05000456/LER-2010-007, Regarding 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
| ML110180428 | |
| Person / Time | |
|---|---|
| Site: | Braidwood  |
| Issue date: | 01/18/2011 |
| From: | Shahkarami A Exelon Generation Co, Exelon Nuclear |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| BW110006 LER 10-007-00 | |
| Download: ML110180428 (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)(x) 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 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)(B), Prohibited by Technical Specifications |
| 4562010007R00 - NRC Website | |
text
10 CFR 50.73 January 18, 2011 BW110006 U. S. Nuclear Regulatory Commission ATIN: Document Control Desk Washington, DC 20555-0001 Braidwood Station, Units 1 and 2 Facility Operating License No. NPF-72 and NPF-77 NRC Docket No. STN 50-456 and STN 50-457 Subject: 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 The enclosed Licensee Event Report (LER) is being submitted in accordance with 10 CFR 50.73, "Licensee event report system," paragraphs (a)(2)(i)(B), (a)(2)(v)(B), and (a)(2)(vii). On November 19,2010, it was identified that there were several occurrences where both residual heat removal (RH) trains were aligned for shutdown cooling with the reactor coolant system at a temperature that could potentially cause RH system voiding during a shutdown loss of coolant accident. 10 CFR 50.73(a) requires an LER to be submitted within 60 days following discovery of the event. Therefore, this report is being submitted by January 18, 2011.
There are no regulatory commitments contained in this letter. Should you have any questions concerning this submittal, please contact Mr. Ronald Gaston, Regulatory Assurance Manager, at (815) 417-2800.
Respectfully, Amir Shahkarami Site Vice President Braidwood Station
Enclosure:
LER 2010-007-00 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 BY OMB: NO. 3150-0104 EXPIRES: 10131/2013 (10-2010)
, 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 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
- 5. EVENT DATE I
- 6. LER NUMBER
- 7. REPORT DATE
- 8. OTHER FACILITIES INVOLVED FACILITY NAME DOCKET NUMBER MONTH DAY YEAR YEAR ISEQUENTIALIREV MONTH DAY YEAR Braidwood Station Unit 2 05000457 NUMBER NO.
FACILITY NAME DOCKET NUMBER 11 19 2010 2010. 007.
00 01 18 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) 50.73(a)(2)(i)(C) l8J 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) l8J 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) l8J 50.73(a)(2)(i)(B) o 50.73(a)(2)(v)(D)
Specify in Abstract below or in Administrative controls were implemented at Braidwood Station to address the RH issue:
During RCS cooldown at least one train of RH will be aligned for injection until RCS temperature is < 200 degrees F. (Mode 5)
During RCS heatup at least one train, preferably both, will be aligned for injection prior to raising RCS temperature >/= 200 degrees F. (Mode 4)
On November 19, 2010, the review of the past three years was performed to determine if Braidwood may have operated with both trains of RH shutdown cooling in service in Mode 4 (above 200 degrees F). Four occurrences were identified where both RH trains were placed into operation prior to reaching Mode 5 <</= 200 degrees F):
October 12, 2009, Unit 2 refueling outage. The second train of RH was placed in SOC at 0527; Mode 5 occurred at 0532 (2 trains in SOC for 5 minutes with RCS > 200 degrees F)
March 30, 2009, Unit 1 refueling outage. The second train of RH was placed in SOC at 0551; Mode 5 occurred at 0608 (2 trains in SOC for 17 minutes with RCS > 200 degrees F)
April 21,2008, Unit 2 refueling outage. The second train of RH was placed in SOC at 0745; Mode 5 occurred at 0805 (2 trains in SOC for 20 minutes with RCS > 200 degrees F)
October 1, 2007, Unit 1 refueling outage. The second train of RH was placed in SOC at 0510; Mode 5 occurred at 0538 (2 trains in SOC for 28 minutes with RCS > 200 degrees F)
These conditions represent an event or condition that could have prevented fulfillment of a safety function.
