05000456/LER-2011-004, Regarding Non-Conservative Historic Assumptions Identified in Turbine Building High Energy Line Break Analyses Result in Potential for Two Channels in a System to Become Inoperable
| ML113570180 | |
| Person / Time | |
|---|---|
| Site: | Braidwood  |
| Issue date: | 12/22/2011 |
| From: | Enright D Exelon Generation Co, Exelon Nuclear |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| BW110146 LER 11-004-00 | |
| Download: ML113570180 (5) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i) 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded 10 CFR 50.73(a)(2)(viii)(A) 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition 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), System Actuation 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4562011004R00 - NRC Website | |
text
Exelon GeneratlOn Company, LLC Braidwood Station 35100 South Route 53. Suite 84 Braceville.IL 60407-96'9 December 22, 2011 BW110146 www.exeloncorp.com Exelon Nuclear 10 CFR 50.73 U. S. Nuclear Regulatory Commission ATIN: Document Control Desk Washington, DC 20555-0001 Braidwood Station, Units 1 and 2 Facility Operating License Nos. NPF-72 and NPF-77 NRC Docket Nos. STN 50-456 and STN 50-457
Subject:
Licensee Event Report 2011-004 Non-Conservative Historic Input Assumptions Identified in Turbine Building High Energy Line Break Analyses Result in Potential for Two Channels in a System to Become Inoperable The enclosed Licensee Event Report (LER) is being submitted in accordance with 10 CFR 50.73, "Licensee event report system." The potential significance of this condition is being assessed and will be documented in a supplement to this LEA.
There are no regulatory commitments contained in this letter. Should you have any questions concerning this submittal, please contact Mr. Chris VanDenburgh, Regulatory Assurance Manager, at (815) 417-2800.
Respectfully,
~I~~
Site Vice President Braidwood Station
Enclosure:
LER 2011-004-00 cc: NRR Project Manager - Braidwood Station Illinois Emergency Management Agency - Division of Nuclear Safety US NRC Regional Administrator, Region III US NRC Senior Resident Inspector (Braidwood Station)
Illinois Emergency Management Agency - Braidwood Rep
NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION APPRoVED eY OMB: No. 3150-0104 EXPIRES: 10/31/2013
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, the NRC may digits/characters for each block) not conduct or sponsor, and a person is not required to respond to, the information collection.
- 13. PAGE Braidwood Station, Unit 1 05000456 1 of 4
- 4. TITLE Non-Conservative Historic Assumptions Identified in Turbine Building High Energy Line Break Analyses Result in Potential for Two Channels in a System to Become Inoperable
- 5. EVENT DATE
- 6. LER NUMBER
- 7. REPORT DATE
- 8. OTHER FACILITIES INVOLVED YEAR ISEQUENTIALIREV FACILITY NAME DOCKET NUMBER MONTH DAY YEAR NUMBER NO.
MONTH DAY YEAR Braidwood Station, Unit 2 05000457 FACILITY NAME DOCKET NUMBER 11 01 2011 2011 - 004 - 00 12 22 2011 N/A N/A
- 9. OPERATING MODE
- 11. THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR §: (Check all that apply) o 20.2201(b) o 20.2203(a)(3)(i) o 50.73(a)(2)(i)(C) 181 50.73(a)(2)(vii) 1 o 20.2201 (d) o 20.2203(a)(3)(ii) o 50.73(a)(2)(ii)(A) o 50.73(a)(2)(viii)(A) o 20.2203(a)(1) o 20.2203(a)(4) o 50.73(a)(2)(ii)(B) o 50.73(a)(2)(viii)(B) o 20.2203(a)(2)(i) o 50.36(c)(1 )(i)(A) o 50.73(a)(2)(iii) o 50.73(a)(2)(ix)(A)
- 10. POWER LEVEL o 20.2203(a)(2)(ii) o 50.36(c)(1 )(ii)(A) o 50.73(a)(2)(iv)(A) o 50.73{a)(2)(x) o 20.2203{a)(2)(iii) o 50.36{c)(2) o 50.73{a)(2)(v)(A) o 73.71{a)(4) 100 o 20.2203{a)(2)(iv) o 50.46{a)(3)(ii) o 50.73{a)(2)(v)(B) o 73.71(a)(5) o 20.2203{a)(2)(v) o 50.73{a)(2)(i)(A) o 50.73{a)(2)(v)(C) o OTHER o 20.2203{a)(2)(vi) o 50.73(a)(2)(i)(B) o 50.73{a)(2)(v)(D)
Specify in Abstract below or in seconds of the transient; however, no suitable basis for these times was found. The operability evaluation, which was revised to account for additional information regarding actuation delay associated with the fusible links, concluded that there was reasonable assurance the design basis requirements for protecting equipment was met.
This expanded the scope of the detailed engineering review of past operability.
On November 1,2011, following the completion of the detailed engineering review, a review of plant records for past operability was completed. The review identified instances where the HELB rollup doors to the MEERs that contain the instrument inverters were open: the Division 11 rollup door was open on March 09, 2009 for 18 minutes, and March 16,2009 for 7 minutes; and the Division 21, the rollup door may have been opened (the exact times are not known) on April 3, 2010, and November 7, 2010.
