Development of robust steady-state code for s-CO₂ Gen-IV reactor precoolers: application to printed circuit and shell-and-tube designs

Gen-IV reactors under development can use supercritical carbon dioxide (s-CO₂) for power conversion or for decay heat removal. Accurate thermal–hydraulic modeling of s-CO₂ heat exchangers remains a significant challenge due to the strong and highly nonlinear variations in thermophysical properties near the critical point. These variations introduce numerical stiffness and instability in classical solution schemes and create issues for modelling of compact geometries such as Printed Circuit Heat Exchangers (PCHEs). This study presents the development of NODEX, a robust steady-state code, formulated to handle steep temperature and enthalpy gradients and multi-channel heat partitioning for PCHEs and shell and tube heat exchangers (STHXs). The code implements a fully coupled Newton–Raphson scheme with adaptive line research method for relaxation to solve governing energy equations on both hot and cold sides. Furthermore, for STHXs, the cold-side model was modified to account for heat absorption through parallel tubes, requiring the reformulation of the governing equations and the corresponding Jacobian matrix. The methodology was validated through experimental data for PCHE and STHX precooler designs under different operating conditions. Results demonstrated that NODEX provides stable convergence, good numerical accuracy, and flexibility for both heat exchanger geometries. The NODEX code provides a tool for the design and optimization of s-CO₂ cycle precoolers in both non-nuclear applications and Gen-IV nuclear reactors.