Nuclear Operating Company

South TTas Pmect Electnc Generatin$ Staton PO. Bar 289 Wadsworth. Txas 77483 ,

January 11,2012NOC-AE- 12002784STI: 33215081File: G25

U. S. Nuclear Regulatory CommissionAttention: Document Control DeskOne White Flint North11555 Rockville PikeRockville, MID 20852-2738

South Texas ProjectUnits 1 and 2

Docket Nos. STN 50-498, STN 50-499Summary of the South Texas Project Risk-InformedApproach to Resolve Generic Safety Issue (GSI)-191

A summary of South Texas Project's methodology towards a risk-informed resolution of GSI-191, "Assessment of Debris Accumulation on PWR Sump Performance" is provided in theEnclosure. The summary includes the initial quantification and project results to date.

There are no regulatory commitments in this letter.

Should you have any questions regarding this letter, please contact either Jamie Paul, Licensing,(361) 972-7344, or me at (361) 972-7074.

J.W. CrenshawVP New Plant Deployment / SpecialProjects

JLP

Enclosure: Summary of GSI-191 Risk-Informed Closure Pilot Project 2011: InitialQuantification

NOC-AE- 12002784Page 2

cc:

(paper copy) (electronic copy)

Regional Administrator, Region IVU. S. Nuclear Regulatory Commission612 East Lamar Blvd, Suite 400Arlington, Texas 76011-4125

Balwant K. SingalSenior Project ManagerU.S. Nuclear Regulatory CommissionOne White Flint North (MS 8B13)11555 Rockville PikeRockville, MD 20852

Senior Resident InspectorU. S. Nuclear Regulatory CommissionP. O. Box 289, Mail Code: MN116Wadsworth, TX 77483

C. M. CanadyCity of AustinElectric Utility Department721 Barton Springs RoadAustin, TX 78704

Stewart BaileyBranch Chief, Safety Issues ResolutionU. S. Nuclear Regulatory CommissionOne White Flint North (MS 011 F01)11555 Rockville PikeRockville, MD 20852

Donnie HarrisonBranch Chief, PRAU. S. Nuclear Regulatory CommissionOne White Flint North (MS 011 F01)11555 Rockville PikeRockville, MD 20852

A. H. Gutterman, EsquireMorgan, Lewis & Bockius, LLP

John RaganChris O'HaraJim von SuskilNRG South Texas LP

Kevin PolioRichard PenaCity Public Service

Peter NemethCrain Caton & James, P.C.

C. MeleCity of Austin

Richard A. RatliffAlice RogersTexas Department of State Health Services

Balwant K. SingalStewart BaileyDonnie HarrisonU. S. Nuclear Regulatory Commission

Summary of GSI-191 Risk-Informed Closure Pilot Project2011: Initial Quantification

South Texas Project, Wadsworth, TXNOC-AE-12002784

January 11, 2012

EnclosureNOC-AE-12002784 STP Nuclear Operating Company's NRC/Industry. GSI-191 Closure Pilot Project

Executive Summary

The main objective of the Risk-Informed GSI-191 Closure Project is, "Through a risk-informed approach,establish a technical basis that demonstrates that the current design is sufficient to gain NRC approval toclose the safety issues related to GSI-191 by the end of 2013." The results presented in this summary arethe joint work of STP Nuclear Operating Company (STPNOC) Risk Management, Los Alamos NationalLaboratory, The University of Texas at Austin, Texas A&M University, Alion Science and Technology, ABSConsulting, ScandPower, Mike Golay, The University of New Mexico, and KnF Consulting Services, LLC.

In the risk-informed approach, STPNOC would seek exemption from certain requirements of 10 CFR 50.46if the risk associated with the fibrous insulation in STPNOC's containment buildings is not risk significant.If STPNOC determines this insulation to pose a significant risk, STPNOC is committed to investigatingplant modifications including insulation removal and other measures to preserve sufficient margins for nuclearsafety.

The 2011 preliminary results show that the change in risk for fibrous insulation in contain-ment is less than 1.OE-06 in core damage frequency (CDF) and less than 1.OE-07 for largeearly release frequency (LERF), that is, very small per RG 1.174.

This result represents the uncertainties of more than 20 input parameters and the complementary executionof the physics-based (CASA Grande) model, thermal-hydraulics model, and PRA models. Although previousrealistic testing has shown that chemicals are unlikely to affect the head loss in STPNOC debris beds (sumpstrainers and fuel assemblies), the Pilot Project has used an initial methodology that adds pessimistic head-loss estimates from chemicals. Including these estimates is believed to fully addresses NRC concerns raisedin pre-licensing meetings related to sump chemistry.

This preliminary assessment of the CDF and LERF gives us confidence that the issues asso-ciated with fibrous insulation in the STPNOC reactor containment buildings will be furthershown to be non-risk significant with adequate defense-in-depth and safety margins such thatSTPNOC will be able to provide a sufficient basis for GSI-191 closure by the end of calendaryear 2013.

STPNOC will expand upon the project's technical contributions by including the uncertainties of morethan 50 input parameters and the seamless integration of CASA Grande, a new jet formation model, un-certainty propagation in the thermal-hydraulics models, and PRA analysis that takes into account detailedoperational conditions. The resulting framework will provide STPNOC the ability to assess future issues onrisk-informed basis as they may arise.

