TRACE: Modifications in Support of S-CO2 Transient Modeling

Brett Siebert Michael Hexemer Jared Dolatowski Greg Wahl Knolls Atomic Power Laboratory

Slide 1

TRACE Background • TRACE: general thermal-hydraulics computational modeling code • Transient Analysis Code • Form “networks” of “pipes” and “heat structures” • track conservation of mass, momentum and energy • Powerful control system to represent changes in the real set-up (valve motion, Rx kinetics, etc.) • TRACE - developed by the NRC, used in: • Licensing of nuclear power plants • Safety and performance analysis for PWR’s, BWR’s • The NRC also initiated the development of SNAP, a Graphical User Interface for input and output processing. • Does not handle: • CO2 as a working fluid • Brayton Turbomachines (Compressors, Turbines) Slide 2

TRACE Modifications: The Starting Point • Modifications “overlay” a particular version received from the NRC • Currently use the version that has been submitted to the ACRS for Developmental Assessment • Exploit the TRACE test suite and associated automation software • Productivity Driver (automated, overnight execution) • Ensure modifications are appropriately delimited • > 1000 Test Cases • Coding structure of TRACE very amenable to quick changes • Good fit with a technology development project • Consolidates past NRC Plant Analysis Code projects (TRAC-P, TRAC-B, RELAP5)

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TRACE Modifications: What Didn’t Change • The numerics of the code are unchanged • It has worked well for S-CO2 work to date • CO2 in the two-phase dome not yet investigated

• The Link to the SNAP GUI is unchanged • Used to Build models and animate results • SNAP: critical in Technology Development Environment • Immediate feedback on system behavior • Rapid Model evolutions, sensitivity studies • Retain all SNAP functionality in accessing TRACE Modifications • Title Cards Used to Pass Information • Huge Productivity driver Slide 4

TRACE Modifications: Progression to S-CO2 • Los Alamos National Lab • Prior work on Ideal Gas Closed Brayton Cycle (CBC) Components • Baseline TRACE has “Complete” numerics for Turbomachines • Existing Pump & Steam Turbine • Implicit Handling of Turbine sinks required • No new energy/momentum operations needed

• Current work = LANL conclusions holding in non-ideal fluids (CO2) • Brayton Turbomachines are Generalized Pumps • They add/subtract momentum/energy (no impact on mass) • All turbo relationships generalized for real (non-ideal) fluids • Generalized so liquid vs. vapor phase distinction is lost for supercritical conditions • All phase change within the component is suppressed • Delayed til next component (outlet pipe) • Should be zero in intended S-CO2 conditions

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Primary Modifications: Brayton Turbomachines • Need Impact of Rotor on fluid Momentum & Energy • Code does not explicitly model flow through complex, rotating passages • Performance Maps Quantify Impact • Become Source/Sink to Momentum & Energy equations • Maps provide 2 performance parameters • Terms for 2 equations • • Power = m × Δh • Momentum Head = Δp / ρ • Different Parameter combinations viable • Pressure Ratio, Efficiency • Ideal Enthalpy Change, Efficiency • All combinations will convert to equation terms • Rotor Torque = Power / Rotation rate Slide 6

Brayton Performance Characteristics • Performance Maps Supplied by Turbomachine Designers • Maps summarize impact of rotating equipment on the flow • Based on detailed, mean-line analyses • Impact dependent on flow conditions + rotation rate of “implied” wheel • “Corrections” relate off-design to design conditions • Dynamic similarity • Flow angles • Mach numbers • Ability to Scale the maps to handle different baseline conditions • Change size of test rig, relate test rig and reference design • Scoping calculations without waiting for new design • Special treatment when conditions go off Map’s low flow boundary • Keeps operation consistent at start-up • No flow = No Δ’s to energy/momentum equations Slide 7

Example Corrections and Map Scaling Map Scaling (They aren’t really Tables) Defining Extent of Permissible Scaling Requires Test Data

Actual Form of Corrections are Map/Vendor Dependent: Corrected Flow:

⎛ pdes ,tot ⎞ Ttot m& c = m& ⎜ ⎟ p ⎝ tot ⎠ Tdes ,tot Corrected Speed:

Nc = N

Tdes ,tot Ttot Slide 8

Brayton Shaft Configuration C

External

T

• Builds on Pump’s Impeller Rotation Calculation • User specifies the configuration of the shaft • # Number Compressors (Stages) • # Number of Turbines (Stages) • External Torque Applicable • Motor / Generator • Net torque is computed • Time integration to determine rotational response of shaft

∂ω I = ∑ Torque ∂t Slide 9

TRACE: Working Fluid Modifications • TRACE handles a variety of working fluids • Use TRACE FORTRAN Module to extend Fluid Property to CO2 • Simple Interface to call NIST “RefProp” Routines • Small Region around critical point where NIST routines fail • pcrit ± 2000 Pa ;

Tcrit ± 0.1 °K

• Linear interpolation within box (corner values) • Found that NIST Properties can govern run time • Some property calculations are complex, iterative solutions (>75% of code run time) • Potential area for run-time improvement • For SNAP Compatibility, other fluids can be made to act as CO2

Slide 10

Control System Modifications • Several Signal Variables have been added • Insight into Turbomachines conditions • “Corrected” conditions • Map output variables • Map scaled/unscaled conditions • Thermodynamic conditions implied by Map • Pressure ratio • Enthalpy change • Operational Status: On/Off Map • Encroachment on Certain Map Boundaries • Choke Status: in region where maps are multi-valued • Surge Margin

m& − m& surge =1+ m& surge

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Heat Transfer Model Modifications • Over-ride built-in models • Use when existing models are not appropriate • They focus on LWR applications

• Heat Transfer models that are applicable for S-CO2 • Kays and London Compact HX Model • S-CO2 Forced Convection Model

• Activation based on configuration (Heat Structure vs. Hydraulic) • Tied to 1 vs. multiple hydro’s (Pipe vs. HX) • Tied to inner/outer side (internal/external flow models) • Different fluids on different sides (IHX vs. Recuperator) • Powered vs. Unpowered (In Core vs. Out of Core) Precooler

Recuperator

Slide 12

Conclusions • TRACE has been a convenient platform for studies to date • Modifications provide successful Transient Analysis of compressible fluid in a highly, thermal-hydraulically, coupled Closed Brayton Loop • Demonstration of Code Operations Included in Other Presentations (M Hexemer) • Still require experimental data to benchmark the code

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Backup Slides

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Example Turbine Performance Map

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Example Compressor Performance Map

Outlet

Inlet Diffuser

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Turbine/Compressor Thermodynamic Processes

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