ML20099A273
| ML20099A273 | |
| Person / Time | |
|---|---|
| Site: | 05200001 |
| Issue date: | 06/29/1992 |
| From: | Duncan J GENERAL ELECTRIC CO. |
| To: | Kelly G NRC |
| Shared Package | |
| ML20099A263 | List: |
| References | |
| NUDOCS 9207290080 | |
| Download: ML20099A273 (8) | |
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19.11 HUMAN ACTION OVERVIEW 4
Late in the process of conduedng this PRA, several funedons, previously I
pe formed manually, were automated to rduce the dependence on human acdons. The system fault trees were updated and the PRA results were requandfied. In addidon, other -
studies were performed to provide an improved understanding of human acdons in the l
PRA.
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Sensidvity studies of the core damage frequency resuldng from the level 1 analysis j
were conducted (Subsecdon 19D.7.t). From this study, four human nedons afts:
accidentinidadon'were found to be the most important, hey are acdone taken to' provide water iglecdon to the reactor vessel if the several automade (q)eedon features fail e
to accomplish this funcdon.
-I In addition, the PRA was reviewed to compile a list of human acdons wisch were j
asumed in other parts of the analyds (Subsecdon 19D.7J). From this list and the above mendoned sensitivity studies, nedone were idendfied which should be given consideradon as being " CRITICAL TASKS" as defined by the human factors evaluation Design Acceptance Criteria, as noted in Seedon 18E.t. These human factors are listed or.
referenced in Subsecdon 19D.7A. :
ne human acdons lists were also reviewed to ensure consistency with the ABWR :
emergency procedure guidelines (Aprendia 18A). nis review is documented in) o
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- Appendix 18F. Some of the acdons are not appropriate for includon in' the symptom based emergency procedure guidellaes. These are included in the COL applicant nedon item list in Secdon 19.9 " COL License Informadon."
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.e-19.12 PRA INPUT TO Tile RELIABILITY ASSURANCE PROBLEM
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The major results of the PRA were reviewed to detennine the reliability and maintenance aedons that should be considered by the COL applicant throughout the life of the plant. This review is documented in Appendix 19L The level 1 analysis results were reviewed by examining two important measures
("Fussell Vesely" and _" Risk Achievement Worth"). IndMdual systems and components were identified as being most important (Table 19ES 1).
The balance of the PRAwas reviewed (Sections 19K4 through 19L8) to-determine other important features not addressed in the level 1 analysis.
The most important features thus identified were finally reviewed to determine appropriate maintenance and surveillance acdone (Secdon 19L9).
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4 19,15
SUMMARY
OF INSIGHTS GAINED FROM THE FRA ne PRA was conducted with several objecdves in mind:
1)
To ensure that the PRA related goals in the ABWR Ucensing Review Bases established in 1987 were nadened.
2)
To review and improve the design capability for potendal weaknesses or relative vulnerabilldes, not withstanding the achievement of the Ucensing Review Bases goals.
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To idendfy the most important aspects of the design and its operation so that particular attention can be placed in these aspects during cerdScadon, detailed design, plant operadon.
4)
To provide addidonal basic studies which were not anticipated when the Ucensing Review Bases was established.
5)
To provide uncertainty /asitivity studies of key res' uhs.
he objectives were achieved as noted in the following subsecdons..
1 19.13.1 Ucensing Review Bases Goals Her goals were established to ensure that an appropriate balance between accident un.ntion and accident mitigation is achieved by ADWR. The goals'.
L (Table 19.61 provides a summary) focus on prevenden (core damage frequency less than IM per year); mitigation (avoiding containment failure from several potential threats) and offsite consequences (as measured by offsite doses, con equences, condidonal containment failure probability, and the Safety Goal Policy Statement).
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Measurement agalast these goals and the features which are important in achieving the goals are discuned in detail in Secdon 19.6. - (19,6 will later be rettsed in -
the SSAR to redect new numbers.) The goals are satisSed, indicating a very iobust design with an excellent balance between accident prevention and mitigadon features.