The methodologies and results of the pilot project in 2011 are presented in the following documents:analysis of results from the physical process solver, CASA Grande and RELAP5 thermal-hydraulic analyses(Letellier, 2011); LOCA Frequency analysis (Fleming et al., 2011); Uncertainty quantification methodologiesand illustrative examples (Popova and Galenko, 2011); Jet formation research (Schneider et al., 2011); andChemical effects research and experimental design (Sande et al., 2011).

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EnclosureNOC- AE- 12002784 STP Nuclear Operating Company's NRC/Industry CSI-191 Closure Pilot Project

Contents

1 Introduction 11.1 Previous efforts .......... ............................................ 11.2 Risk-informed ........... ............................................. 11.3 Pilot Project .......... ............................................. 2

2 Major assumptions 2

3 Findings in 2011 33.1 Chemical effects ............... ........................................... 43.2 Strainer and downstream performance ......... ............................... 53.3 Destruction zone ............... ........................................... 5

4 CASA Grande 7

5 Plant configuration 7

6 PRA 8

7 Uncertainty Quantification (UQ) 10

8 LOCA frequency 11

9 Quality Assurance 11

10 Licensing 11

11 Conclusions 13

List of Figures

1 Illustration of the core damage (CDF, ACDF) risk associated with as-designed fibrous insu-lation in the STPNOC containment on the Regulatory Guide 1.174 decision-making Regionmap ........... ................................................... 4

2 Illustration showing how decreasing decay heat removal flow requirement over time reducescooling flow requirements that would result in significant reductions in head loss across debrisbeds. Experiments of in-core performance with chemical effects show significantly greatertolerance (higher mass loadings) for fully loaded debris beds at lower flow rates ........... 6

3 Illustration of a CASA Grande (complementary) cumulative distribution and the method usedto develop input for the PRA from it ........................................ 8

4 High level illustration of the inputs and outputs overlaid on a schematic diagram of the PRAevent tree structure. The event tree structure, with the exception of the long term coolingtop event, already exists in the typical nuclear power plant PRA ..................... 9

5 Uncertainty quantification for complex computer models ..................... 106 Illustration of the major elements of the STPNOC quality assurance plan for risk-informed

closure of GSI-191 .......... ........................................... 12

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EnclosureNOC-AE-12002784 STP Nuclear Operating Company's NRC/Industr. GS1-191 Closure Pilot Project

1 Introduction

The purpose of this document is to summarizethe initial quantification performed for the risk-informed closure path in STPNOC's NRC/Industryrisk-informed GSI-191 pilot project (Pilot Project).In this document, significant findings from PilotProject work in calendar year 2011 are summarizedand the overall program approach is described. Whilethe risk-informed approach is not new to investiga-tion of GSI-191 closure, the approach in the PilotProject is more comprehensive than previously con-templated. The closure process developed is gener-ically applicable with site-specific information sup-plied. Each primary area of investigation in the PilotProject is summarized in this document.

1.1 Previous efforts

Since 2001 GSI-191: Assessment of Debris Accumu-lation on PWR Sump Performance has eluded reso-lution despite significant efforts by industry and theNRC. Although recent thought has been given to riskquantification (see for example Teolis et al., 2009) andearly recognition of the need for risk evaluation wasidentified, (e.g. Darby et al., 2000), serious investiga-tion into risk quantification has not been undertakenuntil now. Instead, resolution has followed a classicaldeterministic approach. It is STPNOC's view, follow-ing an initial 2011 PRA quantification, that a risk-informed resolution path should be pursued in prefer-ence to a deterministic approach, thereby quantifyingthe safety margins and identifying any scenarios thatpose significant risk in GSI-191.

The primary issue here is that in a deterministicapproach, examination of the uncertainty required toevaluate risk of plant operation is abandoned and anappeal to a hypothetical "worst case" scenario (as-sumed to encompass all uncertainty) is made. By as-suming a sufficient set of nonphysical processes alongwith assumed equipment performance scenarios, ex-amination of the full range of possible scenarios isavoided. Although this approach can be effective insome cases, there are shortcomings associated withit (NEI, 2009) and, relative to GSI-191 closure, havebecome untenable. That is, the "worst case" is so

limiting that it would indicate no design would be ac-ceptable, especially for in-vessel effects (Baier, 2011).Even the presence of tramp debris (regardless of theinsulation design) may not be tolerated. Unless com-promised by incorporating the time-evolution of acci-dent phenomena and accommodation of realistic be-havior, it may be difficult to provide a satisfactoryclosure path using a deterministic approach.

1.2 Risk-informed

In keeping with the agency's commitment to move to-wards increasing risk informed regulation, the NRCdirected the staff (Vietti-Cook, 2010) to considerGSI-191 resolution strategies including an Option 2:

"The staff should take the time neededto consider all options to a risk-informed,safety conscious resolution to GSI-191.While they have not fully resolved this is-sue, the measures taken thus far in responseto the sump-clogging issue have contributedgreatly to the safety of U.S. nuclear powerplants. Given the vastly enlarged advancedstrainers installed, compensatory measuresalready taken, and the low probability ofchallenging pipe breaks, adequate defense-in-depth is currently being maintained.

The operative words for Option 2 are in-novation and creativity. The staff shouldfully explore the policy and technical impli-cations of all available alternatives for riskinforming the path forward. These alterna-tives include, but are not limited to, how50.-46a might impact this issue, and how theapplication of a no-transition-break-size ap-proach might work."

