AGENT-TH scheme: New thermal hydraulics (TH) code coupled with the AGENT neutronics code for light water reactors modeling and analysis

• AGENT-TH is a new thermal-hydraulic module developed for modeling BWR and PWR coolant properties.
• AGENT-TH code uses the drift flux and HEM fluid models for 1D sub-channel analysis.
• Fluid analysis for single BWR & PWR pins in AGENT-TH closely follows TRACE results.
• AGENT neutronics code is coupled with TH to account for temperature effects.

In this paper a newly developed thermal hydraulics (TH) code coupled with the state-of-the-art neutronics deterministic code AGENT (Arbitrary Geometry Neutron Transport) is presented. This coupled system is called the AGENT-TH scheme. The neutronics AGENT code uniquely combines the method of characteristics (MOC) and R-function geometrical modeling to solve the 2D/3D neutron transport in nuclear reactors of current or future designs. Commonly, the cross sections are prepared using the SCALE code system. The implementation of the TH code with AGENT enables coupled thermal-hydraulic and neutronics analysis in providing the average reactor fuel temperatures. This allows for an accurate overall power profiling within the reactor system, which affects reactor coolant temperature, neutron flux, and other associated reactor parameters.The two main types of LWRs are boiling water reactors (BWRs) and pressurized water reactors (PWRs). The BWRs operate at lower pressures than PWRs, which results in a higher void fraction, or fraction of coolant flow which is vapor versus liquid. Performances of PWRs, with a higher operating pressure, may be obtained using the homogeneous equilibrium mixture (HEM) model, which assumes a slip relation equal to one between the coolant liquid and vapor phases. To account for the higher void fraction in the BWR coolant, the BWRs properties are assessed based on the drift flux model. The thermal hydraulics drift flux model is developed as a part of the TH code, to address the two-phase flow associated with two-phase heat transfer (including subcooled nucleation and saturated boiling) as found in BWR systems. This is accomplished based on the drift flux equations in 1D steady-state conditions, which permits fast computation times without sacrificing final accuracy.The TH code is benchmarked against the reactor simulation code TRACE for a single pin to assess the thermal-hydraulics capabilities of the drift flux model for the fluid void fraction, flow quality, pressure, saturation temperature, and mixture temperature along the axial height of the pin. The drift flux thermal hydraulics module is coupled with AGENT using a radial heat transfer model taking into account heat transfer from the fuel centerline to the bulk fluid. Fuel temperatures calculated in the TH code are used to update cross sections from the SCALE code system to account for thermal feedback. Reasonable agreements are obtained between the AGENT-TH and TRACE for the presented benchmark examples.