It is conservatively concluded that the Division 11 (21 on Unit 2) instrument inverters (instrument bus 111 and 113 on Unit 1 or 211 and 213 on Unit 2) would be inoperable with the associated rollup door open in Operating Modes 1 through 4. Therefore, this condition is being reported in accordance with 10 CFR 50.73(a)(2)(vii) for any event where a single cause or condition caused at least two independent trains or channels to become inoperable in a single system designed to shut down the reactor and maintain it in a safe shutdown condition.
C.
Cause of Event
This event was caused by incorrect input assumptions in the original turbine building HELB analyses completed in the early 1990s. The original analyses, combined with later rollup door closure demonstrations, provided the basis to implement procedural compensatory actions to permit opening of the rollup doors with the unit operating in Modes 1 through 4. This went unidentified until recent power uprate analyses reviews. Due to the age of the original error, that exact cause is indeterminate.
p.
Safety Consequences
There were no actual safety consequences impacting plant or public safety as a result of this event. There were no actual occurrences of a turbine building HELB resulting in physical impact on station equipment. This is a condition of past inoperability caused by past system configurations.
The relevant functions of the AP, DC and IP systems are to store, produce and distribute DC and AC power of the proper voltage and quality for the required durations. The power is used to operate the required pumps, fans, dampers, valves, instrumentation and support equipment as needed to maintain reactor coolant pressure boundary integrity, shutdown the reactor, maintain the reactor in a safe shutdown condition, and prevent or mitigate the consequences of an accident that could result in unacceptable offsite exposures.
A HELB in the turbine building can affect safety related equipment since some areas have ventilation connections to the turbine building. High energy lines in the main steam [SB], feedwater [SJ], or heater drain [SN] systems are routed close to some of the ventilation openings. The ventilation connections would allow steam from a HELB to enter the areas, exposing the equipment to high temperatures and humidity. The HELB also causes a loss of ventilation (when fire dampers drop). The electrical equipment in the room then causes a further heatup. Exposure of electrical components to high temperatures or humidity for sufficiently long periods could degrade the electrical components such that their ability to carry or interrupt the flow of power or signals is degraded.
The loss of Division 11 or Division 21 instrument buses does not represent significant safety consequence or loss of safety function. The Division 11 and Division 12 (Division 21 and 22 on Unit 2) MEERs are separated to maintain safe shutdown capabilities in the event of a fire in one of the divisional MEERs. The loss of the Division 11 instrument buses is bounded by the loss of all equipment in the Division 11 MEER as documented in the Braidwood Fire Protection Report.
NRC FORM 36M (10-2010)
2011 004 00 In the event of a turbine building HELB with the Division 11 rollup door open, it is assumed that both inverters (111 and 113) would fail, de-energizing instrument bus 111 and 113. This would result in actuation of the reactor protection system, due to the loss of two of four instrument channels, and an automatic reactor trip. In accordance with Standard Review Plan 3.6.1, 'Plant Design For Protection Against Postulated Piping Failure In Fluid Systems Outside Containment', if a trip of the turbine generator or reactor protection system is a direct consequence of the HELB, then the effects of a loss of offsite power (LOOP) should be considered. With Division 12/22 instrument buses in a normal configuration, the LOOP would not represent a concern since the inverters would operate off battery power until the Division 12/22 emergency diesel generator (EDG) picks up the associated ESF bus.
The March 16, 2009 occurrence of the Division 11 MEER rollup door being opened is the only occasion where the Division 11 inverters were exposed to potential turbine building HELB effects while one of the Division 12 inverters was considered inoperable. On this date, inverter 112 was considered inoperable based on entry in Technical Specification Limiting Condition for Operation (LCO) 3.8.7. Inverter 112 was taken from the normal configuration and placed on the constant voltage transformer (CVT) to support inverter maintenance. In this alignment, instrument bus 112 would be de-energized during a LOOP and remain de-energized until the Division 12 EDG assumed the ESF loads. Instrument bus 112 would be automatically re-energized through the CVT as soon as the EDG picks up the associated ESF bus. The loss of instrument bus 112 would not affect startup or loading of the EDG since this is controlled by instrument bus 114. This event does not represent a significant safety consequence or loss of safety function since the Division 12 ESF equipment would remain available to achieve safe shutdown.
E.
Corrective Actions
Corrective actions include:
The station plant barrier impairment procedure has been revised to remove the pre-evaluated compensatory actions for the applicable rollup doors. This action has been completed.
The calculations for the turbine bUilding HELB analyses will be revised to reflect the actual plant design and door performance.
Station modifications are being pursued to improve design margins for room temperatures in the post turbine building HELB environments.
Previous Occurrences
Two previous, similar Licensee Event Reports were identified at the Braidwood Station:
Licensee Event Report 2008-001 Technical Specification Non-Compliance Due to Inadequate Design of Auxiliary Feedwater (AF) Tunnel Access Covers Causing AF Valves Within the Tunnel to be Inoperable Licensee Event Report 2010-007 Potential Loss of Residual Heat Removal System Safety Function in Mode 4 When Aligned for Shutdown Cooling Due to Potential for Flashing or Voiding of Coolant During a Shutdown Loss of Coolant Accident
G. Component Failure Data
Manufacturer N/ANomenclature N/A Model N/A Mfg. Part Number N/A