It is worth reemphasizing that a risk-informed anal-ysis includes all scales of postulated accidents andthe full spectrum of possible plant responses. Ideally,there should be no exclusive focus on "bounding" as-sumptions, no exclusive focus on "design basis" or"beyond design basis" assumptions, and no exclusivefocus on "best estimate" assumptions. Every factorin the accident analysis should be described as accu-rately as possible by a statistical distribution that is

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EnclosureNOC-AE-12002784 STP Nuclear Operating Company's NRC/Industry GSI-191 Closure Pilot Project

consistent with available data, physical models, or ex-pert opinion, and so, factors with limited or no datanecessarily have bigger uncertainties. Furthermore,the methods used to sample and propagate these un-certainties should be unbiased.

In our initial quantification numerous conser-vatisms are retained to maintain consistency withregulatory assumptions while developing a robusttoolset and exploring parameter sensitivities.

1.3 Pilot Project

STPNOC is the plant working with the staff to de-velop risk-informed closure strategies while preparinga site-specific licensing submittal. Over the 2011 cal-endar year, several public meetings were conductedto inform the NRC staff of the modeling approachand to solicit feedback on the applicability and useof the approach for resolving GSI-191. The severalmeetings included supporting material so that mem-bers of the public, especially other plants could beinformed as well: Rosenburg (2011), Thadani (2011),Singal (2011c,a,b,d,e,f,g).

All materials provided have been posted by theNRC on their website for public access.

In design of the Pilot Project, STPNOC has beencareful to make the proposed implementation at otherplants as straightforward as possible. In particular,the physical models that typically would be foldedinto the event trees and fault trees of the PRA havebeen extracted into a flexible modeling tool and heldoutside the PRA. In the Pilot Project this part ofthe toolset is called CASA Grande (Letellier, 2011).This way, the methodology is made more generic andonly simple changes should be required in the site-specific PRA. The existing PRA LLOCA, MLOCA,and SLOCA branches are maintained intact and onlytwo relatively simple changes are required in the PRAitself: the sump strainer demand failure likelihood isreplaced by a value that CASA Grande produces asan output; and a top event must be added for longterm cooling relative to in-vessel effects.

The rest of this report summarizes the GSI-191risk-informed closure process and methods that havebeen developed over the past eight months. Severalassumptions were adopted in the initial quantifica-

tion in order to ensure the Pilot Project met 2011schedule commitments. Section 2 provides a sum-mary of the major assumptions made in the initialquantification. Section 3 summarizes some of the sig-nificant insights obtained from the Pilot Project workso far. Section 4 describes the several physical mod-els that have been developed in 2011, others will befurther refined and developed in the second projectcalendar year (2012). A new integration frameworkhas been developed and is described as well in Sec-tion 4. A containment CAD model and toolset usedin the project are briefly described in Section 5. Sec-tion 6 is a short description of the PRA modelingrequired to obtain the change in risk needed for Reg-ulatory Guide 1.174 decision-making. In Section 7the methodology developed in calendar year 2011 foruncertainty quantification is described. The statusof the Pilot Project LOCA frequency analysis is pre-sented Section 8. An overview of the Quality As-surance Plan is provided in Section 9. The licensingapproach is designed around Regulatory Guide 1.174(Section 10). Conclusions are in Section 11.

2 Major assumptions

Several assumptions had to be adopted in calendaryear 2011 to accomplish a quantification result withinthe aggressive Pilot Project schedule. In some cases,the assumptions bias the resulting CDF and LERFhigher, in some cases the assumptions bias the resultslower. The Pilot Project has tried to adopt a policyregarding assumptions such that on balance, the CDFand LERF results would be biased higher than withall assumptions relaxed.

The following list summarizes of the major assump-tions in calendar year 2011.

ZOI: The zone of influence is assumed to be 17D forall fiberglass targets regardless of configurationor the presence of piping restraints. ZOI is nottruncated by the presence of compartment walls.

Chemical precipitants: All chemical precipitants(formed based on the available material in con-tainment) are formed and introduced in bulk at24 hours following the LOCA regardless of the

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EnclosureNOC-AE-12002784 STP Nuclear Operating Company's NRC/Industry GSI-191 Closure Pilot Project

operation of containment spray. In-vessel pre-cipitants are assumed to be of the same formand amount used by Bajer (2011). The headloss due to precipitants on the strainer debrisbed doubles the head loss through the bed. Allchemicals are passed through the strainer if it isnot entirely covered with a 1/8 inch-thick uni-form debris bed.

Debris generation: 60% of the debris is fines, 40%is large. 1% of large debris is eroded to fines with100% spray exposure.

Transport of fine debris: 100% go to the sumppool, 4% are retained on the strainer bed (perstrainer), 13% go to dead volumne (other sumps,elevator shaft, dead volumes inside the bioshieldwall).

Transport of large debris: 1% is eroded to finedebris, 99% retained on gratings.

Upstream effects: Upstream effects are assumedto have insignificant contribution.

Strainer configuration: The strainer meets designat the start of the LOCA transient (open bypasspaths' contribution from damage for example, isassumed negligible).

Scenario event timing: The scenario progressionand associated event timing are assumed tofollow nominal values in each LOCA category(small, medium, large LOCA). This also appliesto Operator actions with the exception of con-tainment spray operation. In the case of contain-ment spray, the Operator is successful in stop-ping the single train (when three trains havestarted) as required but fails to terminate con-tainment spray operation (the remaining twopump trains) after meeting termination criteria.Containment spray is never terminated when lessthan three trains have started.

