ML110320643

1/27/11 Presentation on Risk-Informed GSI-191 Project Overview

ML110320643
Person / Time
Site:	South Texas
Issue date:	01/27/2011
From:	Grantom C South Texas
To:	Office of Nuclear Reactor Regulation
Purnell, B A, NRR/DPR, 415-1380
References
GSI-191
Download: ML110320643 (22)

Text

Risk Informed GSI-191 Project Overview 01/27/2011 Slide 1 NRC Public Meeting January 27, 2011 South Texas Project C. Rick Grantom P.E.

Project Manager

Deterministic Evaluation Attributes Probabilistic Evaluation Attributes Predetermined scenarios are analyzed assumed to be worst case.

Full spectrum of scenarios is analyzed that covers wider range of possibilities. There is solid evidence in the scientific literature that probability is the best measure of uncertainty.

Decision making is absolute no Uncertainty is integral to decision 01/27/2011 Slide 2 Decision-making is absolute - no uncertainty in the decision-making process.

Uncertainty is integral to decision-making. Risk-based methods quantify both the uncertainty of the state of our knowledge and the variability in physical phenomena.

Need for detailed analysis and full phenomenology understanding is avoided by assuming conservative values for parameters.

Detailed modeling and analysis is needed to properly characterize uncertainty.

Primary Project Objectives

• Obtain core damage frequency distribution for hypothesized LOCAs that require ECCS recirculation.

• Compare core damage frequency & large, early release frequency results for Potentially Sump Blocking Insulation & Non-Sump Blocking I

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f 01/27/2011 Slide 3 Insulation designs against the criteria of RG1.174

• Employ RG 1.174 strategy to provide risk informed closure of GSI-191

• Finalize plan for GSI-191 closure by mid 2012 for STP

• Develop a repeatable risk informed GSI-191 Closure Method

PRA Perspective LOCA ECCS Response CDF Injection

+ Recirculation CDF 01/27/2011 Slide 4 Accums + SI + RWST SI + Sump Debris + Transport + NPSH Documented probabilistic basis for risk quantifications Area of uncertainty, need probabilistic distributions and commensurate technical basis

Technical Overview

• The risk-informed approach to GSI-191 closure requires development and integration of five major elements.

Each of these elements has one or more technically challenging subtasks.

- DTSB: generation and transport of debris to the sump. Resulting sump differential pressure 01/27/2011 Slide 5 sump differential pressure

- TH: RCS thermal-hydraulic response.

- DEM: Downstream effects of debris getting through the sump screens and into the core, SI components.

- PRA: A logic model that develops and quantifies the scenarios leading to core damage.

- Uncertainty: The development and subsequent reduction of the multivariate probability distributions needed by the PRA for quantification.

Multiple Physics Models Reactor TH Plant State Point Internal Obstructions Transient Blow Down EOP Response ZOI Formation Probability of Break Fracture Mechanics Jet Expansion Jet Reflection Break Location Containment Blow Down Transport Spray Actuation Environment P&T Wash Down Transport Sump Pool DebrisTransport Sump Screen Debris Accumulation 01/27/2011 Slide 6 Debris Transport Debris Degradation Chemical Product Formation Temperature History Debris Accumulation Thin-Bed Formation Screen Penetration Face Velocity Porous Media Head Loss NPSHMargin Plant PRA LOCA Probability by Size/System Probable Loss of Recirculation CDF and LERF Injection Systems Recirculation Demand NPSHreqd Degraded Pump Performance Valve Wear Operability

01/27/2011 Slide 7

Milestone Plan 2010 October - November Meet with Regulator to propose risk-based licensing strategy*

Obtain plant stakeholder buy-in to risk approach Presented plan to PRT 2010 December 2010 Project Plan developed, contract negotiation for 2011 work.

Contract negotiations for work in 2011 commenced Project Team meeting to develop work breakdown and best-guess schedule.

NRC Commission issues SRM recommending risk informed approach to 01/27/2011 Slide 8 g

pp complete resolution of GSI-191 2011 January - February Final Contract negotiations completed Project Team meeting to finalize inter-model communication (TH, DTSB, DEM)

NRC Public Meeting January 27th at NRC HQ Formal Kick-Off Meeting with NRC to communicate risk-based approach plan Licensing strategy finalized (Regulator concurrence).

Data & Information from STP to Project Team (FSAR data, latent debris loading, water balance, etc.)

TH model development (jet boundary condition, downstream effects models)

• Italics indicates actual completion

Milestone Plan (continued)

• 2011 March - April

- Review Report of available literature - pipe fracture mechanics (Deliverable)

- Best estimate of pipe failure distribution (opening rate, size, geometry)

- Provide break characterization to TH group.

01/27/2011 Slide 9 Provide break characterization to TH group.

• 2011 May - June

- CAD Model description of containment/piping with insulation burden

- Pipe failure distribution (locations, rates, likelihoods) input development for DTSB model.

- Containment response finalized (primarily sump fluid temperature).

Milestone Plan (continued)

• 2011 May - August

- Minimum break size requiring recirculation phase

- TH response spectrum complete (Deliverable)

- Complete uncertainty distributions for all supporting analyses

- Complete all preliminary TH/DEM calculations

- Revise LOCA break frequencies based on the above

• 2011 September November 01/27/2011 Slide 10

• 2011 September - November

- PRA Model incorporation complete.

- INITIAL QUANTIFICATION (DELIVERABLE).

- Evaluate results & recommend path forward (risk informed or not)

• 2011 December

- Incorporate feedback from internal reviews.

- Executive report for Regulator/Industry review.