19.18.2 The Search for Vulnerabilities As noted in detail in Seedon 19.7, the PRA proceu was used extensively _ o improve :
t the design, even though it'could be argued that satisfying the goals of Secdon 19.6 was sufficient. Improvements were made in many areas, including for example: the-aatomation 'of several accident prevendon functions, the addition of a combusdon '
turbine generator to improve power supply diversity, the addidon of an ac independent water addition system to improve accident prevention and midgadon, and theiddidon of
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- two panive accident mitigadon featu:es (the lower drywell flooder and the containment a overprenure protecdon system) which substantially addren uncertainties associated with severe accident progression. Proceduralimprovements'were also identifv? Many other :
e.umples are cited in Secdon 19.7 to illustrate the mar.ner in which PRA tedmiques were used throughout the design process to improve the design.1 19.13.3 The Most!mportant Aspects of the Design L
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The ABWR design and its operation was reviewed to determine the featuret, and; operator acdons which are mostimportant from a PRA perspecdve. Applying additional --
focus in these aspects cari provPe con 6dence that ABWR operation will be as accident :
resistant as characterized by tne PRA.
The key design festures were identiBed for input to ths deE l design description, a--
y key certi6 cation effort. These key features are provided in Section 19.8..
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l T1 e potential for human error was reviewed extensively (Secdon 19.11) to ensure.
that " CRITICAL TASKS" were identified for the human factors Design Acceptance Criteria and to ensure that human acdons are covered by :he emergency procedures guidelines or other, more specific procedures.
De PRA results were reviewed to determine which surveillane d maintenance activities are most important throughout plant life (Section 19.12).
19.13A AdditionalStudies a
Several additional studies which were not anticipated in the original Licensing Review Bases were cc,nducted to furt'[er review the robustness of the ABWR design..
The potential for internal fires to lead to core damage is studied in Appendix 19M. The basic ABWR features of separating the three safety divisions into individual fire -
zunes and the abiRv to control key systems from outside the control room are the major reasons that very low core damage frequencies are calculated.
Internal flooding is investigated in detail from both a deterministic and probabilistic perspective in Appendix 19R. Divisional and building separation along with other key flooding mitigation features are idendfled which lead to the conclusion that -
there h a very rmall threat posed by internal flooding. General guidelines for addressing the potential for severe external flooding are provided in Section 19.9.
- A sdsmic margins analysis (Appendix reference later) was conducted to assess the potenGal for seisml'. ewr.ts beyond the design basis to lead to core damage. It was
' determined tMt there is high confidence in a low failbre probability, ewm at ground accele mtiore approximately two times the plant seismic design basis. Key components and their seismic capacities are identified so that the COL applicant can review the design capability against those wumed in this margins snalysis.
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1 An assessment of the potential for cure damage to result kom ABWR operadons a
while shutdown is documented in Appendix 19Q Potendal precursor events are reviewed for their applicability to ABWR and several ABWR features are noted which reduce the risk from activities conducted while shutdown.- A decay heat removal reliability study is conducted to provide input to the COL applicant as to which complements of decay heat removal and water addidon system could be kept avaliable -
while shutdown to reduce the risk of core damage resulting from the loss of an operadngi RHRsystem.
19.13.5 Uncertainty and Sensitivity Studies After the plant system fault trees were updated to reflect several design improvements, the level I results were requantitled. Then a data uncertainty study was -
performed (reference subsecdon later). The resulta show a mean core damage frequency of about 1.5F 7 events per year. The 95th percentile value is about three times this value, or 4.5E 7 per year. Thus, the effect of data uncertainty is retadvely minor. The most--
important contribution to the uncertainty is the RCIC maintenance activity. This acdvity is addressed in the PRA input to reliability assurance (Appendix 19K).
s A comparison of the requantified level I results to those for Grand Gulf was also developed (Due 7/15) to document the major reasons for reductions in the Requency of--.
the various accident classes. The sensitivity of the resuha to equipment outage times and!
surveillance intervals was considered (Due 7/15). The contribution' of human errors was.
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- compared to the contribution from an operating plant (Duc date not yet established)..
certainties associated with severe accident progression were examined in detall-
. through the use of containment event trees supplemented by decomposition event treesc The later were used to study the potential for different outcomes of various severe.
accident events. The results show that the ABWR design is very robust. Analysis of -
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s' phenomena such as direct containment heung were performed which indicate that the
. probability of occurrence with sufHelent magnitude to fail the containment is very small.
The design is not sensitive to anumptions affecting debris coolability due to its high strength and lower drywell/ pedestal design. The studies also demonstrated that the features of the ABWR design substantially reduced the uncertainty anociated with many severe accident phenomena. In many areas, these studies were conducted in greater depth than studies with similar objectives reported in NUREG 1150 and its supporting documents. In addition, the basis for thejudgements made is described in detall.
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