LOCA frequency: The existing STPNOC PRALOCA branch frequencies for small, medium,and large LOCA are preserved in the calcula-tion of location-specific frequency calculations

(to avoid over-estimating or under-estimatingthe LOCA frequency in the source term). Thatis, in each category of LOCA, the location-specific values for that category closely sum toits PRA initiating event frequency. For the pur-pose of in-vessel hydraulic effects on core flowand head loss calculations, the LOCA frequencyis split evenly between hot and cold leg locations.

Qualified coatings: At time zero (at the time ofthe LOCA), 33 lb of Epoxy and 553 lb of IOZcoating particulate is assumed to arrive in bulkto the sump pool. This amount is present in allbreak scenarios (regardless of break size).

Unqualified coatings: At 24 hours, 247 lb ofAlkyd, 843 lb of IOZ, 268 lb of enamel, and294 lb of epoxy coating particulate arrives in thesump pool.

Latent debris: 170 lb of particulate and 12.5 ft 3 of

fiber arrive at the sump pool at the start of theLOCA (time 0).

Strainer bypass: Strainer bypass is a function offace velocity and the number of operating trainsonly (Zigler, 2011). All debris bypassed is de-posited in the core and evenly spread over thefuel assemblies.

Strainer head loss: Head loss follows the behaviordescribed by Zigler et al. (1995). The strainersdo not collapse from overload due to differentialpressure.

3 Findings in 2011

In calendar year 2011, a primary objective was toobtain an initial PRA quantification of the risk as-sociated with fibrous insulation in containment. Theresults of the initial quantification were included inthe project plan as a criteria for going forward orterminating the project and taking aggressive correc-tive action to correct any high risk scenarios revealedin the risk assessment. A major finding of the 2011

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EnclosureNOC-AE- 12002784 STP Nuclear Operating Company's NRC,/Industr'y GSI-191 Closure Pilot Project

1

Illustration of 2011 "Initial Quantification" on RG 1.174 Regions•........... . . Region I

" No changes allowed

Region II. Small changes

LL* Track cumulative impacts

Region III< -.Very small changes

. More flexibility withrespect to baseline CDF

. Track cumulative impactst-5 W&.i:: :... .•.". .:•

U

Region II

10-6 Fibirou's aisuiation risk

at Sip (conservative)'.Region III

10-6 10-5 104 CDF

Figure 1: Illustration of the core damage (CDF, ACDF) risk associated with as-designed fibrous insulationin the STPNOC containment on the Regulatory Guide 1.174 decision-making Region map.

quantification is the risk is "very small" in the lan-guage of Regulatory Guide 1.174 as illustrated in Fig-ure 1. Regulatory Guide 1.174 is concerned with theneed to make changes guided by risk.

The risk of ongoing operation is maintained in thesite-specific PRA average risk model for CDF andLERF. The STPNOC PRA meets the ASME/ANSPRA Standard as Capability Category II and hassuccessfully provided the technical basis for severalrisk-informed applications at STPNOC, for exampleRMTS (Yilmaz et al., 2009; EPRI, 2008). A changeto the plant may increase risk (CDF and/or LERF)and is expressed as ACDF and ALERF. Clearly, if achange results in a large change in risk, it could affectthe plant average risk. But if the ACDF is less than1.0X 10-6 and the ALERF is less than 1.0x 10', thechange in risk is considered to be very small and thecumulative risk to CDF and LERF do not need to beconsidered in decision-making. On the other hand,the effect on plant average risk must be considered if

the changes are not very small. In this case, basedon the "very small" risk evaluation, STPNOC man-agement has authorized the project to go forward ascommunicated in the calendar year 2011 Pilot Projectplan.

3.1 Chemical effects

Based on early prototypical experiments performedby Dallman et al. (2006) and (much later) based onstudies by Sande et al. (2011), STPNOC has main-tained that in the initial quantification, chemical ef-fects could be minimized. STPNOC understood fromthe start of the project that any assertions aboutchemical effects would require experimental verifi-cation. However, based on the feedback from theNRC staff during the Pilot Project meetings, (Sin-gal, 2011g), chemical effects were added to the initialquantification. The approach was modified to includethe results found by Baier (2011) in experiments in-tended for deterministic evaluations.

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EnclosureNOC-AE- 12002784 STP Nuclear Operating Company's NRC/Industrgy GSI-191 Closure Pilot Project

With the inclusion of Baier's chemical effects us-ing mass of fiber as the success criteria, the ACDFincreases from extremely low to roughly 5 x 10'.The ALERF remains unchanged. Based on this re-sult and based on earlier NRC staff Pilot Projectfeedback, an important finding is that chemical ef-fects on debris bed head loss needs to be better un-derstood. STPNOC has accelerated and expandedthe risk-informed chemical effects experimental pro-gram in order to better understand chemical effectson head losses in ECCS strainer and fuel assemblydebris beds. The chemical effects experiments willalso be designed to produce results that can be usedto develop correlations for realistic physical modelsof chemical performance for use in CASA Grande.

3.2 Strainer and downstream perfor-mance

As mentioned earlier, the deterministic approach usesa hypothetical "worst case" scenario assumed to en-compass all uncertainty assuming nonphysical mod-els and scenarios. Although it does create a bound-ing case in some sense, the deterministic approachhas assumed chemicals as precipitants are producedeffectively at the start of the LOCA and during a hy-pothetical equipment alignment that produces max-imum ECCS flow rate through the strainer. None ofthese conditions would ever exist simultaneously inthe unlikely event of a LOCA requiring recirculationswitchover with fibrous debris present.

