Thermal Fluid Characteristics for Pebble Bed HTGRs. Frederik Reitsma IAEA Course on High temperature Gas Cooled Reactor Technology Beijing, China Oct 22-26, 2012
Overview Background Key T/F parameters Key T/F characteristics Heat transfer modeling T/F modeling challenges Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 2
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 Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 3
Basic principles Need to solve the governing equations in: Conservation of mass Conservation of momentum Conservation of energy Heat transfer Conduction Convection Radiation Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 4
Typical thermal-dynamic cycles The T/F conditions of the reactor are determined from the type of thermodynamic cycle used Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 5
Typical reactor T/F parameters Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 6
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 Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 7
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 Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 8
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 Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 9
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 Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 10
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 Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 11
Passive heat transfer path description Centre Reflector Pebble Bed Side Reflector Core Barrel RPV RCCS Citadel Conduction Radiation Conduction Conduction Radiation Convection Convection Conduction Radiation Conduction Radiation Convection Convection Conduction Convection Radiation 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. Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 12
Effect of Different Residual Heat Removal Mechanisms on Peak Fuel Temperature Active and passive heat removal CCS is an active heat removal system Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 13
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 Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 14
Bypass flow Needs to predicts leak flows Use systems code like or detailed CFD Core flow Bypass Leakage LRD Pebble Bed CROD channel Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 15
Effects of modelling bypass flows Bypass flows could increase thermal gradients and thus stresses in components Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 16
Example of test facility and required Modelling Proximity refinement Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 17
Analysis Requirements (A typical picture needed) Reactor Power Profile Detailed Component Temperatures Reactor Neutronics and Thermal Fluid Analysis Reactor Flow Distribution and Temperatures Computational Fluid Dynamics (CFD) Fluid/Structure Interaction Structural Analysis Detailed Flow Distributions Cycle Flow Conditions Detailed Flow Distributions and Neutronic Data Thermal Fluid Analysis
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) Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 19
Heat transfer in the Pebble Bed Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 20
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) Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 21
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) Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 22
Power [% of Nominal] Critical thermal heat transfer modelling Core Power (% of full) in PBMR400 / HTR-Modul after Large Reactivity Insertion 180 160 Homog SS Triso - no gap 140 120 100 80 60 40 20 0 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Time [sec] 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) Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 23
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 Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 24
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 Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 25
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 Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 26
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 UO 2 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 Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 28