ML070510087
| ML070510087 | |
| Person / Time | |
|---|---|
| Site: | Davis Besse  |
| Issue date: | 02/15/2007 |
| From: | Michael Case NRC/NRR/ADRA/DPR |
| To: | Pederson C Division of Reactor Safety III |
| PETERS, SEAN, NRR/PSPB, 415-1842 | |
| References | |
| TAC MD2830, TIA 06-003 | |
| Download: ML070510087 (7) | |
Text
February 15, 2007 MEMORANDUM TO: Cynthia D. Pederson, Director Division of Reactor Safety Region III FROM:
Michael J. Case, Director /RA/
Division of Policy and Rulemaking Office of Nuclear Reactor Regulation
SUBJECT:
FINAL RESPONSE TO DAVIS-BESSE NUCLEAR POWER STATION-TASK INTERFACE AGREEMENT (TIA) 2006-003 RE:
SERVICE WATER SETPOINT FOR SWAPPING FROM NON-SAFETY RELATED TO SAFETY RELATED DISCHARGE PATH (TAC NO. MD2830)
On August 4, 2006, Region III, Division of Reactor Safety, requested assistance from the Office of Nuclear Reactor Regulation (NRR) in determining the licensing basis for Davis-Besse relative to use of the non-safety related discharge flow path for the service water system (SWS).
Specifically, Region III requested that NRR provide answers to the following questions:
1.
Does the Davis-Besse licensing basis include analysis of a simultaneous LOCA [loss-of-coolant accident] with a seismic event?
2.
Is it acceptable for the Davis-Besse licensee to rely on non-safety related equipment during a LOCA or is the licensee required to have an automatic swapover to the safety related piping for a LOCA signal? Is manual operation as described in their abnormal procedure acceptable?
If automatic realignment in response to a LOCA is not required, how should the adequacy of the procedural controls be determined (e.g., partial blockage vs. total blockage).
3.
Section 9.2.1.2 of the FSAR [final safety analysis report] states that if a seismic event causes blockage of service water as a result of the failure of the non-seismic line to the cooling tower, an automatic swapover to the safety related piping occurs. With respect to blockage, does this mean any blockage (i.e., blockage that results in reduced flow) or total blockage (no flow) and does the 50 psig setpoint appear to provide appropriate protection?
The NRR staff's conclusions are documented in the attached assessment.
Docket Number: 50-346
Attachment:
As stated.
CONTACT:
Sean Peters, DPR Holly Cruz, DPR 301-415-1842 301-415-1053 The NRR staff's conclusions are documented in the attached assessment.
Docket Number: 50-346
Attachment:
As stated.
CONTACT:
Sean Peters, DPR Holly Cruz, DPR 301-415-1842 301-415-1053 DISTRIBUTION:
PUBLIC RidsRegion1MailCenter TMartin PSPB Reading File RidsRegion2MailCenter JSegala RidsNrrDpr RidsRegion3MailCenter TWengert RodsNrrDprPspb RidsRegion4MailCenter SGardocki RidsNrrLADBaxley RidsAcrsAcnwMailCenter JTatum RidsNrrPMSPeters RidsNrrPMHCruz RidsNrrAdra RidsNrrDss RidsNrrDssSbpb ADAMS ACCESSION NO: ML070510087
- No significant changes to the SE.
OFFICE PSPB/PM PSPB/PM PSPB/LA DSS/SBPB*
DSS/SBPB*
SBPB/BC*
DSS/D*
PSPB/BC DPR/D NAME HCruz SPeters CHawes for DBaxley JTatum SGardocki JSegala TMartin SRosenberg MCase DATE 1/22/07 1/23/07 1/23/07 12/14/06 12/13/06 12/20/06 12/26/06 1/ 25 /07 2/ 15 /07 OFFICIAL RECORD COPY
ATTACHMENT STAFF ASSESSMENT BY THE OFFICE OF NUCLEAR REACTOR REGULATION SERVICE WATER SETPOINT FOR SWAPPING FROM NON-SAFETY RELATED TO SAFETY RELATED DISCHARGE PATH AT DAVIS-BESSE NUCLEAR POWER STATION TASK INTERFACE AGREEMENT (TIA) 2006-003
1.0 INTRODUCTION
On August 4, 2006, the Nuclear Regulatory Commission (NRC) staff in Region III submitted TIA 2006-003, Request for Technical Assistance - Davis-Besse Service Water Setpoint for Swapping From Non-Safety Related to Safety Related Discharge Path" (Agencywide Documents Access Management System (ADAMS) Accession No. ML062190343). The TIA requested assistance from the Office of Nuclear Reactor Regulation in determining the licensing basis for Davis-Besse relative to use of the non-safety related discharge flow path for the service water system (SWS).