Debris beds in the core and on the strainers in-crease resistance to flow as the bed builds up thick-ness, but more importantly (based on experiment re-sults for deterministic evaluations), the resistance in-creases more when chemical precipitants collect onthem also. Generally, head loss is higher order (nor-mally second order) with flow in a closed channel sys-tem.

Zigler et al. (1995) utilized porous media correla-tions where head loss, H, equals to H = aQ + bQ2 ,where Q is the flow, and the coefficients a and b aredetermined by the fluid and debris bed conditions.

Referring to Figure 2, one can imagine how dra-matically the head loss would increase at constantflow rate as the coefficients a and b increase (caus-

ing the system characteristic to steepen) due to fiberloading and chemical precipitant addition. In fact,NRC staff has stated in Pilot Project review meetings(Singal, 2011e,f) this behavior has been observed dur-ing experiments as chemical precipitants were addedat constant flow and the head loss increased signif-icantly (Baier, 2011). However, and again as illus-trated in the figure, this behavior might not be re-alized in an actual scenario because in the unlikelyevent of a LOCA that produces an in-vessel debrisbed, the required flow decreases dramatically (lin-early with decay heat which is effectively an expo-nential function). Additionally, chemical precipitantssimply can't be created while the required flows arehigh (say within the first day to one week).

A significant result of the risk-informed analysisthat takes into account the evolution of the physi-cal processes is that: there are no ECCS failures dueto strainer blockage (this needs to be examined forsome infrequent potential sequences that, for exam-ple may cause mechanical collapse of the strainer);all hypothesized small LOCA events succeed; almostall medium LOCA events succeed; and most largeLOCA events succeed. All scenarios resulting in coredamage were associated with in-vessel effects, notstrainer blockage. That is, in a realistic setting thatretains significant conservative assumptions, chemi-cal effects turn out to be less important than previ-ously thought. For example, a 15 grn/FA acceptancecriteria would cause failure for essentially all casesbased on the latent debris alone. But in a realis-tic setting, it was demonstrated (again with conser-vatisms) that most hypothesized LOCA events wouldnot cause core blockage.

3.3 Destruction zone

A somewhat surprising result from the initial quan-tification is the insensitivity to the "zone of influence"or ZOI for use in for example, ANSI (1988). A greatdeal of effort in experimentation and study by the in-dustry has been devoted to understanding if the ZOIcould be reduced to much lower volumes of damagethan the widely accepted deterministic value definedby a sphere of 17D, where D is the break diameter.In fact Schneider et al. (2011) have been studying jet

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EnclosureNOC-AE-12002784 STP Nuclear Operating Company's NRC/Industry GSI-191 Closure Pilot Project

Earlier, when debris/chemical luads ae small,and the cooling flow requirement is high, thesystem characteristic is flatter: lower head lossesat higher flows.

Later, increasing debrisichemicalloads steepens system characteristic:however, the cooling flow requirementdecreases rapidly as well.

500 400

Flow rate needed to cool the core (gpml

o

0

700

600

Sao

400

300

200

100

0

4

g

V_610 rr1 9 11

261 gpm ~1'

46 gpm

4 f 12 16 20

Time (hr)24 708 712 716 720

Figure 2: Illustration showing how decreasing decay heat removal flow requirement over time reduces coolingflow requirements that would result in significant reductions in head loss across debris beds. Experimentsof in-core performance with chemical effects show significantly greater tolerance (higher mass loadings) forfully loaded debris beds at lower flow rates.

formation for use in the risk-informed methodology.The sense is that these destruction volumes are muchtoo large. For example, a 31 inch pipe break wouldcreate a sphere of destruction about 44 feet in diam-eter. A commonly accepted practice is to limit theZOI such that it can't extend beyond the concretewalls of containment compartments.

In this initial risk-informed quantification, the 17Ddestruction volume was assumed in all break loca-tions without restriction for compartment walls. Asensitivity study was conducted to see the effect of re-

ducing the ZOI to about 1/2 17D. While some reduc-tion was observed, this change did not significantlyaffect the risk. This surprising result can be explainedby understanding the scenarios leading to core dam-age. Because the STPNOC strainer design was mod-ified to install very large strainers compared to theoriginal strainer design, risk for loss of ECCS NPSHis effectively eliminated. However, the sump perfor-mance gain comes at a price for core damage risk,especially when deterministic-based chemical effectsare used for success criteria.

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The amount of fiber arriving on the fuel is almostentirely governed by the strainer face velocity at thetime of recirculation switchover and for a few min-utes following recirculation switchover. Except forthe smallest breaks, the fiber load is governed by thenumber of ECCS trains running and the train config-uration. That is, the core fiber load (which as men-tioned above is limiting) is (almost) independent ofbreak size and ZOI.

4 CASA Grande

The risk analysis examines the full spectrum ofLOCA accident conditions ranging from predomi-nant, but small accidents that are easily managed upto and including extreme, but unlikely accidents thatchallenge the design basis. The analysis framework,CASA Grande, ultimately develops cumulative distri-bution functions (cdf) for exceedance of a particularvalue used in success criteria (for example, fiber load-ing per fuel assembly) for each of the standard PRAinitiating event LOCA categories, LLOCA, MLOCA,and SLOCA, and for each of the possible equipmentconfigurations as analyzed in the PRA. Once de-veloped, the failure likelihood for each scenario canbe determined knowing the associated value as illus-trated in Figure 3. Note that if a value coming from abounding deterministic experiment is available, thenbelow that amount, success is assured.

