ML20133K806
| ML20133K806 | |
| Person / Time | |
|---|---|
| Site: | 05200003 |
| Issue date: | 01/16/1997 |
| From: | Huffman W NRC (Affiliation Not Assigned) |
| To: | Liparulo N WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP. |
| References | |
| NUDOCS 9701210346 | |
| Download: ML20133K806 (5) | |
Text
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January 16, 1997 Mr. Nicholas J. Liparulo, Manager
{
Nuclear Safety and R gulatory Analysis Nuclear and Advanced Technology Division Westinghouse Electric Corporation P.O. Box 355 i
Pittsburgh, PA 15230
SUBJECT:
REQUEST FOR ADDITIONAL INFORMATION (RAI) CONCERNING THE AP600 IMPLEMENTATION OF THE REGULATORY TREATMENT OF NONSAFETY-RELATED SYSTEMS (RTNSS)
Dear Mr. Liparulo:
WCAP-13856, "A:600 Implementation of the Regulatory Treatment of Nonsafety-Related Systems (RTNSS) Process," is a summary report on the detailed
~
application of the RTNSS process to the AP600 design that was originally submitted by Westinghouse to the NRC in September of 1993.
Subsequent.to the issuance of WCAP-13856, the NRC provided policy guidance on the RTNSS process for passive plant designs in SECY-94-084 dated March 28, 1994, and SECY-95-132 dated May 22, 1995. However, the NRC staff never provide any formal comments or feedback to Westinghouse on the acceptability'of detailed implementation of the RTNSS process for AP600 as documented in WCAP-13856. Westinghouse has always considered that its RTNSS process is consistent with the NRC policy.
Westinghouse is now nearing completion of the RTNSS process and is' revising WCAP-13856, as appropriate, to reflect the current status of the AP600 review.
Westinghouse letter NSD-NRC-96-4843, dated October 14, 1996, submitted a draft 4
revision to Section 3 of the initiating event frequency evaluation of WCAP-13856. The staff has determined that it needs additional information in order to complete its review of the initiating event frequency section of the RTNSS WCAP. The requests for additional information are contained in the~
enclosure to this letter.
If you have any questions regarding this matter, you can contact me at (301) 415-1141.
l Sincerely, original signed by:
William C. Huffman, Project Manager Standardization Project Directorate i
Division of Reactor Program Management j
Office of Nuclear Reactor Regulation j
Docket No.52-003 g
Enclosure:
As stated Il r
cc w/ enclosure:
See next page g
hf DISTRIBUTION:
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'PUBLIC WHuffman TKenyon DJackson JSebrosky JMoore, 0-15 B18 WDean, 0-17 G21 JFlack, 0-10 E4 RJones, 0-10 E4 ACRS (11)
NSaltos, 0-10 E4 Alevin, 0-8 E23 GHolahan, 0-10 E2 DOCUMENT NAME: A:RTNSS-IN.EVT (IA AP600 DISK)
Ta sessive e copy of thle doownent, hemoste la the ben: *C" = Corty without ettschment/encloswo "E" = Copy with attachment / enclosure
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NAME WCHuffman:tsg)WJFlack %by TRQuay DATE 01/B/97 01//,/9M 01/ /97 9701210346 970116
~ ~ lAL RECORD COPY PDR ADOCK 05200003 A
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o Mr. Nicholas J. Liparulo Docket No.52-003 Westinghouse Electric Corporation AP600 cc: Mr. B. A. McIntyre Mr. Ronald Simard, Director Advanced Plant Safety & Licensing Advanced Reactor Programs Westinghouse Electric Corporation Nuclear Energy Institute Energy Systems Business Unit 1776 Eye Street, N.W.
P.O. Box 355 Suite 300 Pittsburgh, PA 15230 Washington, DC 20006-3706 Mr. John C. Butler Ms. Lynn Connor Advanced Plant Safety & Licensing Doc-Search Associates Westinghouse Electric Corporation Post Office Box 34 Energy Systems Business Unit Cabin John, MD 20818 Box 355 Pittsburgh, PA 15230 Mr. James E. Quinn, Projects Manager LMR and SBWR Programs Mr. M. D. Beaumont GE Nuclear Energy Nuclear and Advanced Technology Division 175 Curtner Avenue, M/C 165 Westinghouse Electric Corporation San Jose, CA 95125 One Montrose Metro 11921 Rockville Pike Mr. Robert H. Buchholz Suite 350 GE Nuclear Energy Rockville, MD 20852 175 Curtner Avenue, MC-781 San Jose, CA 95125 i
Mr. Sterling Franks U.S. Department of Energy Barton Z. Cowan, Esq.