2.0 BACKGROUND
On February 26, 2003 (ADAMS Accession No. ML030630314), a non-cited violation for the Davis-Besse plant was issued to the First Energy Nuclear Operating Company (FENOC, or the licensee) in Inspection Report 05000346/2002-014 for failing to establish an appropriate basis for the SWS discharge pressure setpoint that is used for automatically transferring the SWS discharge from the non-safety related to the safety related flow path. Subsequently, on March 3, 2004 (ADAMS Accession No. ML040680070), a Notice of Violation (NOV) was issued to FENOC in Inspection Report 05000346/2003-010 because the licensees evaluation found that the established setpoint was not adequate to ensure required flows to safety related equipment without relying on the non-safety related discharge flow path to some extent. The licensees views on the licensing basis for the SWS discharge flow paths were provided in a letter dated April 2, 2004, and FENOC responded to the NOV in a letter dated April 5, 2004 (ADAMS Accession Nos. ML040970308 and ML040980494, respectively).
Issue Summary The SWS at Davis-Besse consists of four discharge flow paths: two that are non-safety related and two that are safety related. Upon a failure of the non-safety related SWS discharge flow paths, the SWS flow rate could be restricted to the point where it is not sufficient for mitigating the design basis loss-of-coolant accident (LOCA). The licensee established a SWS high discharge pressure setpoint of 50 psig for automatically transferring the SWS discharge flow to the safety related flow paths in order to preserve the capability of the SWS to perform its safety functions. The NRC inspectors questioned the capability of the SWS to perform its functions for the situation where the non-safety related discharge flow paths fail in such a way that the SWS discharge pressure is higher than normal but does not exceed the 50 psig setpoint that was established (as could occur due to partial blockage). The licensee maintains that the 50 psig setpoint is adequate for a seismic event alone or for a LOCA alone, but that the licensing basis for Davis-Besse does not include assuring adequate SWS flow for a LOCA that occurs concurrent with a seismic event. The licensee also believes that the Davis-Besse licensing basis only includes full blockage failures of the SWS non-safety related discharge flow paths and that partial blockage failures that limit the elevated SWS discharge pressure to less than 50 psig are not included and do not need to be addressed.
3.0 EVALUATION By this TIA, Region III requested answers to the following questions:
1.
Does the Davis-Besse licensing basis include analysis of a simultaneous LOCA [loss-of-coolant accident] with a seismic event?
No. The Davis-Besse licensing basis does not include analysis of a simultaneous LOCA with a seismic event. The NRC does not require licensees to postulate the occurrence of a LOCA concurrent with a seismic event because the likelihood of both of these occurring at the same time is extremely remote. However, in order to assure that structures, systems, and components (SSCs) important to safety are robust and capable of performing their assigned safety functions, the design criteria for these SSCs includes the capability to withstand seismic events. This is reflected in Section 3.2.1.1, Definitions, of the Davis-Besse Updated Safety Analysis Report (UFSAR) which states that those SSCs that are necessary to ensure the capability to prevent or mitigate the consequences of accidents are designed to remain functional in the event of a maximum possible earthquake and are classified as nuclear safety-related. Consistent with this, Section 9.2.1.1, [Service Water System] Design Basis, of the Davis-Besse UFSAR states that the portion of the SWS required for emergency operation is Seismic Class I. Furthermore, as discussed in Appendix 3B of the Davis-Besse UFSAR, General Design Criterion 2, Design Bases for Protection Against Natural Phenomena, the licensee indicated that SSCs important to safety are designed to withstand the effects of earthquakes without the loss of capability to perform their safety functions.
