ML20210T722
| ML20210T722 | |
| Person / Time | |
|---|---|
| Issue date: | 09/26/1986 |
| From: | Hoyle J NRC OFFICE OF THE SECRETARY (SECY) |
| To: | Rehm T NRC OFFICE OF THE EXECUTIVE DIRECTOR FOR OPERATIONS (EDO) |
| Shared Package | |
| ML20210T528 | List: |
| References | |
| FOIA-87-10 NUDOCS 8702180287 | |
| Download: ML20210T722 (4) | |
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'{ -[ WASHIN GTON, D.C. 20555 kg*****,hk September 26, 1986 OFFICE OF THE SECRETARY' MEMORANDUM FOR: T. A. Rehm Assistant for Operations Office of the-Executive Director for_ Operations FROM: John,j le, Acting Secretary
SUBJECT:
MEE@INGWITHCOMMISSIONERS' TECHNICAL ASSISTANTS This is to confirm the September 26, 1986 telephone discussion for an Assistants briefing scheduled for 9:00
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a.m., Wednesday, October 8, 1986, in H-1136. The topics on the agenda are as follows:
(1) SECY-86.228, " Introduction of Realistic Source Term Estimates Into Licensing," and 7 (2) Staff's ongoing work.on BWR Mark-I containment.
The briefing is-to be informal with discussions on Source Term in.the morning, followed in the afternoon with discussions on Mark-I containments'.
Attached are some questions the Commissioners' Assistants would like addressed during the briefing on the Source Term effort.
Attachment:
As Stated Copies:
7 John Larkins Charles Ader John Austin Jim Meyer Jim Milhoan Denwood Ross fro I AF - F 7- O/ O Harold Denton Robert Bernero 6'
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MEMORAMotM FOR: Chairman Pallaoino FROM:
Victor Stello, Jr.
Executive Director for Operations SUthltCT: CONTAllMENT INTEGRITY 13, 1986 to H. R. Denton on the This is in response to your memorandum of JuneMr. Denton's remarks be above subject.
Institute (EPRI) conference at Brookhaven National Laboratory on June 2, 1986 cited a conditional probability of containment failure for a swr Mark I containment of about 905, given a core melt and reactor vessel breach, and was These remarks were intended to encourage owners of Marx !
taken from WASH-1400.
containments to focus more clearly upon the issues of containment integrity rather than accident probability and citeo a containment failure probability that. 08 sed upon our present knowledge, is higher tnan we believe Since it ooes not reflect the safety improvements implemented since the Three Mile Better estimates of containment failure probabilities island (TMI) accident. It may be appropriate.
are being developed, but are presently unavailable.
therefore, to discuss containment integrity in a broader context, including what the staff is doing to ensure that we have a high confidence in containment integrity in the event of an accident.
During the licensing review for all plants, a large accidental fission procuet release from the core is required to be postulated, and an applicant for a license must demonstrate that his containment, designed to accommodate the i
temperature and pressure conditions associated with a large Loss-of-Coolant l Acciaent (LOCA), together with certain other engineered safety features such j as sprays ano tilters, can acceptably mitigate potential racioactive releases l to the environment.
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Hence, the Nm, nas a high confidence that all containment buitaings The can 1MI accident prevent a major release for many negraded core events.
where the containment atmosphere had a high concentration et radioactivity, l
but little was released bears this out.
However, there are very low probability severe accident conditions undercon-Although
- which a containment may be unable to prevent a major release.
tainment structures are conservatively designed to withstand the substantia i
' temperatures and pressures associated with a major pipe rupture, and several research stuotes have shown that they retain their integrity at temperatures and pressures weit in excess of these conottions, they are not designed to l
withstand the additional challenges that might be associated with a complete core-melt. Such challenges include phenomena sucn as increased pressures l
from an uncontrolled hydrogen detonation or release of large quantities of non-condensible gases from core-concrete interactions. )Q1)) 57-/ 6 3^//.
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'/ 2 The NRC began to give attention to severe accidents even before the TMI acci-dent, and has increased its emphasis in this area since the accident. With ,
regard to containments, one of the first requirements introduced after IMI was intended to reduce the challenge to containment integrity from a hydrogen detonation. Anong these was t1e provision of a system to safely acecaundate ydrogen generated by the equivalent of a 1001 tuel-clad metal water reaction, i and led to containment inerting or hydrogen igniter systems.
In a complete core-melt, including failure of the reactor pressure vessel, selten core materials may interact with the concrete within containment, causing production of non-condensible gases such as cartion dioxide (CD ).
This may represent a pressure ena11enge to containment integrity, and Ihe important factor in this regard is containment volume. Studies nave indicated that the larger volume containments, suen as for most PWR's and the BWR Mark III, are tolerant and,- if they fail at all, do so only at relatively long times
. (of the order of days) after core-melt. A key insignt emerging from r$ cent research on accident releases or " source terms" is that the timing of contain-ment failure is important; early containment failure without other mitigating factors can result in large radioactivity releases, while delayed failure of even several hours reduces the releases significantly. Hence there is l
' considerable continence that major releases for the larger volume containments
' are unlikely, for many core-melt events.
i i The smaller containments, such as the invR Mark I and Mark II, fall more readily as a result of core-concrete interactions. However, an important potential
. mitigating factor for ali tne BWR's is that they contain large suppression pools (111ed with water, wnich are capable of scrubbing out and retaining a
! large portion of any released activity. As a result of such studies, the NRC l 1s pursuing potential improvements in containment capability under severe accident conditions. These include a possible venting system to preserve containment integrity under severe accident conditions while assuring scruboing of fission products by the suppression pool, as well as improved arywell spray performance.
i the additional hydrogen control measures implemented since the TMI
' Clearly, accident have reduced the conditional containment failure probabutty from the value given in WASH-1400. In addition, NRC's goal is to reduce the frequency of occurrence of severe accidents, thereby reducing the challenge to containment i integrity, as well as to mitigate potential releases. Reductions in 'the frequency of occurrence of severe accidents have resulted from changes such as isproved protection against hydrogen detonation, reduction of anticipated transients without scram (ATWS rule), and the proposeo rule on station blackout. In conjunction with the nuclear industry, each individual plant will also be examined 'for severe accident vulnerabilities. This program, now in its final phase, will begin the individual examinations next year, and is expected to result in the elimination of severe accident vulnerabilities, where these are identified.
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l' In suusnary, our licensing reviews have consistently emphasized Increased emphasis containment on severe i
performance against postulated accidents. accidents since TM! h well as isproved conta'nment performance under such conditions.
start is vigorously puresing a number of additional initiatives aimed at ensuring that the probability of large releases is kept extremely low.
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Victor Stello, Jr.
Executive Director for Operations cc: Comriissioner Roberts Cocnissioner Asselstine Connissioner Bernthat Commissioner Zech SECY OGC OPE i
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