NE-50 Eckert Seamans Cherin & Mellott 19901 Germantown Road 600 Grant Street 42nd Floor Germantown, MD 20874 Pittsburgh, PA 15219 Mr. S. M. Modro Mr. Ed Rodwell, Manager Nuclear Systems Analysis Technologies PWR Design Certification Lockheed Idaho Technologies Company Electric Power Research Institute Post Office Box 1625 3412 Hillview Avenue Idaho Falls, ID 83415 Palo Alto, CA 94303 Mr. Frank A. Ross Mr. Charles Thompson, Nuclear Engineer U.S. Department of Energy, NE-42 AP600 Certification Office of LWR Safety and Technology RE-50 19901 Germantown Road 19901 Germantown Road Germantown, MD 20874 Germantown, MD 20874
1 J
RAIs ON SECTION 3 (INITIATING EVENT FREQUENCY EVALUATION) 0F WCAP-13856 (AP600 IMPLEMENTATION OF RTNSS) 720.366 Westinghouse uses three criteria (p. 3-2) for selecting nonsafety-related SSCs, which contribute to initiating event frequencies, for RTNSS. Only those nonsafety-related SSCs which satisfy all three criteria will be considered for RTNSS. These criteria are applied to each of the initiating event categories documented in the baseline PRA. Criterion 1 is clear, it simply states that an SSC which was not considered in the calculttion of any initiating 4
event frequency will not be considered for RTNSS.
Criterion 2 states that an SSC which does not contribute sianificantly to the frequency of any initiating event will not be considered for RTNSS.
Criterion 3 states that an SSC which has a "significant" contribution to any initiating event frequency (determined by Criterion 2) but it does not sianificantly affect the focused PRA core damage frequency (CDF) and large release frequency will not be considered for RTNSS.
The staff have concerns regarding criteria 2 and 3.
a.
The word "significantly" in criterion #2 should be defined in terms of CDF and large release frequency (i.e., connected to criterion #3). This is necessary to prevent screening out impor-tant SSCs before criterion #3 is applied.
b.
Criterion # 3 has been revised (relaxed). The criterion for deciding whether an SSC (found to have a "significant" contribution to an initiating event frequency) significantly affects the focused PRA CDF or large release frequency, has been changed to require 10 percent contribution (instead of 1 percent).
This change is justified, according to Westinghouse, based on
" risk significance definitions suggested by EPRI and those that are used in maintenance rule applications" and the fact that "....
the nonsafety-related system failure probabilities are already modeled conservatively in the AP600 PRA.."
The staff cannot endorse this definition, particularly that nonsafety-related system failure probabilities are already modeled conservatively in the AP600 PRA. Since the screening criteria was originally es-tablished at 1 percent, Westinghouse should explain what impact relaxing the criteria has on the screening of potential RTNSS SSCs.
c.
Usually " significance" is quantified using more than one impor-tance measures. The reliability / availability of nonsafety-related systems, such as the Chemical and Volume Control System, the AC power and the CCW/SW systems, could be lower than that of same systems at operating plants (failure rates used in the AP600 PRA for these systems are from operating plant histories where such systems are safety related).
If it is assumed that a particular SSC (contributing to be frequency of an initiating event) has Enclosure
a j 1 l
high reliability / availability, then it may not be as a "signifi-cant" contributor to the focused PRA CDF and large release fre-i
_ uency but still be important to plant risk. An example is the q
j RCS leak event followed by failure of the CVCS. As the CVCS reliability / availability (assumed in the PRA) increases the contribution of the RCS leak to the CDF and large release 4
j frequency decreases. Therefore, while it can be argued that the 1
contribution to the CDF (or large release frequency) is not l
significant, it cannot be concluded that maintaining the assumed i;
reliability / availability of CVCS is not important.
Please address this issue in screening SSCs, contributing to initiating event
)
frequency, for RTNSS.
d.
The criteria proposed by Westinghouse do not account for cumula-l tive effects.
In determining " risk importance," the contribution of a nonsafety-related SSC to the total (i.e., from all initiating i
events) focused PRA CDF should be considered and not individual initiating event contributions.
720.367 Westinghouse's implementation of the proposed approach for the RCS leak initiating event (page 3-4) indicates that RCS leaks are not an important contributor to the focused PRA CDF or large release frequency (criterion #3). Although this is in agreement with what is reported in the latest revision (Revision 8) of the focused PRA, it is a major change from the previous focused PRA results (which showed the RCS leak initiating event to be a very important contributor to CDF).
Please explain the reason and bases for this change.
720.368 In reference to nonsafety-related SSC contribution to LOCAs, it is stated (page 3-5) that "the contribution of DAS to spurious ADS actuation is much less than that of PMS."
Please explain the i
reason and bases for this statement by referring to design fea-tures and related analyses.
In addition, the qualitative "much less" statement must be quantified in order to show that DAS-related spurious ADS actuations are not significant contributors to the focused PRA CDF and large release.
720.369 The contribution of nonsafety-related SSCs to the focused PRA CDF and large relaase through their contribution to the frequency of transient initiating events (incNding "special" initiators, such as loss of CCW/SW and loss of instrument air) was evaluated without accounting for transfers to ATWS and LOCA event trees.
In pages 3-9 and 3-10, it is stated that "Since the failure of safety-related SSCs determines if an initiating event develops into an ATWS event, nonsafety-related SSCs do not significantly affect the calculation of the probability of an ATWS...."
The staff disagrees with this statement and the rationale used to screen out most transient initiating events.
Please re-evaluate the above statement and account,for transfers from " transient" to ATWS and LOCA event trees.
/
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720.370 The staff believes that the external events risk analyses (fires, l
floods and seismic) could reveal additional potential important i
contributors to initiating event frequencies and should be evalu-i ated in identifying candidates for RTNSS.
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