The fluids on both sides of the helical coil steam generator of the high-temperature gas-cooled reactor (HTGR) are helium and water respectively, whose physical properties are quite different and the transient response time is different. Traditional semi-implicit finite difference scheme applied to thermal hydraulic analysis code can only apply small time steps due to Courant-Friedrichs-Lewy (CFL) condition, which will decrease the computation efficiency.

This study aims to develop and verify a new transient analysis code using all-implicit algorithm for the helical coil steam generator of the high-temperature gas-cooled reactor.

Firstly, based on homogeneous flow model, a fully implicit finite difference scheme for the convection and diffusion term combined with the full coupling solution algorithm of flow and heat for thermal conductivity process to develop a new transient analysis program, named NUSOL-HTGRSG, for the helical coil steam generators of HTGR. Then, verification of the code was conducted in the four aspects: the design condition of the HTR-PM steam generator used for steady-state calculation validation, the number of grids changed for spatial sensitivity analysis, the time step changed for time sensitivity analysis, and the transient calculation carried out under the same disturbance (the flow rate of the primary side of the steam generator reduced by 10%). and results were compared with that of NUSOL-SG calculation.

Steady-state validation results show that the relative errors of outlet temperatures and pressure drops in the primary and secondary sides are generally within 1%. Transient validation results indicate that, under identical transient conditions, the maximum relative deviation between the transient responses of the two codes is 1.4%.

The validation results demonstrate that the NUSOL-HTGRSG code can effectively predict the operating parameters of the helical coil steam generator in HTGRs under steady-state conditions and accurately capture its transient characteristics with a relatively large time step (5 s).