ML20212Q077

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Discusses Focus of Proposed 870123 Meeting Re Mark I Containments & Identification of Possible Regulatory Decisions to Assure Use of Best Scientific Info.Containment Improvements Can Reduce Likelihood of Core Melt Sequences
ML20212Q077
Person / Time
Issue date: 01/13/1987
From: Harold Denton
Office of Nuclear Reactor Regulation
To: Beckjord E
NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES)
Shared Package
ML20209C630 List:
References
FOIA-87-10 NUDOCS 8704030552
Download: ML20212Q077 (6)
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January 13, 1987

w T>u MEMORANDUM FOR

Eric S. Beckjord, Director

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Office of Nuclear Regulatory Research i

FROM:

Harold R. Denton, Director Office of Nuclear Reactor Regulation

SUBJECT:

MEETING ON MARK I CONTAINMENTS I

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JANUARY 23, 1987 9:00 A.M., ROOM P-422 We have set the indicated times and place for a review of the integrity of Mark I containments. By this memorandum the others designated as proposed attendees are requested to participate in the subject meeting. The focus of our attention at this time is on possible regulatory decisions regarding these containments and seeking to ensure that such decisions take into account the bestavailablescientifiginformation. As you know estimates of BWR Mark I containment performance indicate that somewhere between 10% and 90% of large scale core melt sequences are expected to result in large releases of radioactive material because of failure of the containment in such a way that little mitigation of releases is expected. The expectation of such high fractions of large releases is troubling in terms of defense in-depth a

philosophy even if the likelihood of such large scale core melts is quite low.

2 It has been suggested that by incorporating certain improvements to these j

containments, the fraction of core melt sequences expected to result in large release can be reduced to the range of 1% to 10%, a range which might be accepted as reasonable assurance of mitigating the consequences of a core melt.

Again, we are focusing here on containment performance, we expect to leave to i

individual plant exanination the identification of the best ways, if any are needed, to reduce tbc frequency of core melt accidents.

We are also presently engaged in a review process with the BWR Owners Group of Emergency Procedure Guidelines, Rev. 4, which affords an excellent opportunity to improve accident management procedures in 8WRs.

It is desirable at this time to determine what design changes are warranted, if any, to go with improved emergency procedures.

I have rearranged and amplified the agenda 2

items you proposed in order to explore the options in the proposal and their J

scientific basis.

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1 Draft NUREG-1150, !DCOR Summary Report, etc.

2 Memorandum from R. Bernero to E. Beckjord et al. of December 4,1986, 1

transmitting proposed generic letter.

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I would like Bob Bernero, Director of BWR Licensing, to chair the meeting for us.

In addition to the proposed attendees listed below, I suggest that you and I keep to a minimum the attendance of additional staff in order to maximize j

the effectiveness of the meeting. After this meeting we will pursue an appropriate course of action on this matter through regulatory staff and 1

Commission review.

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Harold R. Denton, Director j

Office of Nuclear Reactor Regulation

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Enclosures:

1.

Proposed Attendees 2.

Proposed Agenda and Discussion Leaders i

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V. Stello, Jr., EDO

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PROPOSED ATTENDEES Eric Beckjord, RES Denny Ross, RES Joe Murphy, RES Harold Denton, NRR Bob Bernero, NRR Themis Spets, NRR Tom Murley, RI Rich Starcstecki, IE Steve Hodge, ORNL Rich Denning, BMI AlanBenjamin,SNL 1

Trevor Pratt, 8NL George Greene, BNL e

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PROPOSED AGENDA AND DISCUSSION LEADER $*

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1.

What core melt accident sequences may be expected to be significant in 4

BWRs with Mark I containments? (J. Murphy) 4 l

2.

Do current analyses indicate containment failure preceding core melt?...

and causing core melt? (J. Murphy) 3.

What are the approximate time scales for significant sequences?

e.g.,

time to core uncovery, time to core melt, time to melt through, time to containment failure.

Is this generic or very plant specific? (J. Murphy) j 4.

Do high pressure melts (ADS failure) have a significant effect on the

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physical behavior of the core melt in a 8WR? (S. Hodge)

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Do current models indicate substantial differences between PWRs and BWRs 4

in meltdown times?...in meltthrough times? (R. Denning) j 6.

Are the physical properties of the " core-on-tty-floor" for a 8WR expected

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to be significantly different than for a PWR7 e.g., thermal conductivity, viscosity, etc.

(G. Greene) j 7.

In a typical Mark I, initiation of drywell spray before meltthrough can cover the drywell floor with up to 1 foot of water before core material begins to drep.

Is the presence of such a water layer beneficial.

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(S. Hodge) 8.

In a typical Mark I the drywell spray can distribute up to 20,000 gpm in the area outside the reactor pedestal area. If this spray is operating at the time of meltthrough can it inhibit corium movement toward and attack of the outer wall of the drywell? Would success be proportional to water flow rate? (T. Pratt) 9.

Given the presence of drywell spray, would a short diversion barrier 1

which could double or triple the path length to the outer wall j

significantly reduce the Itkelihood of itner seltthrough? (T. Pratt)

10. If a substantial barrier of refractory character could be provided to 1

j hold most of the corium in the reactor pedestal area, would this be preferred? Would attack of the reactor vessel pedestal be a significant concern? (G. Greene) i

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11. Is any release attenuation expected from the biological shield surrounding

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the Mark I drywell?...is it treated in current models? (R. Denning)

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Discussion leader is expected to initiate discussion on the topic with 1

a 3 5 minute statement, viewgraphs can be used. Discussion leaders may 4

exchange topics by agreement.

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12. In a typical Mark I containment available or practically adaptable vent paths have an effective diameter of about 10-12 inches which is sufficient to pass water vapor at I to 1 times design pressure equivalent to 1-2%

decay heat. What effect on significant accident sequences can be expected if there are assured means to open this vent path? (R. Denning)

13. Calculations now available indicate that although noble gas doses can be high (see attached Figure) deliberate release of those gases appears to be better to avoid the far greater releases that might occur with an uncontrolled release. Do present models indicate that deliberate venting of noble gas activity may not be justified? (A. Benjamin) 14.

To what extent could reliable containment spray alone, without venting, 3

substantially reduce containment failure in the station blackout sequence? (A. Benjamin) 1 15.

Is there any other practical change to the Mark I containment system which can significantly improve its performance in core melt?

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(R. Bernero)

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