For example, if the amount of fiber is the crite-ria value, and say 75 gm/FA is the amount of fiberfrom an LLOCA, two available ECCS trains, and thatvalue falls below the intersection of the distributionon the abscissa, conceptually one would draw a verti-cal line from that value to the distribution curve andlook up the split fraction value (the failure probabil-ity) on the ordinate. It is important to understandthat thousands of samples for different scenarios pro-duce the complementary distribution and it is pos-sible that the same likelihood would come from dif-ferent scenarios. For example, a small break with alot of fiber nearby may result in the same value (sayfiber loading) as a larger break having less damageopportunity. From CASA Grande, each scenario canbe traced from where the break occurs to, for exam-

ple, the strainer loading and the downstream (coreFA) loading.

The inputs to CASA Grande are a combinationof conservative, yet realistic, treatments of accidentphenomenology and decision criteria based on pre-cursors to possible system damage. This methodol-ogy enables risk-informed insights without compro-mising the traditional safety basis. In the initial riskquantification for calendar year 2011, the plant per-formance metrics of head loss across the recirculationstrainers and fiber deposition per fuel assembly wereused to assess the risk of flow blockage leading to coredamage during recirculation scenarios. Detailed de-scription of the models and methods that constituteCASA Grande can be found in Letellier (2011).

5 Plant configuration

The basis for the insulation source term and locationsfor all welds and plant components in containmentis the current STPNOC plant design drawings anddesign configuration database. Although other for-mats may be equally useful, the Pilot Project choseto exploit the availability of computer aided designtools (CAD) as the method to capture and integratethe spatial information required to accurately calcu-late the weld LOCA locations, debris quantity (het-erogeneously distributed insulation) and type of de-bris. Additionally, specialized CAD interface toolswere developed that efficiently and reliably interro-gate the CAD model. The output of the tool set re-duces the spatial information contained in the CADto a database accessible by CASA Grande such thatthe debris source term can be calculated accuratelyfor all locations, break orientations, and break sizes.

Although not credited in the initial quantifica-tion, compartment wall information is included inthe database so that when wall truncation is imple-mented in 2012, the ZOI will be properly shaped andlimited by their presence. A configuration database isrequired because CASA Grande samples all welds atevery location in each replication of the total calcula-tion. For each LOCA category and at each location,10 to 15 samples, random in size and direction, aretaken for the source term calculation. Each sample

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1.0

00

If the threshold of concernvalue increases, the exceedance

ea,

valueiincreaseseorese(smallerthe efcodnc likeihoo folowalng pcm lativ frato o ailurev fo xape

At sufficiently high valuesthe likelihood goes to 0.0

- ------ ------ -- ---

U.uI

Performance measure (e.g., core fiber mass)

Figure 3: Illustration of a CASA Grande (complementary) cumulative distribution and the method used todevelop input for the PRA from it.

always includes the largest possible break (double-ended guillotine, DEGB) at each location. A fullspherical ZOI shape is used for the DEGB sample.Otherwise, a full hemispherical ZOI shape is used.The configuration database is designed to support au-tomatic calculation of the debris source terms (thou-sands of calculations in each CASA Grande repeti-tion).

6 PRA

The typical PRA will have initiating events forLLOCA, MLOCA, and SLOCA. It is interesting tounderstand that before GSI-191 became an issue,each of the LOCA initiating event branches alreadycontemplated a sump screen plugging event (mostlikely as in the STPNOC PRA) represented as ademand failure of the containment sump screens at

the time of recirculation. Additionally, in each ofthese branches, there are the preexisting scenarios forthe different combinations of ECCS equipment de-pending on support system and component successesor failures. The Pilot Project exploits this existingstructure by replacing failure likelihoods based on theresults of CASA Grande as described in Section 4 in-stead of the simplistic demand failure likelihood.

As described in Section 1.3, the physical mod-els have been extracted out of the PRA to enhanceportability, simplify incorporation of different plantinformation and characteristics, and facilitate adop-tion of the methodology. Another inherent complex-ity fundamental to typical PRA design is overcome byperforming the uncertainty quantification in CASAGrande. In the Pilot Project, time-dependent andmultivariate uncertainty distributions (described inSection 7) have been identified in the physical models

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Distributions: Scenariol Scenario2 . . .

LLO L LLiiMLOC L L~

The recirculation failure eventis actually a top that includesother failures such as valvesfailing to transfer. However,all the failure scenarios includethe sump blockage failure.

Figure 4: High level illustration of the inputs and outputs overlaid on a schematic diagram of the PRAevent tree structure. The event tree structure, with the exception of the long term cooling top event, alreadyexists in the typical nuclear power plant PRA.

required to understand GSI-191. CASA Grandealso provides the platform to propagate the time-dependent and multivariate uncertainties outside thePRA. In fact, this is a basic requirement of a risk-informed analysis because current commercial PRAmethodology is not fully capable of propagating therequired uncertainties.

Referring to Figure 4, the preexisting event treestructure and LOCA initiating event frequencies arenot changed by the Pilot Project methodology. Thedistributions from CASA Grande described in Sec-tion 4 are used in the recirculation switchover eventto substitute specific scenario likelihoods for the (in-variant) simplified demand recirculation likelihood (asingle basic event in the preexisting model). A new

top event was required in the STPNOC PRA to ad-dress the possibility of core flow blockage (long termcooling). This is the only structural change requiredin the PRA and is considered to be a relatively minorchange.

