ML20202E777

Documents Closeout of NRC Staff Review of Millstone Nuclear Power Station,Unit 1 Isap Topic 1.108, Torus to Reactor Building Vacuum Breakers

ML20202E777
Person / Time
Site:	Millstone
Issue date:	11/28/1997
From:	Stephen Dembek NRC (Affiliation Not Assigned)
To:	NRC (Affiliation Not Assigned)
References
NUDOCS 9712080139
Download: ML20202E777 (3)

Text

MEMORANDUM TO: Docket File Noveraber 28, 1997 e

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FROM: Steph:n Dcmb3k, Proj::ct Man::ger,y Special Projects Offico - Licensing b (

Office of Nuclear Reactor Regulation

SUBJECT:

CLOSEOUT OF STAFF REVIEW OF INTEGRATED SAFETY ASSESSiAENT PROGRAM (ISAP) TOPIC 1.108, MILLSTONE NUCLEAR POWER STATION, UNIT NO.1 (TAC NO. M86399)

This memorandum documents the closecut of the NRC staff's review of the Millstono Nuclear Power Station, Unit No.1 ISAP Topic No.1.108, " Torus to Reactor Building Vacuum Breakers." This 7.em involved a potential conflict betwcen the two post-accident 2

safety functions of the torus vacuum breaker butterfly valves. Millstone Unit No.1 has two parallel 20-inch torus vacuum relief lines, each containing a normally closed check valve and a normally closed air-operated butterfly valve. On loss of actuating air (or AC power), the butterfly valves fail open to assure that the external pressure limit of the containment is not exceeded during a design-basis accident. Thus, af ter a loss of actuating air, the containment isolation function would rely solely on the check valves.

This is an apparent violation of the intent of General Design Criterbn 56 of 10 CFR Part 50, Appendix A, which states, in part, that "la] simple check valve may not be used as the automatic isolation valve outside containment." However, the criterion also states that "upon loss of actuating power, automatic isolation valves shall be designed to take the

, position that provides greater safety."

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This issue applied to several other boiling water reactois with Mark l contairments. The Containment Systems and Severe Accident Branch in NRR has reviewed this issue and 1

concludsd that the design is acceptable because it meets the current plant licensing bases,

, regulations, and guidance dealing with containment isolation, in addition, the staff 1

concluded that the safety risk from this desigr is low; therefore, the staff concluded that a safety enhancement backfit would not be cost beneficial. For further information on this issue, see the August 14,1997, memorandum from James W. Clifford, Acting Director, Project Directorate IV 1 to Arthur T. Howell, Director, Division of Reactor Safety, 3

, Region IV.

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-p g WASHINGTON. D.C. 20666 4001 g**.**j Ntw mber 28, 1997 MEMORANDUM TO: Docket File FROM: Stephen Dembek, Project Manager - 4 l Special Projects Of fice - Licensing Office of Nuclear Reactor Regulatic,n *

SUBJECT:

CLOSEOUT OF STAFF REVIEW OF INTEGRATED SAFETY ASSESSMENT PROGRAM (ISAP)10PIC 1.108, MILLSTONE NUCLEAR POWER STATION, UNIT NO.1 (TAC NO. M86399)

This memorandum documents the closeout of the NRC staff's review of the Millstone Nuclear Power Station, Unit No.1 ISAP Topic No.1.108, " Torus to Reactor Building Vacuum Breakers." This item involved a potential conflict between the two post accident safety functions of the torus vacuum breaker butterfly valves. Millstone Unit No.1 has two parallel 20-inch torus vacuum relief lines, each containing a normally closed check valve and a normally closed air-operated butterfly valve. On ioss of actuating air (or AC power), the butterfly valves fail open to assure that the external pressure limit of the containment is not exceeded during a design basis accident. Thus, after a loss of actuating air, the containment isolation function would rely solely on the check valves.

This is an apparent violation of the intent of General Design Criterion 56 of 10 CFR Part 50, Appendix A, which states, in part, that "[a] simple check valve may not be used as the automatic isolation valve outside containment." However, the criterion also states that "upon loss of actuating power, automatic isolation velves shall be designed to take the position that provides greater safety."

2 This issue applied to several other boiling water reactors with Mark I containments. The Containment Systems and Severe Accident Branch in NRR has reviewed this issue and concluded that the design is acceptable because it meets the current plant licensing bases, regulations, and guidance dealing with containment isolation, in addition, the staff concluded that the safety risk from this design is low; therefore, the staff concludet that a safety enhancement backfit would not bt. cost beneficial. For further inforn.ation on this issue, see the August 14,1997, memorandum from James W. Clifford, Acting Directe.

Project Directorate IV 1 to Arthur T. Howell, Director, Division of Reactor Safety, Region IV.

Docket No. 50 245 CONTACT: Stephen Dembek 415-1455

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• WASHINGTON. D.C. 20665-0001

[ Noveraber 28, 1997 MEMORANDUM TO: Docket File FROM: Stephen Dembek, Project Managey .pwds Special Projects Office - Licensing Office of Nuclear Reactor Regulation

SUBJECT:

CLOSEOUT OF STAFF REVILW OF INTEGRATED SAFETY ASSESSMENT PROGRAM (ISAP) TOPIC 1.108, MILLSTONE NUCLEAR POWER STATION, UNIT NO.1 (TAC NO. M86399) -

- This memorandum documents the closeout of the NRC staff's review of the Millstone Nuclear Power Station, Unit No.1 ISAP Topic No.1.108, " Torus to Reactor Building

_ Vacuum Breakers." This item involved a potential conflict between the two post-accident safety functions of the torus vacuum breaker butterfly valves. Millstone Unit No.1 has two parallel 20-inch torus vacuum relief lines, each containing a normally closed check valve and a_normally closed air operated butterfly valve. On loss of actuating air (or AC

--power), the butterfly valves fail open to assure that the external pressure limit of the containment is not exceeded during a design-basis accident. Thus, after a loss of actuating air, the containment isolation function would rely solely on the check valves.

This is an apparent violation of the intent of General Design Criterion 56 of 10 CFR Part 50, Appendix A, which states,in part, that "[a] simple check valvo may not be used as the autornatic isolation valve outside containment." Hov.wer, the criterion also states that "upon loss of' actuating power, autoinatic isolation valves shall be designed to take the position that provides greater safety."

This issue applied to soveral other boiling water reactors with Mark I containments. The Containment Systems and Severe Accident Branch in NRR has reviewed this issue and concluded that the design is acceptable because it meets the current plant licensing bases, regulations, and guidance dealing with containment isolation in addition, the staff conciuded that the safety risk from this design is low; therefore, the staff concluded that a safety enhancement backfit would not be cost beneficial. For further information on this issue, see the August 14,1997, memorandum from James W. Clifford, Acting Director, Project Directorate IV-1 to Arthur T. Howell, Director, Division of Reactor Safety, Region IV.

Docket No. 50 245 CONTACT: Stephen Dembek 415-1455

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