2012 - 2013 Plan

• Will be based on 2011 initial quantification results and interactions with NRC

• Emphasis and scope will be on areas where highest uncertainties remain 01/27/2011 Slide 11 where highest uncertainties remain

• May require additional testing and/or experiments

Requirements for a Robust Risk Analysis Realistic models are needed to properly characterize uncertainty in our state of knowledge and variability in phenomenology.

Additional effort is required to identify and subsequently reduce uncertainty (if possible) in otherwise acceptable (mean value) failure scenarios with large tails.

01/27/2011 Slide 12 An enormous number of scenarios (as opposed to a single deterministic methodology) are required to be analyzed and understood for a complete risk analysis.

Additional information (important measurements, ranges, types) may be required to focus on phenomenology contributing most to uncertainty and/or levels of risk.

SUMMARY

• A risk informed approach for closing GSI-191 has been developed which will employ robust probabilistic methods

• A highly qualified and specialized team has been assembled to undertake this project 01/27/2011 Slide 13 p

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• A project plan has been developed with milestones for regulatory/industry communication and project completion

• The intent of the project will be to develop a risk informed GSI-191 closure process that can be replicated by others

TECHNICAL BACKUP SLIDES 01/27/2011 Slide 14

Spatial, Temporal, Complexity Scales Risk Worker Very Small Chemical Product Formation Micro Physical Debris Generation Porous Media Flow Debris Transport Debris Bed Penetration 01/27/2011 Slide 15 Very Large ZOI Jet Formation Containment Spray Sump Pool Flow Reactor System Behavior Sump Screen Loading Plant-Wide PRA

Primary Objectives: TH & DEM

• Jet Model Boundary Conditions (BCs) : To execute the jet model, the fluid conditions at the break location as a function of time are needed.

These BCs will be evaluated via system codes.

• Floor for recirculation: The break size(s) below which recirculation would not be required (scope analysis

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01/27/2011 Slide 16 study) over the mission time (24 - 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br />).

• Accomplished by solving conservation equations of mass, momentum and energy for the RCS. Use of extensive sensitive study of the coefficients of the constitutive relations to evaluate Uncertainty Quantification (UQ).

• Use computer system code: RELAP5-3D (validated for LOCA)

Primary Objectives: TH & DEM

• Downstream effects refer to the material that passes through the sump strainer and enters the RCS system (i.e., reactor vessel).

• The transported debris that bypasses the sumps can lead to near and long term cooling degradation (coolable t

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01/27/2011 Slide 17 geometry).

• Similar to primary TH model, additionally requires particle transport models and associated constitutive support.

• Required to identify success criteria (i.e.,failed fuel pins).

• No software currently exists to fully treat downstream effects from a first principles viewpoint. Will require model development and incorporation into the analysis.

Primary Objectives: DTSB

• Determine the material ablation due to jets formed during the energetic blow down phase of applicable LOCA (and other applicable events) events (i.e. zone-of-influence, ZOI).

• Determine the transport of ablated material to the SI sump strainers and a function of time for applicable events.

- Requires momentum and non-Newtonian transport equations. Uses modified computational fluid dynamics (CFD) code package to produce the necessary boundary conditions for particle transport equations.

f 01/27/2011 Slide 18

- The jets formed during energetic blowdown phases are solved using CFD. Specific constitutive models are required for material ablation using jet models for boundary conditions

• Expansion models are required to determine the energy distribution of jets formed and require knowledge of the materials in the jets path(s).

- Used in conjunction with CAD drawings of piping and insulation results for the different pipe locations will be generated. Will also use existing data from industry and NRC ZOI tests.

Primary Objectives: PRA

• Modify the existing STP PRA to reflect realistic sump performance. Currently, the sump blockage is a single probability fraction and is not based on actual data or plant-specific analysis as will be developed by this project.

• Requires knowledge of the core damage probability for each possible LOCA scenario where recirculation is 01/27/2011 Slide 19 each possible LOCA scenario where recirculation is required. The probability is a random variable having a distribution that must be developed from the models undertaken in this project.

• Requires modification of the STP PRA event tree structure to accommodate a sump model and redesign of the LOCA event tree logic to incorporate a non-transition break size model.

Primary Objectives: Uncertainty

• Develop a stochastic wrapper that would integrate the different models developed in the core damage frequency analysis in such a way that the appropriate distributions are made available to the event tree logic structure.

• The primary result of the uncertainty analysis will be a collection of multivariate distributions and may require redesign of the PRA input and quantification 01/27/2011 Slide 20 redesign of the PRA input and quantification methodology.

• The PRA software (RISKMAN) is used to solve the logic model and quantify and bin the resulting scenarios.

• Requires recasting the multivariate distributions developed in the models (DTSB, TH, DEM, etc.) to single parameter distributions that PRA software such as RISKMAN or others can use.

This problem will have many multivariate uncertainties possibly important to uncertainty quantification Uncertainty in containment pressure is dependent on the uncertainty in break flow rate.

Uncertainty in sump differential pressure is dependent on the uncertainty in fluid temperature (in turn, is Primary Objectives: Uncertainty 01/27/2011 Slide 21 y

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dependent on containment temperature - break flow).

Uncertainty in fuel pin failures is dependent on uncertainty in ECCS flow and uncertainty in sump screen particulate penetration.

• Uncertain parameters will be varied and propagated through physics models to generate distributions on PRA metrics

• All physics models will initially be conservatively approximated and refined as needed to address dominant uncertainties

Multivariate uncertainties and their mapping to PRA models

• Standard inputs for PRA analysis are discrete conditional probabilities

• They are computed from conditional distributions (densities)

• Conditional densities are computed from the joint distribution function (density) 01/27/2011 Slide 22 (density)

• Example shows two uncertain variables with a positive correlation

• We propose conditional probability of NPSHmargin loss as one important interface to the PRA