Development of a steady-state sub-channel code for small reactor on the basis of combined cross momentum and non-linear conduction

•Combining equations to have a more stable and faster convergence solution.•Taking account of non-linear conduction of fuel rods.•Validating code with COBRA code.•Applying code to the small reactor “MUTSU” and comparing result with its design conditions.

For purpose of thermal hydraulic analysis in small nuclear reactors, a sub-channel code with an improved convergence has been developed based on the homogenous flowing model. A combined lateral momentum equation coupling with continuity and axial momentum equation has been used to substitute the original lateral momentum equation. The Gauss iteration method has been adopted to solve the Kirchhoff's transformation equation of nonlinear heat conduction of fuel rod, a temperature dependent conducting has been considered. The code has been validated by using experimental data from the NUPEC PWR Sub-channel and Bundle Tests (PSBT) and then applied to the “MUTSU” reactor. Results show that the code can predict the experimental data with acceptable accuracy and has ability to analyze the small PWR reactor.