
OF 5
failure is apparently a random phenomenon, and not related to age, environment, or preventative maintenance. Data reviewed thus far indicates that tantalum capacitors are not susceptible to age related failures. Most failures are random in nature, and no clear pattern has been established to predict failure. Since the problem is random, no common mode failure mechanism exists in the other UPS's installed in the plant.
The cause of the RCIC turbine exhaust trap high water level alarm has been attributed to the specific operational cycle that occurred in this event. The discharge line contains vacuum breakers (**VACB**) to preclude siphoning water from the suppression pool. In this event, the short duration of RCIC operation caused the maximum vacuum to develop. The vacuum breakers do not have sufficient flow area to compensate for this scenario. Water was siphoned up the exhaust line from the pool, and was sensed in the drain traps.
CORRECTIVE ACTIONS TO PREVENT RECURRENCE The failed circuit board in the UPS was replaced, and the UPS was restored to operation. During troubleshooting and maintenance, the instrument bus had been shifted to a separate UPS where it remains. While the malfunction that caused this event does not represent a common-mode failure mechanism, as a conservative measure, plans are being developed to replace the static switch SCR drive board in the other Elgar UPS applications. This will be tracked in the station corrective action program.
The RCIC system was determined to be capable of performing its design function despite the presence of water in the turbine exhaust line. A documented evaluation of this condition has been completed. Plans are being developed to install a nozzle check valve in the RCIC turbine exhaust line. This action is being tracked in the station corrective action program.
Regarding the difference between the responses of the reactor water level indicators in the plant compared to the simulator, a modification has been installed in the simulator to eliminate this disparity.
Engineering design work has been initiated to improve the robustness of both the UPSs and their supplied loads. Detailed single-point vulnerability reviews will be conducted on the UPSs. Methods for hardening the circuits supplied by the UPSs will be examined. Both of these actions are being be tracked in the station corrective action program.U.S. NUCLEAR REGULATORY COMMISSION (1.2001)
LICENSEE EVENT REPORT (LER)
FAILURE CONTINUATION
: 1. FACILITY NAME
: 2. DOCKET
: 6. LER NUMBER
: 3. PAGE I
NUMBER lNUMBER River Bend Station - Unit 1 05000-458 2004 005 01 l
5 OF 5
PREVIOUS OCCURRENCE EVALUATION A review of reportable events over the last three years found no occurrence of a similar event.
 
==SAFETY SIGNIFICANCE==
This event is bounded by analyzed transient for a decrease in reactor recirculation flow in the RBS Updated Safety Analysis Report. As detailed above, the RCIC system was capable at all times of performing its design function.
Thus, this event was of minimal safety significance.
(NOTE: Energy Industry Component Identification codes are annotated as
(**XX**).)
}}
 
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