A calculation can be performed (at any particu-lar plant) by summing the current frequencies as-sociated with each LOCA initiating event that gothrough recirculation to success. With each of theseinitiating event frequencies in hand, one can multiplyeach by its associated failure likelihood from CASAGrande. The resulting frequency sum is the new suc-cess frequency and the difference between the origi-nal frequency (the unchanged frequencies) sum fromthe new frequency (multiplied by the CASA Grande

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A

Figure 5: Uncertainty quantification for complex computer models

likelihoods) sum is a close approximation to the newACDF due to having fibrous insulation in contain-ment.

7 Uncertainty(UQ)

Quantification

faced by the Pilot Project during the 2011 were thechoice of probability distributions for the input pa-rameters of CASA Grande and how to sample fromthem.

One of the inputs to CASA Grande is the LOCAfrequency table, Fleming et al. (2011). The proba-bilities associated with LLOCA are extremely smalland simple Monte Carlo sampling approach will al-most never produce that observation. In order tohave the LLOCA in all of the scenarios, nonuniformLatin Hypercube Sampling was applied, Helton andDavis (2003). This methodology generates observa-tions from the whole space (i.e. the LLOCA is al-ways included) and weights them by the correspond-

The modeling and propagation of uncertainties forthe GSI-191 project involves several steps. Figure 5shows the collection of information flow, methodolo-gies and challenges for UQ of computer models thatapproximate a complex real system like the one beingconsidered in this pilot project. The main challenges

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EnclosureNOC-AE-12002784 STP Nuclear Operating Company 's NRC/Industry GSI- 191 Closure Pilot Project

ing probabilities. The Pilot Project will expand uponthis approach in 2012 to be able to generate corre-lated samples and estimate quantiles using the datagenerated by CASA Grande.

Another major task during the 2012 will be the ver-ification and validation of the methodology appliedto solve the GSI-191. The verification part will makesure that all the code written is "bug-free" and repre-sents correctly the theory and methods implemented.The validation part is challenging since it will haveto be shown that the approximation produced is veryclose to the real system. Popova and Galenko (2011)provide a detailed description and illustrative exam-ples of the UQ methodology applied to the initialquantification.

8 LOCA frequency

In order to explore all possible scenarios, the risk-informed analysis requires knowledge of the likeli-hood of a pressure boundary failure for all possiblelocations and possible sizes of failure. Per design,significant rupture failures of Class 1 piping in do-mestic light water reactors have not been observed.Obtaining the appropriate likelihoods where there isno evidence presents a challenging problem for theGSI-191 risk-informed closure investigator.

In the initial quantification, Fleming et al. (2011)performed a substantial study designed to buildupon the established EPRI risk-informed In-ServiceInspection program (EPRI, 1999). EPRI (1999)methodology was used as primary basis to developthe size and location-specific rupture frequencies forthe quantification. In the Pilot Project study, Flem-ing et al. (2011) showed the total frequency inthe standard PRA methodology (SLOCA, MLOCA,and LLOCA) were preserved. Although the over-all methodology is considered to be sound based onpeer review (Mosleh, 2011), and reasonableness ofthe values obtained, NRC review feedback in thePilot Project has resulted in further review of theapproach. In 2012 an alternative to the LOCA fre-quency methodology will be performed on a new basisto fully address NRC concerns.

9 Quality Assurance

A quality assurance plan has been developed in cal-endar year 2011. The plan includes regularly sched-uled (nominally weekly) technical review teleconfer-ences supplemented at critical product developmentsteps with on-site review. The STPNOC PRA ana-lyst (Technical Team Lead) is responsible for reviewand verification of the PRA inputs developed. How-ever, the STPNOC PRA analyst review is supple-mented by independent critical peer review intendedto help disclose any overlooked technical gaps thatwould compromise results and also help ensure thatthe overall product is academically defensible eventhough it is developed for the industrial setting. In-dependent technical oversight was a part of the STP-NOC 2011 efforts and was used to further focus anal-ysis efforts.

The overall quality assurance plan is illustrated inFigure 6 as a flow chart. Due to the diverse technol-ogy required to be implemented in the GSI-191 scope,the PRA inputs originate with products developed byexperts in their respective field. The CASA Grandeintegrating framework uses the inputs to generate thetwo main inputs to the PRA, the sump demand fail-ure likelihood and the in-vessel cooling failure like-lihood (for each category of LOCA and all possibleequipment configurations). These elements are doc-umented by the vendor and the normal vendor doc-ument review process is followed to assure they aresuitable for use as input to the PRA. The overall PilotProject quality assurance methodology is expected tobe similar to most utilities' processes for PRA activ-ities although the details are most likely different.

