ML20197F082

Provides Recommendations Re Credit for Containment over- Pressure to Provide Assurance of Sufficient Net Positive Suction Head for ECC & Containment Heat Removal Pumps, Initially Discussed During 442nd ACRS Meeting in June 1997

ML20197F082
Person / Time
Issue date:	12/12/1997
From:	Seale R Advisory Committee on Reactor Safeguards
To:	Shirley Ann Jackson, The Chairman NRC COMMISSION (OCM)
References
ACRS-R-1739, GL-97-04, GL-97-4, NUDOCS 9712300132
Download: ML20197F082 (4)

Text

[

UNITE 3 8TATES -

/*

NUCt EAR REGULATORY COMMIS$10N POR

{

pi ADVISORY COMMITTEE ON REACTOR SAFEQUARDS i

4 wwworow. o. c. mw December 12, 1997 i

The Honorable Shirley Ann Jackson Chairman U.S. Nuclear Regulatory Commission Washington, D.C.

20555 0001

SUBJECT:

CREDIT FOR CONTAINMENT OVERPRESSURE TO PROVIDE ASSURANCE OF SUFFICIENT NET POSITIVE SUCTION HEAD FOR EMERGENCY CORE COOLING AND CONTAINMENT HEAT REMOVAL PUMPS We %'tially discussed this subject during the 442nd meeting of the Advisory

. AHtte on Reactor Safeguards in June 1997.

After that meeting, and an exchange of letters with the EDO, we requested additional discussions with the NRC staff to address some of our continuing concerns.

We addressed these concerns during our Plant Operations Subcommittee meeting on December 2, 1997, and during the 447th meeting of the ACRS on December 3 6, 1997, We also had the benefit of the documents referenced, Recommendation in our June 17,1997. letter to the Executive Director for Operations regarding our review of Generic Letter (GL) 97 04, " Assurance of Sufficient Net Positive Suction Head for Emergcncy Core Cooling and Containment Heat Removal Pumps," we stated that giving credit for containment overpressure to provide assurance of sufficient - net positive suction head (NPSH) should not be considered en acceptable corrective action for any deficiencies found by the reviews requested by GL 97 04, As a result of further review of this issue, we now concur with the NRC' staff position that selectively granting credit for small amounts of overpressure for a few cases may be justified.

We recommend that instead of

.using qualitative arguments _and restricting attention-to a. limited range of accident sequences, the decisionmaking process should consider the time variation i

of.NPSH for a broad range of accident sequences such as typically found in a probabilistic risk assessment (PRA).

g Dl g;

c audb$

] ?(K b 97y2 g2971212 l

l lfl I R-1739 PDR

, g,3 BCC 5

l 2

i Discussion We recognize that previous NRC staff actions have established a precedent for 3

giving credit for containment overpressure to assure sufficient NPSH.

These actions have been based on the judgment that the consequences of reduced margins are small.

These actions, however, depart from the basic guidance found in Regulatory Guide (RG).1.1, " Net Potitive Suction Head for Emergency Core Cooling and Containment Heat Removal System Pumps." and the associated Standard Review Plan Section 6.2.2. " Containment Heat Removal Systems."

RG 1.1 states that

" emergency core cooling and containment heat removal systems should be designed 50 that adequate NPSH is provided to system pumps, assuming maximum expected temperatures of pumped fluids and no increase in containment pressure from that present before any postulated LOCA."

The language in Section 6.2.2 of the Standard Review Plan backs away slightly from this guidance by stating that the NPSH analysis should be based on the assumption that the containment pressure equals the vapor pressure of the sump water.

The saidance in RG 1.1 was developed from a design basis approach that utilizes surrogate design basis accidents (DBAs) with conservative assumptions so that the resulting design is robust (has acceptable risk) for all accident sequences.

Ideally, the justificatinn for any departures from such guidance should be grounded in the proposed Regulatory Guide 1.174 (formerly DG 1061). "An Approach for Using PRA in Risk Informed Decisions on Plant Specific Changes to the Current Licensing Basis." risk informed process.

In the absence of such justification, we believe that the original RG 1.1 guidance should be the appropriate regulatory position.

Because of the nature of the DBA approach however, it is difficult to quantify the impact on risk caused by the minor departures from this guidance associated with granting credit for containment overpressure to assure adequate

'NPhH. Therefore, the staff has relied on engineering judgment and qualitative arguments.

i The potential for loss of adequate NPSH results from overheating of the. sump water.

The qualitative arguments used by the staff involved evaluation of i

selected accident sequences to show that when the sump is hot.- overpressure will exist'. and when overpressure 11s not-available, the sump temperature will be a

limited to approximately 212' F.

We conclude that these arguments do demonstrate anl acceptable qualitative approach to assuring that the risk is small. We are concerned..however, with the completeness of the. staff's evaluations with respect

-to the full spectrum of accident sequences. We recommend that future decisions i

o 3

be guided by a more extensive PRA evaluation of the NPSH status for the specific plant of interest over a broader range of accident sequences, rather than relying completely on the above qualitative arguments.

The margins in NPSH afforded by the DBA approach constitute a level of defense in depth. Allowing more credit for containment overpressure reduces defense in depth. The staff's justification for this was that the consequences of losing NPSH would not be catastrophic, i.e., the particular pumps at issue would not suffer damage and the discharge flow rates would remain sufficiently high. We believe that the evidence to support these assertions needs to be identified as a part of the decisionmaking process.

in general, where departures from DBA requirements are being considered, we continue to_ believe that such departures should be justified using the risk-informed approach described in the proposed Regulatory Guide 1.174 (formerly DG-1061).

Sincerely,

$ )4

,n R. L. Seale Chairman

References:

1.

Letter dated June 17, 1997, from R. L.

Seale, Chairman, ACRS, to L.

Joseph Callan Executive Director for Operations NRC,

Subject:

Proposed Final Generic Letter, " Assurance of Sufficient Net Positive Suction Head for Emergency Core Cooling and Containment Heat Removal Pumps."

2.

Letter dated August 15, 1997, from L. Joseph Callan Executive Director for Operations, NRC, to R.

L.

Seale, Chairman,ACRS,

Subject:

Proposed Final Generic Letter

• Assurance of Sufficient Net Positive Suction Head for Emergency Core Cooling and Containment Heat Removal Pumps."

3.

Memorandum dated September 9,1997, from John T. Larkins, Executive Director, ACRS, to L. Joseph Callan, Executive Director for Operations.

NRC,

Subject:

Proposed Final Generic Letter " Assurance of Sufficient Net Positive Suction Head for Emergency Core Cooling and Containment Heat Removal Pumps."

4 4.

U.S.

Nuclear Regulatory Commission, Generic letter 97 04: Assurance of Sufficient Net Positive Suction Head for Emergency Core Cooling and Containment Heat Removal Pumps, dated October 7. 1997.

5.

Memorandum dated November 24, 1997, from H. Wayne Hodges. Office of Nuclear Regulatory Research and Gary H. Holahan, Office of Nuclear Reactor Regulation, to John Larkins, Executive Director. ACRS

Subject:

General Regulatory Guide (DG 1061) and Standard Review Plan (SRP Chapter 19) for Risk Informed Regulatory Decisionmaking for Plant Specific CLB Changes.

--n z-r

,-,