ML20052C785
| ML20052C785 | |
| Person / Time | |
|---|---|
| Site: | Zion File:ZionSolutions icon.png |
| Issue date: | 01/15/1982 |
| From: | Davis P INTERMOUNTAIN LABORATORIES, INC., INTERMOUNTAIN TECHNOLOGIES, INC. |
| To: | Griesmeyer J Advisory Committee on Reactor Safeguards |
| References | |
| RTR-NUREG-0850, RTR-NUREG-850 ACRS-CT-1409, NUDOCS 8205050531 | |
| Download: ML20052C785 (6) | |
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ADMSORY COMunitt 04 January 15 '1982 EACTOP 3ArtguARDS, U.SAR.C.
Dr. J. Michael Griesmeyer G' ' N 19 m
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Subject:
FIRST ROUND REVIEW OF ZION PRA
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Re: Letter, J. Michael Griesmeyer to P. Davis, Dec. 15. 1981 s
Dear Dr. Griesmeyer:
Pursuant to your letter referenced above I have reviewed portions of the Zion PRA study and y concerns are contained herein. % review was restricted to the documents which you provided.II' 2) plus NUREG-0850,I3) and infohnation presented at the Class 9 Subconnittee meeting in Denver on December 16 and 17.
To the extent possible, I have tried to pattern y review after the enclosure to your referenced letter (ACRS Review of the Zion PRA), which appears to delineate most of the important issues associated with PRA's. However, due to the limited information available and the very restrictive time provided, it was not possible to do :nore than an overview of the documents. No in-depth assessment could be made of questionable or sketchy areas. As such, it is possible that y present concerns are adequately considered in other parts of the study, and some of g concerns may be due to a lack of understanding of design details for the Zion plant. On the other hand. I have found several areas which appear to be of questionable validity, as described in the following items. I have made no attempt to rank or group the items, but '. have, per your instructions, tried to avoid broad, generalized concerns as well as what are obviously minor or trivial deficiencies. I hope g limited review is of some use to you and the ACRS.
I consider this effort to be a very important step in the increasing use of PRA in the regulatory process, and I look forward to continued participation in the Zion PRA review. % concerns are as follows:
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1.
The computed core melt probability for Zion (s 4 x 10-5/ reactor year) is quite close to core melt probabilities predicted in other PRA's, including
..the RSS.. However, the computed Zion risks from core melts are substantially less than RSS computed risks. One of the apparent reasons for the difference is the increased integrity ascribed to the Zion containment.
I have therefore concentrated a significant portion of sty review on various aspects of contain-ment integrity, and have the following related concerns:
A.
There appears to be no consideration in References 1 or 2 of gross containment leakage due to failure of the isolation system. This 1
event does not appear en the event trees (as it did in WASH-1400 and other PRA's), nor is it considered in discussions of potential containment leak paths.
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B.
While a deriviation of the containment failure pressure does not appear in the documents provided, it is considerably higher for ZionthanSurry(theRSSPWR). Determination of containment failure pressure is extremely uncertain, as has been noted by other reviewers of this issue.
It would be of interest to determine if the following factors have been considered in the Zion assessment of containment integrity:
Penetration seal failures due to high temperature /high a.
pressure conditions. Seal materials begin to seriously degrade at 400*F. This mode of containment failure has been found to be significar.t in other studies (5. 6) but appears not to be considered in the documents provided.
l b.
Detrimental influence on concrete structure from thermal effects. These include added stress due to thermal expan-sion of the liner, stress from thermal gradient in concrete walls, strength deterioration from elevated concrete tempera-ture (such deterioration can begin to occur at 100*C for I4}).
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c.
Uncertainties in thermal resistance between liner and concrete structure. Such uncertainty can have a large effect on thermal factors described in previous items.
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d.
Interfacing systems failure.
During most accident sequences, either the LpIS or CSIS (or both) are operating or at least lined up to provide flow to the containment. Since much of the equipment (pumps,' valves, piping) for these systems is located outside containment, failure 6f this equipment (including pump seal failure) can provide a leakage path bypassing containment. These systems were designed to provide flow at containment conditions corresponding to the calculated maximum design basisaccident(42psigII), modest temperature and radiation levels in coolant), and not Class 9 accident conditions (pressure up to 135 psig, high temper-ature and radiation levels in coolant). The signi-ficantly more severe Class 9 conditions may produce non-negligible failure probabilities in this equip-ment, and the high radiation levels may preclude its repair and maintenance. This potential is not consid-ered iri the documents provided.
e.
