VHTR Thermal Fluids: Issues and Phenomena

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1 VHTR Thermal Fluids: Issues and Phenomena Technical workshop at PHYSOR 2012: Advanced Reactor Concepts April 15, 2012 Knoxville, TN Gerhard Strydom Idaho National Laboratory (INL)

2 Overview Thermal Fluid Phenomena in PMRs Typical Current Approach AGREE Example Some of the Remaining Challenges

3 Important Thermal Fluid Phenomena in PMR NRC-PIRT: Prediction of the bypass flow is the most important thermal fluids phenomena Prediction of the steady state temperature distribution is influenced by: Core flow distribution Thermophysical properties on material (graphite fluence dependence) Multi-scale heat transfer (UO 2 kernel to TRISO to sphere surface to reflector to vessel to final heat sink) Multi-phenomena heat transfer (conduction/convection/radiation), as well as non-local fast neutron and gamma deposition

4 PMR Bypass Flow Paths Vertical gaps form bypass flow channels Control rods coolant flow (engineered) Horizontal gaps between stacked graphite elements form leakage paths to/from primary coolant flow path Large temperature gradients and fast neutron irradiation result in uneven block expansion (uncertainty in gap geometry) 4

5 PMR Thermal Fluids Modeling Requirements Capturing the bypass flow is essential 10%~25% of total coolant flow moves between the blocks (ANL- GenIV-071) Momentum flux terms should be included in conservation equations Large DT across core causes significant localized fluid acceleration Significant for depressurization event (blowdown) Ability to capture flow reversal Forced circulation flow is from top of core to bottom of core Natural circulation flow (LOFC) is from bottom to top Complex low velocity, buoyancy-driven flow in upper plenum during LOFC: limiting safety case for core barrel top plate. 5

6 Approach to Core Thermal Fluids Modeling (1D System Codes e.g. RELAP5) Reactor is represented by a series of 1-D pipes Pros Flexible History of successful use in LWRs Cons Ability to model 3-D bypass flow phenomena is questionable Existing codes would require substantial modification for use with HTRs Modeling concepts have been borrowed from these codes (i.e. single and time dependent junctions/ volumes) for use in AGREE code 6

7 Core Heat Transfer Methodology Traditional thermal fluid approaches 2-D porous medium approximation for the PBR 1-D pipe flow networks for the PMR These are fast enough for core transient analysis but details are lost THERMIX model RELAP5 model

8 Approach to Core Thermal Fluids Modeling: (CFD) Full 3-D representation of core using many (~10 6 ) cells Pros High fidelity solution Cons Computationally expensive Parametric studies may be cumbersome/ slow Plan to use CFD as a verification/ validation tool (Image: ANL-GenIV-121) 8

9 Core Heat Transfer Methodology: CFD CFD can get the detail but only for a piece of the core, and only at steady state Wanted: Fast whole core solid heat conduction/gas dynamics transient solver with some local resolution

10 Core Heat Transfer Methodology: Full House Ideal would be to investigate, for example, bypass flow with all of the relevant physics: graphite thermo-mechanics, neutronics, fluid flow. CFD model of block intersection

11 Effect of Gap Width on Block Temperatures: CFD 0 mm gap 5 mm gap Can this be done with RELAP5??

12 Example: AGREE PMR Fluids Modeling A 3-D core is represented by a series of cross-connected 1-D subchannels Subchannel method is based on proven LWR core thermal-hydraulic analysis methodologies (i.e. COBRA/VIPRE) 1-D subchannels flow into and out and slide

13 Thermal Fluid Challenges Effect of the gamma + neutron heating on the thermal fluids calculation Approximately 6% of the power is generated via gamma + neutron heating ~2% of this is deposited in the inner and outer reflector which may have non-negligible effect on the temperature distribution (research is currently ongoing) Changes in the by-pass and cross flow channels (gaps) due to graphite irradiation and thermal expansion/contraction. Changes in the thermo-physical properties of graphite due to graphite irradiation and thermal expansion/contraction.

14 Thermal Fluid Challenges cont. 3D Core heat transfer Hot streaking in the lower plenum Bypass flow Low flow phenomena (natural circulation plumes on top plate) Ex-core heat removal

15 Presented by: Gerhard Strydom (INL) Content: Gerhard Strydom (INL) Volkan Seker (U-Michigan) (208)