ML20009B911
| ML20009B911 | |
| Person / Time | |
|---|---|
| Issue date: | 04/09/1981 |
| From: | Rowsome F NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES) |
| To: | Minogue R NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES) |
| References | |
| NUDOCS 8107200008 | |
| Download: ML20009B911 (3) | |
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M MLB.k28P April 9, 1981 f.rW [$ ell g
lief!ORANDUtt FOR: Pobert B. Minogue, Director M 2519g7
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Frank H. Rowsome, Deputy Director Q/
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Office of Nuclear Rejulatory Research
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SUBJECT:
TECHNICAL ISSUES IN COlt!!ISSIONER AHEARNE'S QUERY OF f1 ARCH 24, 1981, ENTITLED, " ROLES OF COPPONENT/HARDilARE BEHAVIGR IN ACCIDENT SEQUENCES" The subject letter cites a nunber of exampics in which the treatment of accident process phenomenology and the consequent equipment challenges or failures are afforded rather shplistic or presunptuous treatnent in our IREP studies. Many of these are an expected consequence of the scope and objectives of IREP. A few are deficiencies even within the tems of IREP objectives.
IREP is intended as a nodel of a " quick and dirty" survey of the susceptibility of a plant to core melt accidents that could be perfomed by licensees cutekly and without great cost. Many of the refinements we would like to see in state-of-the-art risk assessments are deliberately sacrificed to mininize the skills and nan-hour requirenents for such s tudies.
The IREP progran is predicated on our belief that it is preferable to survey many plants quickly than a few plants thoniughly.
There are several reasons we believe extensive application is better than intensive application:
(1) l'any reactor owners / operators will be directly involved in a widespread IREP progran.
This would not be the case if a few licensees hired consultants to do nassive PRA studies. We believe the widespread owner involvement is an intrinsic benefit.
(2) Many of the safety weaknesses responsible for the historical close calls were unique to the subject plant or to a few similar plants.
Intensive studies of a few plants could miss such weaknesses in design or operation of individual plants.
(3)
IREP should serve as a building block for more intensive, subsequent PRA refinements. The investment of resources in IREP should not be lost if and when follow-up studies tre done. Thus we can think of IREP/NREP as the first ohue of a UmuuW, ini.cn> ive appi h.a tiun vi grubobiHshe safety s u re
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Robert B. !!inogue -
APR 9 1981 The sinplifications in IREP adopted to minimize the resource requirenents (analyst skills and time) include the omission of thermal hydraulic analysis of core uncovery, containment challenge analysis, and consequence analysis.
In the Connissioner's first example, the accident sequence TilB' entails the failure of all bulk AC power supplies. There is no core cooling, and no containment heat removal. We know that the containment pressure will rise until the rise is limited by containment failure. We can infer from previous studies the time of and pressure at failure within a factor of 2 or so.
It is out of scope for IREP to calculate the details of containnent atmosphere response or of containment failure, as this is not relevant to the goal of identifying how the plant might get in this fix and roughly estinating its likelihood.
It would be interesting, though peripheral to IREP objectives, to calculate the time and pressure at failure of the containment and to detemine the isotopic release. This can be done later if desired.
IREP will give a cursory, qualitative treatment of the possibility that AC power might be restored before core damage or in the interval after the core is lost but before the containnent bursts on overpressure.
Repair of faulted equipment will be addressed in IREP accident analysis, although not in the initial screening of accident sequence likelihood used to sort important accident scenarios from the less important ones.
Conmissioner Ahearne's second example entails reactor vessel rupture due to thermal shock. There will be a niche in the event tree analysis in IREP appropriate to reactor vessel rupture but the analysis of the structural mechanisms and likelihood are outside IREP scope. That, too, can be added later.
The third example involves ATWS. The treatment of scran failure in the IREP BWR studies include the explicit consideration of whether or not HPCI, RCIC, SLC, etc. function, and likelihoods are developed for each variant sequence, but not a consequence analysis.
In the PWR analysis of scram failure, the effects of the reactor coolant pressure spike are not being nodeled. We will instruct the IREP teams to make sure that their event trees can accommodate each of the possible outcones of the pressure spike, in order to preserve the applicability of the IREP building block concept.
IREP teams are not, however, expected to calculate the pressure spike or the likelihood of the several kinds of failure it might cause.
The Commissioner's letter suggests that the phenomenological analyses that are missing from IREP would improve its value as a source of technical and regulatory insights. This is unquestionably true. Such complementary, phenonenological analyses can be done as a follow-up.
It will be essential to do this in the applications of PRA to rulemaki.1gs and regulatory
, guide development such as the degraded core, mininun engineered safety feature, and energency planning rules.
Sone of this supporting phenomeno-logical analysis is already being done under the Severe Accident Sequence Angysis program or the wrk under the SAPffR decision unit in DAE (nee.
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c Robert B. Minogue APR 91981 Nonetheless, the coordination between DSPR and R3R has not been as good as it should becone. 1!e need to develop a nechanism to fold our PRA-based needs for input fron DAE into their hopper along with outside user needs. Bob Bernero and I will work with Sam and Charlie to work out the details.
I Frank H. Ro some. Deputy Director Division of Systens & Reliability Research Office of Muclear Regulatory Research cc:
R. Bernero J. 11urphy Dave Carlson C. Kelber S. Bassett Distributipn:
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