Therefore, this event is being reported pursuant to 10 CFR 50.73(a)(2)(i)(B), any operation or condition which was prohibited by Technical Specifications; 10 CFR 50.73(a)(2)(v)(B) as a condition that could have prevented the fulfillment of a system's safety function; and 10 CFR 50.73(a)(2)(vii), any event where a single cause or condition caused two independent trains to become inoperable in a single system.
~'
Cause of Event
The cause of this event was the failure to consider bounding conditions when calculating the temperature limit to prevent flashing in the RH system.
Based on the history of this industry issue, deficiencies existed in the original RH system design and system operating guidance provided by the equipment manufacturer. Contributing to this was that the Braidwood historical review of NSAL-93-004 was less than adequate in that the review did not address the full scope of the issue when it was originally evaluated in 1994.
D.
Safety Consequences
There were no actual safety consequences impacting plant or public safety as a result of this event. This issue addressed the potential for the development of steam voiding in the RH pump suction shutdown cooling piping of the RH system if the RH system had to be transitioned from shutdown cooling to the ECCS mode due to a LOCA occurring in Modes 3 or 4 at temperatures above 200 degrees F (Modes 3 and 4).
The potential exists for flashing/steam voiding of an RH system hot leg suction line if the RH system is aligned for ECCS recirculation, following termination of shutdown cooling with an RH temperature that exceeds 200 degrees F, and suction of the RH system is transferred to the ECCS recirculation sump during a LOCA. Therefore, this is considered a safety system functional failure.
In the event that the RH system became inoperable, abnormal and emergency procedures exist that provide guidance to immediately secure any RH pumps aligned for shutdown cooling to prevent pump damage, to restore core cooling through alignment of a high head safety injection pump [BO] in injection mode, and restoration of the medium head safety injection pumps [BO] if necessary. Existing procedures also include steps to vent and refill the
RH loops if necessary. In Modes 3 or 4, at least one charging (CV) [CB] pump is available and would be aligned to the RWST. Additionally, the steam generators would be available with auxiliary feedwater providing a heat sink to aid in decay heat removal.
When this issue was first identified by Westinghouse in NSAL-93-004, Westinghouse performed an Assessment of
Safety Significance
Referencing prior work documented in WCAP-12476, "Evaluation of LOCA during Mode 3 and Mode 4 Operation for Westinghouse NSSS," which looked at the probabilities of a LOCA in Modes 3 or 4 and available gUidance for actions to cope with a shutdown LOCA, Westinghouse concluded that this issue was not risk significant in regard to large LOCAs in Mode 3 and the relative risk was not much different whether or not flashing occurs in Mode 4. In the more recently issued NSAL-09-8, Westinghouse states that the conclusion of the previous safety significance assessments for this issue was based on the low risk of this event. This conclusion remains valid since no new information changes this condition. The consequences of RH system failure due to suction flashing in Modes 3 or 4 remained bounded by the core damage consequences of the Mode 1 LOCA events. This is because of the reduced pipe break probability due to the relatively low temperatures and pressure that exist for the majority of time the plant is in these modes. It is also reflective of the time the plant is in these modes, which is very short relative to the time it is in Mode 1. Therefore, the risk significance of this event is considered to be low.
E.
Corrective Actions
The corrective actions include:
A standing order was implemented for the RH system to establish controls that ensure at least one train of RH be aligned for injection when RCS temperature is > 200 degrees F.
Revise the associated operating and emergency procedures to reflect the more restrictive 200 degrees F temperature limit for restoration/alignment of an RH Train for ECCS injection following termination of shutdown cooling.
Revise Technical Specification Bases 3.5.3, "ECCS - Shutdown," to reflect the new temperature limit.
Previous Occurrences
There have been no previous, similar Licensee Event Reports identified at the Braidwood Station.
G.
Component Failure Data
Manufacturer N/ANomenclature N/A Model N/A Mfg. Part Number N/A