Containment purge system. According to Reference 7, the Zion containment design includes a purge system for hydrogen control which is actuated when H2 **"C'"'
tration reaches 3%. Failure or inadequate operation of this system under severe Class g accident conditions could provide a containment leak path. Such a possibility is not considered in the documents provided.although much discussion of containment integrity is included.
f.
Fan coolers. Containment fan coolers appear to be considered operable and effective for most accident sequences. Reference 3 concludes that fan cooler operation during Class 9 accidents may be, precluded due to high filter aerosal loadings.
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2.
The rates and magnitudes of mass and energy loading on the containment are crucial factors in any assessment of containment integrity. Many aspects of containment loading during Class 1 accidents are complex and uncertain.
In this regard, 1 r,ote the following:
A.
The assumptions regarding extent of core melt (pg. II 5-8, Reference 1) and metal-water reaction appear more optimistic than generally used in PRA's.
(In this regard, the production and combustion of H were said to be the most important differences 2
between the Zion and NRC's assessment (3) during the Denver ACRS meeting. Thisissueneedscontinuedscrutiny.) While such assumption may be more realistic, they need to be carefully assessed and justified, and their influence on accident progression investigated.
In view of the lack of even basic physical infoma-tion necessary for a confident understanding of core melt accident progression details, it seems prudent that pessimistic assumptions be considered to provide a bounding result.
B.
The MRCH code, used in the Zion study, contains inherent uncer-tainties, limitations, errors, and questionable assumptions, some of which are delineated in Reference 8.
A few of these problems lead to non-conservative results.
It is not clear that these problems have been acknowledged and accounted for in the Zion assessment.
(In this regard, I note that the concrete erosion rate quoted for the Zion study is some 5 times greater than that which is more generally acceptedI3) based on recent experiments and analyses).
3.
The V accident sequence (which ranks second in terms of risk contribution) probability (1.1 x 10 ) assessed for Zion is considerably less than for Surry (4 x 10-6) or Sequoyah ( 5 x 10-6). While the system design for Zion is different, I am unable to reconcile these differences even when consid-ering infomation in Reference (10).
4.
The documents provided do not contain a discussion of the basis for and uncertainties in event tree functional success criteria. Such information is essential in order to evaluate the basis and validity of associated plant system success definition.
5.
The P3tential for and adverse influence of cascading effects has become a recent ACRS concern.
I do not see any consideration of such effects in the Zion study.
6.
The probability of loss of off-site power is much lower (E a factor of 10) than used in WASH-1400 or in other generic assessments. Further1mor't, the *.
probability of off-site power recovery is ilso much better (about 7 times higher at 1 min) than assumed in NASH-1400. While cogent arguments are provided for such high grid reliability, actual data are not. When such a large probability difference exists in a critical accident initiator, detailed justification is essential, including the prospects that such probabilities will remain constant in the future when grid reserves are likely to diminish.
(Ialsofoundapparentinconsistenciesinthe-presen-tation.of power restoration probability - Sect. II.4.5.2.1.3, Reference 1).
7.
Indefinite operation of the steam driven auxiliary feedwater pump appears assumed.
Depending on system design, steam driven systems require a minimum steam supply (produced in this case by decay heat) for successful operation.
Thus, indefinite bperation of these systems cannot be sustained.
8.
In WASH-1400, it was assumed that switchover from cold to hot leg injection with the RHR system would be required for cold leg LOCA's after 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> of RHR operation. A similar requirement does not appear considered for Zion although similarities in system design seem to indicate a similar requirement may exist.
9.
In view of the very small risks calculated for the Zion plant, it is not clear that the risks from Class I through VIII accidents are negligible by comparison (as is apparently assumed based on the discussion on pa'ge I-14 of Reference 1). Reference 11 does imply that Class I through VIII hisks are small compared to WASH-1400 results, but it is not obvious that the same conclusion is valid for the Zion result.
