NGNP Program NGNP Methods: Summary of Approach and Plans Richard R. Schultz

NGNP Program November 19, 2008 Overview NGNP Methods: Objectives Discussion of Evaluation Models & NGNP Methods Approach Overview of NGNP Reference Reactors How we are defining needs in NGNP Methods Discussion of Phenomena Requiring Analysis Standard Problem Other potential standard problems Potential issues

NGNP Program November 19, 2008 Objectives of NGNP Methods Ensure that tools are available to analyze the behavior of the NGNP for DOE programs (and NRC). Analysis tools must have demonstrated capability and low calculational uncertainty Analysis software adequacy will be demonstrated using guidelines expressed in Reg Guide Hence software used for NGNP analyses are being assembled to create NGNP Evaluation Models

NGNP Program November 19, 2008 NGNP Evaluation Models (EMs) The calculational framework for evaluating the behavior of the NGNP during postulated transients or design basis accidents. This includes the: –Development of practices and procedures for treating the input and output for software in EMs. –Specification of the calculational envelope for the software to be used for various portions of scenarios of importance. –Specification for how the software shall be used by the plant analysts so several analysts analyzing the same problem using the same software calculate the same answer. –The validation procedures and requirements—including the requirements for designing and performing validation experiments. NGNP methods is focused on best estimate models aimed at calculating plant behavior at most challenging conditions, e.g., identify localized hot spots and unacceptable thermal gradients when plant is operating at maximum outlet temperatures and efficiency.

NGNP Program November 19, 2008 NGNP Methods Development & Validation The R&D Process is based on… –Identifying the most demanding scenarios for candidate plant design –Isolating key phenomena in scenarios –Determining whether analysis tools can be used to confidently analyze plant behavior scenarios (Validation) –Performing R&D to upgrade analysis tools where needed Scenario Identification: Operational and accident scenarios that require analysis are identified PIRT: Importantphenomena are identified for each scenario (PhenomenaIdentification & RankingTables) Validation: Analysis tools are evaluated to determine whether importantphenomena can be calculated Development: If important phenomena cannot be calculated by analysis tools, thenfurther development is undertaken Analysis: The operational and accident scenarios that require study are analyzed No Yes

NGNP Program November 19, 2008 The Calculation Process… Requires the analysis tools to have reasonable † agreement with data for key phenomena. † Reasonable agreement: calculated value sometimes lies within data uncertainty band and shows same trends as data. Consists of seven steps It’s assumed to be equally likely that the plant will be either pebble-bed or block-type reactor a. Material Cross Section Compilation and Evaluation b. Preparation of Homogenized Cross Sections c. Whole-Core Analysis (Diffusion or Transport), Detailed Heating Calculation, and Safety Parameter Determination d. Thermal-Hydraulic and Thermal-Mechanical Evaluation of System Behavior f. Fuel Behavior: Fission Gas Release Evaluation g. Fission Gas Transport e. Models for Balance of Plant Electrical Generation System and Hydrogen Production Plant a. Material Cross Section Compilation and Evaluation b. Preparation of Homogenized Cross Sections c. Whole-Core Analysis (Diffusion or Transport), Detailed Heating Calculation, and Safety Parameter Determination d. Thermal-Hydraulic and Thermal-Mechanical Evaluation of System Behavior f. Fuel Behavior: Fission Gas Release Evaluation g. Fission Gas Transport e. Models for Balance of Plant Electrical Generation System and Hydrogen Production Plant

NGNP Program November 19, 2008 Includes Software such as:

NGNP Program November 19, 2008 The Very High Temperature Gas Reactor is Reference Design Utilize inherent characteristics –Helium coolant - inert, single phase –Refractory coated fuel - high temp capability, low fission product release –Graphite moderator - high temp stability, long response times Simple modular design: –Small unit rating per module –Low power density –Silo installation Passively safe design: –Annular core –Large negative temperature coefficient –Passive decay heat removal –No powered reactor safety systems Prismatic Pebble-bed

NGNP Program November 19, 2008 NGNP Methods: Focused on Work Generally Applicable to Both Prismatic & Pebble-Bed Design Since design has not been chosen yet (It is planned to choose design in fiscal year 2009 though.) Work scope applicable to either design is still large and includes: –Performing PIRTs to identify most challenging scenarios and highly ranked phenomena for prismatic and pebble-bed designs. – Selecting overall software repertoire compatible with known design characteristics, most challenging scenarios, and calculational requirements for highly ranked phenomena.

NGNP Program November 19, 2008 From General Perspective… Most challenging scenarios are the same for both prismatic and pebble-bed type reactors. Highly-ranked phenomena for the most challenging scenarios are similar. Differences rest with (for example): –Core geometry: moveable vs stationary; core cross- flow vs no core cross-flow; definition of bypass flow; nonuniform core flow area vs uniform core flow area –Quantity of dust generation –Geometry of RCCS. –IHX designs

NGNP Program November 19, 2008 Many of the Phenomena that Must Be Quantified & Analyzed Require Advanced Tools System behavior, i.e., comprehensive model of reactor plus balance-of-plant, will be calculated using RELAP5-3D RELAP5-3D will be used to provide boundary conditions to CFD calculations for transient scenarios where localized hot spots must be identified and studied.

