ML20197J987
| ML20197J987 | |
| Person / Time | |
|---|---|
| Site: | 05000000, Diablo Canyon |
| Issue date: | 07/02/1985 |
| From: | Novak T Office of Nuclear Reactor Regulation |
| To: | Fraley R Advisory Committee on Reactor Safeguards |
| Shared Package | |
| ML082840462 | List:
|
| References | |
| FOIA-86-197 NUDOCS 8507170460 | |
| Download: ML20197J987 (3) | |
Text
T' l'
7 o arog%,
UNITED STATES
[ V,,,.,,. " (,E g
NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 20555 o Q: '.l
- /
JUL 2 1985 Docket Nos.: 50-275 50-323 MEMORANDUM FOR: Raymond F. Fraley, Executive Director Advisory Comittee on Reactor Safeguards FROM:
Thomas M. Novak, Assistant Director for Licensing Division of Licensing
SUBJECT:
DIABLO CANYON PRA At its meeting on April 11, 1985, the Committee requested the staff and PG&E to address the suitability of performing a probabilistic risk assessment (PRA) at Level 1 rather than a Level 2 PRA for Diablo Canyon. The staff and the PG&E responses are provided as Enclosures 1 and 2, respectively.
Q
^
r Thomas M. Novak, Assistant Director for Licensino Division of Licensing
Enclosures:
As stated cc: w/ enclosure RRothman LReiter 58rocoum RMcMullen Sisrael AThadani Glear VNoonan LChandler CONTACT:
H. Schierling X27100
[ _.
T 5b? hp M xh(apo N
v' q
i.
L
NRC Staff Response The ACRS has asked the staff and PG&E to address the suitability of performing a Level 1 Probabilistic Risk Assessment (PRA) rather than a Level 2 PRA in the Diablo Canyon seismic reevaluation program. PG&E's response to that request is contained in an enclosure to PG&E Letter No.: DCL-85-210 from James D.
Shiffer to Harold R. Denton, dated June 11, 1985. The following is the staff's response to that request.
The purpose of the PRA is to assess the significance of the differences, if any exist, between the existing seismic design basis of Diablo Canyon and that obtained from the seismic reevaluation program and to identify any possible " weak links" in the plant.
The level 1 PRA provides a systematic examination of the plant and its operation. This results in estima~ted core damage frequencies for various potential plant damage states, such as, early core melt with no containment cooling and containment failure prior to core melt. The systematic characterization of the potential severe accident sequences provides a framework for judging the significance of a particular aspect of the study.
The staff's accumulated experience has indicated that potential off-site consequences are only significant for containment failure prior to core melt and for core melts with no containment cooling. Previous siting study work on consequences at reactor sites, NUREG/CR-2723, can be used to obtain an estimate of off-site consequences without conducting an exhaustive consequence / containment failure analysis. This is a very appropriate e
m
. Enclosure 1 approach since the thrust of the license condition is to obtain a better understanding of the seismic aspects of the plant / site and not a quantita-tive investigation of risk.
Another reason for limiting the effort to a Level 1 PRA !s the state of flux in the containment analysis area due to the evolving implementation of the results of the source term studies. It would be a questionable expenditure of resources to perform a consequences analysis in this envi-ronment which could have the potential to misorient the program away from the plant / site features to generic analysis problems that would not and should not be resolved in this particular forum.
=e
'I
- sey
+
" ' ~ ~
/
NOTE: Partial enclosure only.
PACIFIC GAS AND E LE C T RIC C O M PANY
}M~2 17 ecue staccr san <unc:sco.cauroam mes. om na:
..., m: u s:
June 11,1985 JAM E 5 O. SMsPPE R vect e.tuotwf sevCLgae sowta Graeteaticas PGandE Letter No.: DCL-85-210 Mr. Harold R. Denton, Director Office of Nuclear Reactor Regulation U. S. Nuclear Regulatory Comission Washington, D.C.
20555 Re: Docket No. 50-275, OL-DPR-80 Docket No. 50-323, OL-DPR-81 Diablo Canyon Units 1 and 2 Long Term Seismic Progran Plan
Dear Mr. Denton:
The enclosure to this letter contains PGandE's responses to NRC Staff coments on the Diablo Canyon Long Tenn Seismic Program Plan. This Program Plan was submitted to the NRC Staff for review and approval on January 30, 1985.
The Staff's comments were forwarded to PGandE by MtC letter dated May 9,1985. On May 22,1985, PGandE met with the Staff and verbally responded to the comments.
At that meeting, the Staff requested that PGandE provide a documented response.
These responses consist of supple. mental ciarifying information for items that tre currently described in the Program Plan. There are no new studies or investigations proposed in these responses. As the program progresses,
additional pertinent information will be identified..PGandE intends to provide the Staff with that information in progress reports and at meetings held during the course of the program.
Kindly acknowledge receipt of this material on the enclosed copy of this letter and return it in the enclosed addressed envelope.
Sincerely, J.
iffer Enclosure cc:
S. T. Al germissen D. Perkins (3)
D. Bernreuter (2)
A. J. Philippacopoulos R. D. Brown M. Reich
-[/
ACRS (5)
C. J. Costantino R. P. Savio, ling (5)
H. E. Schier J. Davis f(MI -~
, _ ~
R. T. Do dde D. B. Slennons M
J. B. Martln 8506 5
~
~
P
J Chapter 10 - PROBABILISTIC RISK ASSESSMENT Comment 1 The ACRS has requested additional justification for limiting the PRA to a Level 1 assessment.
Res ponse PGandE believes that a Level 1 PRA will satis fy the fourth element of the seismic licensing condition which requires that "PGandE shall assess the significance of conclusions drawn from the seismic reevaluation studies in elements 1, 2, and 3, utilizing a probabilistic risk analysis and deterministic studies as necessary..."
As recommended by the fourth element, PRA was chosen as a tool to use in assessing the significance of conclusions drawn from the seismic reevaluation s tudies. A Level 1 PRA will give a clear indication of whether the seismic initiator is dominant relative to plant damage, i.e., core mel t.
Fur thermore, a Level 1 PRA will allow determination of major contributors to core melt.
The establishment of whether the seismic initiator dominates core melt and the determination of major contributors to core melt will allow for assessment of the significance of conclusions drawn from the first three elements of the seismic licensing condition.
As part of the Level 1 efforts, PGandE will be determining the frequencies of A plant damage state is a category to which sequences plant damage states.This assignment is made in such a way that, when assembling can be assigned.
the risk model, a manageable number of end states is being assembled, rather than the millions of end states that would be assembled if each individual scenario was accounted for.
There is a correlation between accident sequence and the ultimate consequences, and this correlation is factored into how the plant damage states' are defined. Thus, there are insights and general conclusions
' concerning consequences that can be drawn from Level I results.
When a thorough understanding of the Level 1 PRA tool has been gained, and when the current controversy concerning Level 2 core and containment analyses has been resolved, the merits and cost-effectiveness of extending the PRA to Level 2 and/or 3 will be evaluated. This evaluation and possible extension will be performed outside of the Long Term Seismic Program.
=m 7*g, 0363S/0033X.
l
-