Thermal Fluid Characteristics for Pebble Bed HTGRs

Thermal Fluid Characteristics for

Pebble Bed HTGRs.

Frederik Reitsma

Oct 22-26, 2012

IAEA Course on High temperature Gas Cooled Reactor Technology

Beijing, China

Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology

Overview

• Background

• Key T/F parameters

• Key T/F characteristics

• Heat transfer modeling

• T/F modeling challenges

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Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 3

Background

• Analysis of thermal-fluid systems – Often complicated because of the complex nature of fluid

flow and heat transfer

• Characteristics of thermal-fluid systems – Time-dependent – Multidimensional – Complex geometries – Complicated boundary conditions – Coupled transport phenomena – Turbulent flow – Structural and phase change – Energy losses and irreversibilities – Variety of energy sources


Basic principles

• Need to solve the governing equations in:

– Conservation of mass

– Conservation of momentum

– Conservation of energy

• Heat transfer

– Conduction

– Convection

– Radiation


Typical thermal-dynamic cycles

• The T/F conditions of the reactor are determined from the type of thermodynamic cycle used

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Typical reactor T/F parameters

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Key T/F characteristics • Helium is a single phase coolant

– No phase change in the cycle to deal with – Helium has excellent heat transfer properties – Compressible gas

• Large ΔT across reactor inlet to outlet – Requires a smaller coolant mass flow rate resulting in lower pumping

requirements

• High coolant outlet temperatures – Allows for higher thermal efficiency in power conversion cycles and process heat

applications

• Small ΔT between fuel and coolant (~70 °C) • Large temperature margins in the fuel (~600-1000 °C) • Slow thermal transients

– Large thermal capacitance in the fuel and graphite combined with a low power density results in slow transients

• Pebble bed is one flow channel – Strong coupling in the pebble bed does not require throttling of flow channels or

adjusting for flow distribution through the core


Thermal fluid considerations

• In the Thermal-Fluid design of a pebble bed core, the following aspects need to be considered:

– Positions of heat generated

– Flow path design to keep the metallic components cool

– Identification of all intentional and unintentional flow paths

– Pressure zoning to prevent hot gas impingement

– Temperature stratification in the outlet flow

– Component Temperatures

– Needs to design both an active (forced flow) and passive (natural) heat transfer path


Heat generation input

• Heat is generated in both local (in the fuel) and non-local sources

Heat sources: – Fuel – Reflectors – Control rods – Lateral restraints – Core barrel – Reactor vessel


Coolant flow design

• The coolant flow path design needs to consider the following aspects: – cool the metallic

structures where necessary

– reduce bypass flows – provide a uniform

temperature distribution – mix the bypass flows to

lower the thermal lower the thermal stratification in the outlet gas


Secondary flow paths

• Engineered

– Control rod cooling flow

– Central reflector cooling flow

– Pressurisation flow

• Leakage paths

– Across side reflector

– Inlet-to-outlet

– Along side reflector


Passive heat transfer path description

• Inherent post-shutdown decay heat removal is achievable through conduction, natural convection and radiation heat transfer. Design choices include core geometry, low power density and high thermal capacity of the core structures.

Centre Reflector Pebble Bed Side Reflector Core Barrel RPV RCCS Citadel

RadiationConduction

Conduction

Conduction

Convection

Radiation

Convection

Conduction

Radiation

Convection

Conduction

Convection

Radiation

Convection

Conduction

Radiation


Effect of Different Residual Heat Removal Mechanisms on Peak Fuel Temperature

• Active and passive heat removal • CCS is an active heat removal system


T/F Correlations • Helium properties

– Given by KTA 3102.1 Calculation of the Material Properties of Helium

• Heat transfer from sphere to gas – Given by KTA 3102.2 Heat Transfer in Spherical Fuel Elements – Function of ΔT, sphere diameter, Pr, Re, coolant properties, bed porosity

• Pressure loss through a pebble bed – Given by KTA 3102 3 Loss of Pressure through Friction in pebble bed cores – Function of bed porosity, sphere diameter, coolant properties, bed

height, bed diameter, mass flow

• Effective thermal conductivity of a pebble bed – Given by Zehner-Schlünder correlation – Function of bed porosity, sphere material properties which in turn is

dependent on temperature and dose


Bypass flow

LRD

Pebble Bed CROD

channel

Core flow Bypass

Leakage

Needs to predicts leak flows Use systems code like or detailed CFD

Oct 22-26, 2012 15


Effects of modelling bypass flows

• Bypass flows could increase thermal gradients and thus stresses in components


Example of test facility and required Modelling

Proximity refinement

Oct 22-26, 2012 17

Reactor Neutronics and Thermal Fluid

Analysis

Reactor Power Profile

Reactor Flow Distribution and Temperatures

Structural Analysis

Thermal Fluid Analysis

Computational Fluid Dynamics

(CFD)