NGNP Program November 19, 2008 RELAP5 Model Summary: Reactor Vessel Model Coolant active and stagnant volumes Structures in the core region –inner and outer reflectors –upper and lower reflectors –core barrel –upper plenum shield –reactor vessel wall and upper head Structures below the core are being ignored

NGNP Program November 19, 2008 VHTR Vessel Hydraulic Nodalization— Bypass Not Shown

NGNP Program November 19, 2008 Other Candidate Coupled Calculations Reactor cavity cooling system Coupled through heat transfer boundary

NGNP Program November 19, 2008 RELAP5 Model Summary: Ex-vessel Model Containment air volume Reactor cavity cooling system (RCCS) –Inlet plenum/downcomer piping –Lower distribution plenum –Riser/outlet plenum –Riser, downcomer, and outer metal walls Containment concrete wall and surrounding soil (behind RCCS downcomer) Other structures/walls neglected

NGNP Program November 19, 2008 VHTR Reactor Cavity Nodalization

NGNP Program November 19, 2008 RELAP5 Model Summary: Heat Transfer Modeling with RCCS Model Core Inner reflector conduction Outer reflector Reactor vessel RCCS riser wallRCCS downcomer wall Containment wall radiation convection, radiation convection He coolant Axial conduction in core and reflectors convection

NGNP Program November 19, 2008 Thermal-Hydraulic Phenomena: RELAP5-3D Normal operation at full or partial loads – Coolant flow and temperature distributions through reactor core channels (“hot channel”) – System behavior at operational conditions – Calculation of heat balance between reactor vessel and confinement volume, confinement walls, and cooling water flow behavior in walls. Loss of Flow Accident (LOFA or “pressurized cooldown”) – Modeling of all 1-D systems – Boundary conditions for CFD calculation of fluid behavior in plena – Calculation of two-phase conditions in water cooling passages in RCCS. – Prediction of system behavior during transient. Loss of Coolant Accident (LOCA or “depressurized cooldown”) – Prediction of reactor core depressurized cooldown - conduction and thermal radiation – Rejection of heat by natural convection and thermal radiation at the vessel outer surface

NGNP Program November 19, 2008 Flow in Lower Plenum Objective: The flow in the lower plenum of the VHTR will involve multiple jets entering from the core into a crossflow moving toward the exit duct and having to negotiate the presence of rows of support pillars. The modeling strategies (e.g. turbulence model, grid characteristics, time stepping, etc.) for simulating this complex turbulent flow must be validated. Courtesy of Fluent

NGNP Program November 19, 2008 Coupling of RELAP5-3D © & CFD Software –RELAP5-3D and Fluent presently coupled: however parallel calculational capability using multiple CPUs must be improved. –Embarking on coupling with STAR-CD and STAR-CCM+ this fiscal year. –STAR CFD codes have immense potential due to their focus on the calculation of massive problems. –Mesh cell requirements for the plena are on the order of tens of millions of cells.

NGNP Program November 19, 2008 Blowup of Fluent model linked to RELAP5-3D © model

NGNP Program November 19, 2008 RELAP5-3D © coupled to enable detailed analysis of lower plenum flow patterns Core Upper Plenum Lower Plenum Balance Of Plant CFD model RELAP5-3D model RELAP5-3D model CFD model RELAP5-3D coupled to Fluent, GAMMA, and STAR

NGNP Program November 19, 2008 INL Will Develop Standard Problems for Systems Analysis Codes (RELAP5) Based on the need to calculate highly ranked phenomena identified in PIRT. Potential standard problems include: –Mixed convection heat transfer in core –Two-phase flow behavior in reactor cavity cooling system (RCCS) water cooling channels: natural circulation with flow stagnation and both forward and backward flow. –Coupled problems: RELAP5-3D and CFD Will follow same process presently being undertaken with CFD for the lower plenum turbulent mixing problem.

NGNP Program November 19, 2008 Capabilities of CFD & Systems Analysis Codes Validated Using Standard Problems… Standard Problem Committee defined by GIF Methods Project Management Board: specifies required problems Problem Oversight Committee: industry experts assign problems to participants and evaluate results of participants Problem participants Publish results

NGNP Program November 19, 2008 Experimental R&D Needs—to Validate RELAP5-3D are Under Development… Major efforts are shown in planning schedule.

NGNP Program November 19, 2008 Conclusions Application of RELAP5-3D to analyze flow in gas-cooled reactor systems must be qualified and applied using appropriate practices and procedures to ensure code verification, minimize numerical error, quantify uncertainty and validate calculations. The NGNP Methods Program is formulating plans to ensure the project objectives are achieved.