a benchmark study

© 2018 Electric Power Research Institute, Inc. All rights reserved.

Aladar A. Csontos and Jeremy Renshaw

Reg Con12 December 2018

Thermal Modeling:A Benchmark Study

2© 2018 Electric Power Research Institute, Inc. All rights reserved.

Background

Project Goal: better understand the actual environment inside a canister – going beyond theory to reality– How accurate are thermal models?– Is improvement needed?

"In theory there is no difference between theory and practice; in practice there is.”

- Yogi Berra


Background: Thermal ModelingNRC ISG-11 R3:

– PCT not to exceed 400°C for normal conditions of storage & short-term loading operations

PCT modeled to 400°C with assumptions to compensate for modeling uncertaintiesDOE best-estimate thermal

modeling identify significant margins in design basis thermal models:– As-loaded PCT << 400°C– All < 325°C– Most < 300°C

PCT: Peak Clad Temperature


Benefits of Improved Thermal Models

Improve Operational FlexibilitySupport Risk Informing Dry Storage and Aging ManagementFacilitate Transportation and Ultimate Disposal


ESCP Thermal Modeling Subcommittee

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CISCC SubcommitteeChair: Shannon Chu (EPRI)

NDE SubcommitteeChair: Jeremy Renshaw (EPRI)

Canister Mitigation/Repair SubcommitteeChair: Dana Couch (EPRI)

Fuel Assembly SubcommitteeChair: Mike Billone (ANL) /

Vice-Chair: Sven Bader (Areva)

Thermal Modeling SubcommitteeChair: Al Csontos / Vice-Chair: Sam Durbin

International SubcommitteeChair: Jose Conde (Enusa)

Phase IBWR DCS Simulator

NRC/DOE Lead

Phase IIHBU Demo

Project

EPRI Lead

Phase IIINext Gen Simulator

Lead TBD


Significant Domestic and International Interest


Phase II: HBU Demonstration Project

Purpose: Blind model benchmarking to demonstration measurements:– Model best estimate steady state temperaturesDOE requested HBU thermal data release by May

2018:– Phase II project met timeline commitmentEPRI collected and compared all modeling results

to demonstration temperaturesFull report on Phase II project expected early 2019

– Will be freely-available to the public


Thermal Transient Thermocouple Data: LoadingPeak Cladding Temperature (Cell 14) = 237°C

Video provided by ORNL

50 100 150 200Degrees Celsius


Phase II: Thermal Modeling

Parameter FSAR LAR Best-Estimate

Peak Clad Temperature (model) 348°C 318°C 229°C

Heat Loadouts 36.96kW 32.934kW 30.456kW

Ambient Temperature 100°F 93.5°F 75°F

Design Specifics Gaps Gaps Gaps

Purpose: Blind model benchmarking to measurements:– Model best estimate steady state temperatures

Key Inputs to NRC, PNNL, and TN Modelers:– Decay heat profiles and assembly map with composition and burnup– Design specifics (gaps), ambient temperature, and internal pressure

Modeler Software

S1 ANSYS Fluent

S2 STAR-CCM+

S3 COBRA

S4 ANSYS APDL


Steady State Thermocouple Data

1 2 3 45 6 7 8 9 1011 12 13 14 15 1617 18 19 20 21 2223 24 25 26 27 28

29 30 31 32

S1S2S3S4Measured

DLB DLB

DLB DLB DLB DLB

DLB

DLB: Design Licensing Basis


Steady State Thermocouple Data: Deviations

1 2 3 45 6 7 8 9 1011 12 13 14 15 1617 18 19 20 21 2223 24 25 26 27 28

29 30 31 32

S1S2S3S4Measured


Steady State Thermocouple Data: Cell 14 (Hot Cell)


Peak Cladding Temperature (Simulation Results only)

Max value is in Cell 14 of 32 for all modeling results Max fuel cladding

temperature from design basis evaluation (on the pad) is 318°C (605°F) – Limiting factor for HBU

demo heat load was resin temperature, not PCT

– Design basis heat load about 8% higher than actual fuel assemblies


External Thermocouple Results – Column A


External Thermocouple Results – Column B


External Thermocouple Results – Column C


Phase II: Key Takeaways

HBU Demo project provided valuable early resultsThermal Model Trends/Results:

– Reasonable axial distribution shape and surface temperatures– Reasonable best-estimate thermal model results:However, consistently biased to higher temperatures

1. Decay Heat Penalties (often 17-20%)2. Ambient Temperatures (often assumed much higher than actual) 3. Design Gaps (FSAR assumes large gaps poor thermal cond.)

Why such large differences between Design Basis and Reality?


Phase II: Key Takeaways

Steady state PCTs from all models and measurements significantly lower than the design licensing basis:

Parameter FSAR LAR Best-Estimate

HBU Cask Measurements

PCT (model vs data) 348°C 318°C 254-288°C 229°C

Heat Loadouts 36.96kW 32.934kW 30.456kW 30.456kW

Ambient Temperature 100°F 93.5°F 75°F 75°F

Design Specifics Gaps Gaps Gaps No Gaps?


Thermal Modeling StrategyDetermine temperatures more

accurately via thermal modelingEvaluate what the true limits should

be based on fuel performance

Extracting Benefits:– Improve inputs to models (models

perform well when given proper inputs) Realistic gaps, accurate decay heats,

and reasonable ambient temperatures

– EPRI facilitating reassessment of ISG-11 R3 400oC PCT limit with NRC and DOE for performance-based criteria 400350300250

Temperature, °C

Axia

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Actual Margin

PCTMargin

Overall Thermal Margins


Together…Shaping the Future of Electricity


Phase III: Proposal for Next Generation Simulator Purpose:

– Blind validation and benchmarking to experimental dataProposed Next Generation Simulator:

– 1/3 Scaled down system– Pressurized system (up to 24 bar)– Convective and conductive systems– Horizontal and vertical orientations– Different overpack designs– Modern basket designs with varied fuel loading patterns– Ability to assess vacuum and drying operationsDOE FY 2018 Funding:

– $3.2M Total for Phase I-III / ~$2M for Phase III


Conductive System Sensitivities

4/18/18 Discussion: Thermal Model Sensitivities

Model Sensitivities Sensitivity Knowledge Level Comments

Decay Heat Power Well Known / Well CharacterizedDecay Heat Loading Pattern Well Known / Well CharacterizedGaps Design Dependent / Issue Well KnownAmbient Temperature Known / Controllable / Margin Not UsedMaterial Emissivity Cladding Marginal Improvement in UncertaintiesMaterial Properties Solids Well Known / AcceptedPorous media inputsDecay Heat Axial ProfileMaterial Emissivity Other ComponentsMaterial Properties Gas Well Known / AcceptedPressure (He) Known above a min pressure


Convective System Sensitivities 4/18/18 Discussion: Thermal Model Sensitivities

Model Sensitivities Sensitivity Knowledge Level Comments

Decay Heat Power Well Known / Well CharacterizedDecay Heat Loading Pattern Well Known / Well CharacterizedPorous Media Inputs Testing Storage of fuel in decay heat range usefulOverpack Annulus Air Flow Configuration Specific - additional data usefulFlow Paths (Inside/Outside Basket) Configuration Specific - additional data usefulPressure (He) Known above a min pressureAmbient Temperature Known / Controllable / Margin Not UsedMaterial Emissivity Cladding Accepted Values / Marginal ImprovementMaterial Properties Gas Well Known / AcceptedDecay Heat Axial ProfileMaterial Emissivity Other ComponentsMaterial Properties SolidsGaps

a benchmark study

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