Cycle Flow Conditions

Detailed Flow Distributions

Detailed Component Temperatures

Analysis Requirements (A typical picture needed)

Detailed Flow Distributions and Neutronic Data

Fluid/Structure Interaction


Physical Phenomena • FLUID FLOW

– Very hot helium gas under high pressure flows through an inlet, riser channels, leakage paths, inlet plenum, pebble bed, outlet plenum

– Frictional resistance (mainly in the pebble bed core, riser channels and leakage paths) cause pressure drops

– Heat transfer from the solid through convection (mainly in the pebble bed and riser channels)

– Internal heat redistribution in the gas through heat conduction and “braided” turbulent flow (in the pebble bed)

– Secondary helium circuit for cooling purposes • SOLID HEAT TRANSFER

– Nuclear heat sources (mainly in the pebbles) – Pebble-pebble heat transfer through solid and stagnant gas

conduction, radiation, etc. Heat transfer in the reflector through conduction and radiation

– Heat transfer to the gas through convection (mainly in the pebble bed and riser channels, couples the solid and gas temperatures)


Heat transfer in the Pebble Bed


The Lumped Parameter Approach

• Could also be described as a “macroscopic” approach • Uses a relatively coarse grid for the gas (as opposed to CFD

simulations) • The gas is treated as inviscid (no turbulence models, etc.) • The porous medium approximation is used in the core • Makes use of (non material property) empirical correlations (e.g. for

frictional resistance and heat transfer via convection) because the flow field around each pebble is not resolved and the gas is inviscid

• Programs like RELAP, FLOWNEX and CFD programs using the porous medium approximation are also lumped parameter models

• Many of the earlier codes used for HTR-Pebble-Bed modeling is 2D, which enforces the lumped parameter approach

• In the core these programs predict different temperatures for the gas and solid, this the pseudo-heterogeneous approach (not to be confused with the term heterogeneous which refers to subdivided pebbles and kernels)


Unique features to take into account…

• Fast reactivity transients – kernel modelling – In normal operation very small difference (normally

not modelled at all)

– Essential to model the kernel temperature behaviour explicitly (with all the coatings)


Critical thermal heat transfer modelling

0

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40

60

80

100

120

140

160

180

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Time [sec]

Po

wer [

% o

f N

om

inal]

Homog

SS Triso - no gap

Core Power (% of full) in PBMR400 / HTR-Modul after Large Reactivity Insertion

Because the fuel is dispersed in a matrix, simplistic energy deposition assumptions can lead to large errors when modeling reactivity insertions (e.g., control rod withdrawal or water ingress)


Other pebble specific aspects to remember

• Different fuel spheres of different “batches” in multi-pass have:

– Different heat sources

– Different graphite thermal conductivity (temperature, fluence and irradiation T dependent)

– Thus different surface temperatures

– (may want to include kernel – buffer layer gap and fission product buildup...)

– Variations in pebble packing fractions


Ultimate heat sink

• Significant amount of work was performed to find a passive Reactor Cavity Cooling System (RCCS).

• Different systems were investigated using coupled CFD models: – Direct passive air cooled

– Indirect passive air cooled

– Direct passive water cooled

– Indirect active water cooled

– Direct active water cooled with boil-off


Summary

• Flow phenomena in a pebble bed is straightforward and well characterized

• Thermo physical properties of helium is well understood and characterized

• Modeling challenges stems from defining flow paths with “loosely” packed side reflector blocks that creates leak flow paths

• Modern modeling and calculation methods are used to calculate design inputs for components in lieu of measurements from operating plants

Materials and design shape the core neutronics and thermal flow characteristics

• Graphite is the moderator and structure, not metal and water – high temperature solid

moderator – hard thermal spectrum – fixed burnable poison possible – large physical dimensions – low power density

• Helium is the coolant not water – Coolant is transparent to

thermal neutrons – Coolant has no phase change

• Fuel is carbide-clad, small ceramic, particles not metal clad UO2 – PyC/SiC carbide clad is primary

fission product release barrier – Fuel operates at high temperatures

with wide margin to failure – Double heterogeneity in physics

modelling in fuel

• Heat removal path through core structures – Modular requires metallic vessel – For increased power (and lower

maximum fuel temperature in DLOFC) - have to go to annular core

Oct 22-26, 2012 IAEA Course on High temperature Gas

Cooled Reactor Technology 27


• Source material used: – HTGR Technology Course for the Nuclear Regulatory

Commission, May 24 – 27, 2010

– HTR/ECS 2002 High temperature Reactor School, 2002

– Advanced Reactor Concepts Workshop, PHYSOR 2012

– Coupling of neutronics and thermal-hydraulics codes for the simulation of transients of pebble bed HTR reactors, T. Rademer, W. BERNNAT and G. Lohnert, Paper C22, HTR2004

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Documents

Thermal Fluid Characteristics for Pebble Bed HTGRs