ML20211P038
| ML20211P038 | |
| Person / Time | |
|---|---|
| Issue date: | 07/10/1986 |
| From: | Stello V NRC OFFICE OF THE EXECUTIVE DIRECTOR FOR OPERATIONS (EDO) |
| To: | Asselstine, Roberts, Zech NRC COMMISSION (OCM) |
| Shared Package | |
| ML20210T528 | List: |
| References | |
| FOIA-87-010, FOIA-87-10 NUDOCS 8607160200 | |
| Download: ML20211P038 (3) | |
Text
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NUCLEAR REGULATORY COMMISSION j WASHINGTON. D. C. 20655
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JUL 101996.
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' PEMORANDUM FOR: Chairman Zech
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Commissioner Rcberts
, Commissioner Asselstine Comissioner Bernthal FROM: Victor Stello, Jr. .
Executive Director for Operations
SUBJECT:
CONTAINMENT INTEGRITY
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This is in response to Mr. Palladino's memorandum of June 13, 1986 to H. R. Denton lon the above subject. Mr. Denton's remarks before an Electric Power Research
. Institute (EPRI) conference at Brookhaven National Laboratory on June 2,.1986 cited a conditiona1' probability of containment failure for a BWR Mark I containment of about 90%, given a core melt and reactor vessel breach, which was
. indicated to be based upon WASH-1400. .As a matter of fact. results in WASH-1400 L were that all large scale ccre melt accidents would cause loss of containnent l ' integrity. However,,not all of these core melts would result in a major release.
These remarks were intended te encourage owners of Mark I contabments to focus upon the issues of containment integrity since their designs have smaller
- containment . volumes than others. This value is higher than we currently believe since it does not reflect the safety improvements implemented sinca the Three Mile. Island {TMI) accident. Better estimates of containment failure probabilities'.are being developed, but are presently unavailable. It may be appropriate, therefore, to discuss containment integrity in a broader context, including what.ithe staff is doing to ensure that we have a high confidence in containment integrity in the event of an accident. ,
During the licensing re91ew for all pynts, a large ac'cidental fission product release from the core .is reouired to be: postulated, and an applicant for a license must demonstrate that hist con'dte t. designed to accommodate the temperature and pressure conditicns associated with a large loss-of-coolant accident (LOCA), together with certain other engineered safety features such as sprays and filters, can acceptably mitigate potential radioactive releases to the environment.!
l Hence, the NRC.has a high confidence that all containment buildings can prevent a major release for many degraded core events. The TMI accident, where the containment atmosphere had a high concentration of radioactivity,'
but little was released, bears this out.
However, there are very low probability severe accident conditions under which a containment may be unable to prevent a major release. Although con-tainment structures are conservatively designed to withstand the substantial temperatures and pressures associated with a major pipe rupture, and several i research studies have shown that they retain their integrity at temperatures and pressures well in excess of these conditions, they are not designed to withstand.the additional challenges that might be associated with a complete h
' core-melt. Such challenges from an uncontrolled hydrogeninclude phenomena detonation such or release o as increased pressures' = titin '
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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 TMI was intended to reduce the challenge to containment integrity from a hydrogen detonation. This led to maintaining an inert containment atmosphere such as for the BWR Mark I and Mark II, while others, such as the BWR Mark III, have* elected-igniter systems for hydrogen control.
In a complete core-melt, including failure of the reactor pressure vessel, molten core materials may interact with the concrete within containment causingproductioncfnon-condensiblegasessuchascarbondioxide(C0d.
This may represent a pressure challenge to containment integrity, and the
~important factor in this regard is containment volume. Studies have indicated that the larger volume containments, such 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 insight emerging from recent 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 considerable confidence that major releases for the larger volume containments are unlikely, for many core-melt events.
The smaller containments, such as the BWR Mark I and Mark II, fail more readily as a result of pressure buildup from core-concrete interactions. However, an important potential mitigating factor for virtually all the BWR's is that they contain large suppression pools filled with water, which are capable of
.. scrubbing out and retaining a large portion of any released activity. As a result, the NRC is 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
, scrubbing of fission products by the suppression pool, as well as improved drywell spray performance.
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l NRC's goal is to redu'ceathe frequency of occurrence of severe accidents, thereby I reducing the challenge to containment, integrity,'as well as to mitigate potential releases. Reductions in the frequency of occurrence of severe accidents have resulted from changes such as improved protection against l
hydrogen detonation, and reduction of anticipated transients without scram (ATWS rule). We would expect some further benefits should the proposed rule on station bitckout become final. In conjunction with the nuclear industry, each l
individual plant will also be examined for severe accident vulnerabilities.
This program 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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- ,' 3 in summary, our licensing reviews have consistently emphasized containment performance against postulated accidents. Increased emphasis on severe accidents since TMI has resulted in reduction in severe accident frequency as well as improved containment performance under such conditions. Finally, the staff is vigorously pursuing a number of additional initiatives aimed at ensuring that the probabliity of large releases is kept extremely low.
Original signed by Victor Stellop Victor Stello, Jr.
Executive Director for Operations cc: SE0Y 03C OPE Distribution:
Central File J 00 001836 ED0 r/f VStello JRoe TRehm JSniezek GCunningham RMinogue HDenton RVollmer - '.-
PPAS DMossburg/ Toms TSpeis BSheron ZRosztoczy LSoffer
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