Therefore, while a LOCA is not postulated concurrent with a seismic event, it is necessary for SSCs that are credited for accident mitigation to be nuclear safety-related, Seismic Class I, and parts of SSCs that are not safety-related, Class I cannot be relied upon for accident mitigation.
In the case of the Davis-Besse UFSAR, the confusion on this point stems from the description in Section 9.2.1.2, Service Water System - System Description, which discusses the capability of the SWS to perform its safety functions if a LOCA and seismic event should occur. This description is typical of how UFSARs addressed the seismic design capability of systems important to safety when a part of the system was not designed to seismic Class I standards. The intent was to demonstrate that the seismic boundary would be preserved during a postulated accident, such as through automatic isolation of the non-seismic parts of the system, thereby, demonstrating compliance with the seismic design criteria for that part of the system that is relied upon for accident mitigation.
2.
Is it acceptable for the Davis-Besse licensee to rely on non-safety related equipment during a LOCA or is the licensee required to have an automatic swap over to the safety related piping for a LOCA signal? Is manual operation as described in their abnormal procedure acceptable?
If automatic realignment in response to a LOCA is not required, how should the adequacy of the procedural controls be determined (e.g., partial blockage vs. total blockage)?
No. It is not acceptable to rely on non-safety related equipment to mitigate a LOCA. As discussed in response to Question 1, only those parts of the SWS that are safety related, Seismic Class I may be credited for mitigating the consequences of a LOCA.
In order to preserve the safety related, Seismic Class I parts of systems that are relied upon for accident mitigation, automatic isolation of non-safety related flow paths and swap over to safety related flow paths during accident conditions is preferred. This approach eliminates the need for analyzing postulated failures of the non-safety related parts of the system. The use of operator actions is also acceptable to the extent that it is permitted by a plant licensing basis. In this case, the use of operator actions is permitted in the Davis-Besse UFSAR. However, the licensee must be able to demonstrate that the necessary actions can be taken in accordance with approved operating procedures such that the safety related, Seismic Class I part of the SWS is capable of performing its safety functions without reliance on parts of the SWS that are non-safety related. All failure scenarios of the non-safety related parts of the SWS must be considered and addressed, including failures that result in partial blockage, unless the licensee can demonstrate that resolution of the full blockage scenario is adequate and bounding for partial blockage scenarios as well.
3.
Section 9.2.1.2 of the Final Safety Analysis Report states that if a seismic event causes blockage of service water as a result of the failure of the non-seismic line to the cooling tower, an automatic swapover to the safety related piping occurs. With respect to blockage, does this mean any blockage (i.e., blockage that results in reduced flow) or total blockage (no flow) and does the 50 psig setpoint appear to provide appropriate protection?
The UFSAR states blockage, it does not stipulate full or partial blockage. As discussed in response to Questions 1 and 2, failure scenarios associated with the non-safety related parts of the SWS must be considered when the non-safety related parts of the system are not automatically isolated following an accident.
The setpoint for swapping over from the non-safety related discharge flow path to the safety related discharge flow path must be established based upon preserving adequate flow through the SWS for all postulated accident scenarios, regardless of how much blockage exists in the non-safety related SWS discharge flow paths. As identified by a vendor calculation (Proto-Power Corporation Calculation 03-014, Service Water System Performance with Discharge Line Blockage, March 27, 2003) noted in Inspection Report 05000346/2003-010, a switch-over setpoint of 50 psig will not assure an adequate amount of flow through the SWS following an accident. We agree that the 50 psig setpoint does not provide adequate protection.
4.0 CONCLUSION
The Davis-Besse licensing basis does not include analysis of a simultaneous LOCA with a seismic event. Nevertheless, it is not acceptable for the Davis-Besse licensee to rely on non-safety related equipment during a LOCA.
Failure scenarios associated with the non-safety related parts of the SWS must be considered when the non-safety related parts of the system are not automatically isolated following an accident. The setpoint for swapping over from the non-safety related discharge flow path to the safety related discharge flow path must be established based upon preserving adequate flow through the SWS for all postulated accident scenarios. NRC staff concludes that the 50 psig setpoint does not provide adequate protection.
Principal Contributors: J. Tatum S. Gardocki Date: February 15, 2007