ML20136F841
| ML20136F841 | |
| Person / Time | |
|---|---|
| Issue date: | 03/07/1985 |
| From: | Dircks W NRC OFFICE OF THE EXECUTIVE DIRECTOR FOR OPERATIONS (EDO) |
| To: | Palladino NRC COMMISSION (OCM) |
| Shared Package | |
| ML20136F845 | List: |
| References | |
| FOIA-85-448, RTR-NUREG-1116 NUDOCS 8503140021 | |
| Download: ML20136F841 (3) | |
Text
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, UNITID STATES ENCLOSURE 1
,[ g r, NUCLEAR REGULATORY COMMISSION j wasmuctow. o. c. zosas l
k**..* HAR 0 71985 MEMORANDUM FOR: Chairman Palladino Commissioner Roberts Ccesissioner Asselstine Commissioner Bernthal Commissioner Zech a
FROM: William J. Dircks Executive Director for Operations
SUBJECT:
STATUS OF CURRENT ACTIVITIES REGARDING REVIEW OF THE POTENTIAL FOR CONTAINMENT FAILURE FROM IN-VESSEL STEAM EXPLOSIONS DURING CORE MELT ACCIDENTS IN LWRS The staf? has recently sponsored a review by a group of experts on steam explosions of the current-understanding of the potential for containment failure from in-vessel steam explosions in LWRs. Enclosure 1 is a final draft of the report on this review (NUREG-1116).
The term " steam explosion" refers to a phenomenon following a severe core melt accident in which molten fuel rapidly fragments and transfers its energy to the coolant resulting in steam generation, shock waves and possible mechanical damage. To result in a significant safety concern the interaction must be very rapid and must involve a large fraction of the molten core mass. If such an event were to take place within the reactor pressure vessel missiles could be generated which might penetrate the containment and allow early release of fission products. In the Reactor Safety Study (WASH-1400) this mode of containment failure was denoted by the symbol alpha (a) and is often referred to as the a-sode containment failure or simply the a-mode failure.
The review was organized in response to a letter, dated August 24, 1984, from Dr. D. Ross, Deputy Director of NRC's Office of Nuclear Regulatory Resestch, to several experts from both the United States and Europe on the subject of steam explosions. A copy of Dr. Ross' letter is included in Appendix A of NUREG-1116 together. with a list of the experts contacted (referred to as the Steam Explosion Review Group or SERG) and other correspondence related to this review.
The need for the present review arose from the conclusions given in a report issued by the Sandia National Laboratory ("An Uncertainty Study of PWR Steam Explosions," NUREG/CR-3369, May 1984) of uncertainties associated with estimates of the probability of a-mode failure from in-vessel steam explosions.
That document reflected the author's opinions that such probabilities.must be
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, considered to range from 0 to 1. The implication from the Sandia report is i that it is not possible to provide a meaningful best-estimate of the probability of such events. This conclusion was considered to be at variance with conclusions from a number of other studies including those of the NRC staff and its consultants. (See for example, Meyer, J. F. , et. al. ,
" Preliminary Assessment of Core-Melt Accidents at the Zion and Indian Point Nuclear Power Plants and Strategies for Mitigating Their Effects, Appendix B,"
NUREG-0850, Vol. 1.) It was, therefore, deemed advisable to convene a group of experts to review NUREG/CR-3369 and to obtain from them their own independent assessment document. Inofaddition, the likelihood of such events as well as their comments on that since Sandia had recently submitted a related research proposal,'that merits of proposal.it was appropriate to request comments from these experts A meeting was held at Harpers Ferry, West Virginia on November 27-28, 1984' to discuss the results from these' assessments and reviews. To facilitate discussion at the~ meeting the participants were asked to provide their written responses to the questions posed in Dr. Ross' letter well before the meeting. Their submittals are included in NUREG-1116. The Harpers Ferry meeting was co-chaired by Dr. T. Ginsberg (Brookhaven National Laboratory-BNL) and Dr. M. Corradini (University of Wisconsin). Dr. Marshall Berman of Sandia was provided the opportunity to present his views on NUREG/CR-3369 and to discuss the Sandia research proposal.
The report (HUREG-1116) has been reviewed and concurred in by all members of the SERG. Written comments provided by the various reviewers are attached to the report and, to the extent possible, their opinions have been taken into account in the final version of the report. Independent comments by the authors of NUREG/CR-3369 are also attached to the report.
l The coreSERG concludes (the melt) probability that best the a-mode failure estimate valuesis of low conditional cluster around 10(1 e., g%n a 4 to 10 ).
Tha SERG disagreed with the conclusion reached in NUREG/CR-3369 regarding the need to adopt an uncertainty range of 0 to 1. In general the SERG supports the develooment of a carefully defined steam explosion research program to provide additional confirmation of its estimates of low probability.
The recent report by the American Physical Society (APS) on Radionuclide Release from Severe Accidents at Nuclear Power Plants (February 1985) also noted that " steam explosions large enough to challenge the containment directly are now considered very unlikely" (see Section VIII, B.1). The APS relied on the fact that "the judgement of experts seems unanimous that the probability is low" (see pages 158-159).
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4 An international study published in 1982 on severe accidents also addresses the e-mode failure (Enciosure 2). That study (the Final Report To CSNI on Severe Accidents by the Senior Group of Experts on Severe Accidents (SEN/ SIN (82)49Tdated October 13, 1982) is, at present, classified Restricted by the CSNI, although a continuance of this Group of Experts is expected to result in a public report by November 1985. That report states "Although detailed mechanisms are still very imperfectly understood the balance of available evidence gives increasing confidence that in-vessel steam explosions capable of damaging the containment are extremely inprobable."
(See page 11 of Enclosure 2).
It should be noted that Drs. D. Ross (RES) and T. Speis (NRR) contributed heavily to this .tview. The report also refers to a companion on the subject document at that (SEN/ time. SINThat (82)24) which summarizes the state of knowledge-document provides a list of considerations and reasons explosion. that contribute to the low probability of a damaging steam ]
i The reasons are consistent with those identified in NUREG-ll16.
The Office of Research has sponsored a considerable level of effort in this !
area for several years. In light of recent budget declines and considering Enclosures 1 and 2, and also considering the recent APS comments, we plan t.o modify our program taking these views into account.
j$igntCWilliam J.Dirths William J. Dircks Executive Director for Operations NOTE: -- The Restricted stamp on the NEA report indicates As Stated that the report has not been cleared by NEA for release to the public, cc: OPE SECY 0GC DIST: NRC Central DST Chron TPSpets Rdr .
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