10 Licensing

STPNOC will rigorously quantify the risk contribu-tion to core damage and large early release of recir-culation scenarios that are encompassed by GSI-191.The licensing strategy is based on looking at the dif-ference in risk between a hypothetical containmentdesign that has no fibrous insulation and the existingSTPNOC design. The expectation is that the hy-pothetical containment design would have lower risk

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Responsibility: Contracted service organization

Process: Local quality program

Input development

CASA Framework

I I I I II

Responsibility: LANL

Process: Local Quality ProgramI

Ga30

CL

Responsibility STP ContracrTechnical Coordinator, Project Technical LeadProcessaOTP Technical Document Review ProcessProcedure OPGPO4-ZA-0 108 Approved/Distributed Vendor Document Control Process'Internal review supplemented and supported by outside Peer Revuew

Input to PRA

PRA Quantification/Output

PRA Application

License AmendmentRequest

Inputs to PRA Verified/Reviewed

Responsibility: ABS Consulting

Process: IOCFR5O Appendix B ProgramIResponsibility STP Contract Technical Coordinator, ProjectTechnical Lead

Process' STP PRA Assessoment ProcessProcedure OPcSP04-ZA-r)6O'rProbabiiisitc Risk Assessment Program'

IResponsibility. STP Licensing Engineer

Process: 5fTr License Amendment ProcessProcedure OPGCp5-ZN-•00r'Changes to Licensing Basis Documents and Amend.mentsto the Operating License-

Figure 6: Illustration of the major elements of the STPNOC quality assurance plan for risk-informed closureof GSI-191.

than the existing design. Clearly an ideal outcomewould be the difference in risk between the two de-signs to be roughly zero.

The difference between a comparison of the risk

analysis to the Regulatory Guide 1.174, Region IIIlimit of ACDF< 10-6 yr- 1 and ALERF< 10-7 yr-1will provide a basis for GSI-191 closure via exemp-tion to certain requirements in 10 CFR 50.46. The

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terms ACDF and ALERF in the context of this workmean the difference in CDF and LERF between a hy-pothetically perfect containment building having nofibrous insulation and the existing plant design. Thepurpose of the initial quantification in calendar year2011 was to understand if the risk associated withfiber insulation in containment would exceed the Re-gion III limits.

If the final analysis (calendar year 2013) continuesto support recirculation failure as a Risk Region-Illevent, preventative measures (safety margin, defensein depth) will be identified to address contributingfactors that carry the largest potential impacts inthe analysis. Regardless of the quantitative findings,STPNOC will use the risk analysis to prioritize spe-cific actions and determine the degree of remediationthat may be required. Thus, it is essential that allparties fully understand the theory, implementationmethods and interpretation of a risk-informed deci-sion process. STPNOC intends to continue to com-municate regularly and openly with the NRC staffand industry in calendar years 2012 and 2013, andcontinually refine and enhance communication andcommunication tools as the Pilot Project evolves.

11 Conclusions

The calendar year 2011 initial quantification in thePilot Project has demonstrated the viability of a risk-informed closure path to GSI-191. Even at the earlystage, the usefulness of the risk-informed approachhas been demonstrated in initial findings related toZOI, chemical effects, time-dependency, and LOCAfrequency. In a short amount of time, a comprehen-sive understanding of the GSI-191 important physicalphenomena has been accomplished and has been usedby the Pilot Project to develop a new, robust, risk-informed framework for study and closure of GSI-191.

The Pilot Project has worked closely and effec-tively with the NRC to incorporate feedback and in-form the NRC staff of progress and the technologydeveloped. In some cases, NRC feedback has redi-rected the efforts of the Pilot Project and acceler-ated the schedule (for example, chemical effects andLOCA frequency). The interaction and feedback is

appreciated by the Pilot Project due to the aggressiveschedule and complexity of the issues.

The Pilot Project is designed generically so thatother plants can easily take advantage of it. Thedesign provides for simple integration of site-specificmodels in a method that extracts complexities outof the PRA and integrates them in a flexible model-ing tool, CASA Grande. A straightforward licensingapproach is proposed and generally accepted qualityassurance methods can be applied.

Even with conservative margins retained, the PilotProject in the initial quantification has shown thatthe risk from fibrous insulation in containment atSTPNOC is very small (in Region III) based on thedecision-making criteria of Regulatory Guide 1.174.This helps assure both the NRC and STPNOC thatdefense against ECCS failures can be easily identifiedand reasonable measures can be undertaken to avoidexcessive risk in a risk-informed closure path.

References

ANSI (1988). ANSI/ANS-58.2-1988: Design Basis forProtection of Light Water Nuclear Power PlantsAgainst the Effects of Postulated Pipe Rupture.Design Standard 58.2-1988, ANSI/ANS, Washing-ton, DC.

Baier, S. L. (2011, Septmeber). GSI-191 Fuel Assem-bly Test Report for PWROG. WCAP 17057 Revi-sion 1, Westinghouse PWROG, Pittsburgh, PA.

Dallman, J., B. Letellier, J. Garcia, J. Madrid,W. Roeschy, D. Chen, K. Howe, L. Archuleta,F. Sciacca, and B. P. Jain (2006, December).Integrated Chemical Effects Test Project: Con-solidated Data Report. NUREG/CR 6914, LosAlamos National Laboratory, Los Alamos, NM.

Darby, J., D. V. Rao, and B. Letellier (2000). GSI-191 STUDY: TECHNICAL APPROACH FORRISK ASSESSMENT OF PWR SUMP-SCREENBLOCKAGE. Technical Letter Report LA-UR-00-5186, Los Alamos National Laboratory, LosAlamos, NM.

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EPRI (1999). Revised Risk-Informed In-Service In-spection Procedure. TR 112657 Revision B-A,Electric Power Research Institute, Palo Alto, CA.

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Teolis, D., R. Lutz, and H. Detar (2009). PRA Mod-eling of Debris-Induced Failure of Long Term Cool-ing via Recirculation Sumps. WCAP 16882, West-inghouse Electric Company, LLC, Pittsburgh, PA.

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