- 10. Common cause failures are always an important co'nsideration in PRA's and are very difficult to find and assess. While the documents provided I* U give a good general discussion of the subject, the actual quantitative evaluations were not provided.
I am somewhat concerned about the Zion coninon cause assessment based on failure rates quoted in Tables II.413 4-
l and II.4-15.
For example,, based on the failure rates provided, the failure of electrical buses appear completely independent, contrasted with the WASH-1400 assessment that connon cause contributions will fail two redundant systems 10% of the time.
- 11. Some failure rates appear extraordinarily low co@ared to other PRA results.
For example (from Yable II 4-15) for the HPIS, Zion used a median unavail' ability of 1.2 x 10-6 for 2 of four pumps while WASH-1400 i
computes a value of 1.2 x 10' (2 of 3 pumps). The conditions and requirements assumed don't seem to explain such a large difference.
Similarly, for small LOCA's, the corresponding HPIS differences are 5.8 x 10-9 (one of four pumps) for Zion. vs 8.6 x 10'3 (one of these pumps) for Sur 12.
In WASH-1400, the results are said to' be invalid after 5 years based on consideration of several factors, which appear to be valid. There is no discussion of a similar time limit qualification for Zion. Such factors as increasing site population, changes in the data base, wear out failures, plant design changes etc. can ir. fluence the results as a function of time.
- 13. Over 90% of the Zion risks are calculated to be due to a seismic initiated accident.
I have raised the concern in the past that seismic events of sufficient magnitude to cause nuclear plant accidents will likely minimize evacuation effectiveness due to the high potential for disruption of comunications and destruction of evacuation pathways (bridges, roads etc).
In response to a similar concern raised by Dr. Okrent at the December ACRS meeting, it was stated that no change had been made in the evacuation model to account for these concerns. Unless comunications and traffic arteries necessary for evacuation are (or can be) " seismically qualified", or special evacuation procedures provided in the event of earthquakes, it does not seem realistic or prudent to assume an unaltered evacuation model, especially in view of the dominance of the seismic accident contribution.
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- 14. While the preceding concerns deal mainly with the potential for increasing the Zio.1 risks, it should be noted that many conservative assumptions were made in the assessment. "While the influence of these assumptions is frequently difficult to evaluate, it is clear that some could have a substantial effect.
l Very truly yours, P. R. Davis cc: Dr. David Okrent
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REFERENCES:
1.
Zion Probabilistic Safety Study, Vol.1.
2.
Zion Probalistic Safety Study Sec. O. Probabilistic Risk Assessment Methodology.
e 3.
Preliminary Assessment of Core Melt Accidents at'the Zion'and ~ Indian Point Nuclear Power Plants and Strategies for Nitigating Their Effects.
NUREG-0850. Vol.1. Nov.1981.
4.
Task 2: Concrete Properties in Nuclear Environment
'A~ Review of Concrete Material Systems for Ap)11 cation to Prestressed Concrete Pressure Vessels, ORNL/TM-7632, J. J. Naus. May 1981.
5.
Severe Accident Sequence Assessment of Hypothetical Complete Station Blackout at the Browns Ferry Nuclear Plant, D. D. Yue and W. A. Condon, presented at the International ANS/ ENS Topical Meeting on Probabilistic Risk Assessment, Port Chester, N.Y., September 1981 (paper to be published).
6.
An Evaluation of Selected Accidents Relative to Underground Nuclear Power Plants, P. R. Davis, et al., Decen.ber 1977.
7.
Design Data and Safety Features of Commercial Nuclear Power Plants (Vol. II),
ORNL-NSIC-55, January 1972.
e 8.
MARCH Code Assessment, S. B. Rivard, presented at US/FRG Core Melt and Fission Product Behavior Research Information Exchange Meeting, Columbus, Ohio, Nov. 2 and 3,1981.
9.
Reactor Safety Study Methodology Ap')11 cations Program: Sequoyah i 1 PWR Power Plant, NUREG/CR-1659, Feb.1931.
- 10. Probabilistic Safety Analysis EPRI NP-424. April 1977.
- 11. A Risk Assessment of a Pressurized Water Reactor for Class 3-8 Accidants, NUREG/CR-0603 R. E. Hall, et al., January 1979.
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