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• Tube-in-tube bundles of NHR200-II primary heat exchangers cause thermal stress. • This numerical study optimized the tube diameters to minimize the thermal stress. • Flow and temperature fields in channels and tubes were simulated. • Optimal diameters and thicknesses of inner and outer tubes were identified. • Predicting the nonuniform temperature fields is useful in thermal stress assessment. NHR200-II is a multipurpose small integral pressurized water reactor. The primary heat exchangers of NHR200-II are in-vessel single-phase heat exchangers. The primary heat exchangers adopted a tube-in-tube bundle design to improve compactness; however, a significant weakness of the tube-in-tube bundle is the nonuniform thermal expansion between the inner and outer tubes owing to the inhomogeneous tube-temperature distribution. To minimize the thermal stress, the inner and outer tube diameters were optimized using numerical simulations. The nonuniform mass flow rates and temperature fields in each channel and tube were calculated using the optimal tube sizes, and the axially averaged temperatures of the inner and outer tubes were also calculated. The present prediction of nonuniform temperature fields can be used as the input to estimate the magnitude of thermal stress in the assessment of the structural integrity of the primary heat exchangers of NHR200-II. [ABSTRACT FROM AUTHOR]

• Mathematical model for primary residue heat removal (PRHR) system of NHR200-II. • Increased air cooler elevation suppresses reverse flow phenomenon in PRHR system. • There exists a critical elevation of air cooler. • Reverse flow phenomenon should not be suppressed by changing friction coefficients. The passive residual heat removal (PRHR) system of the NHR200-II nuclear heating reactor is a parallel-channel single-phase natural circulation loop. The reverse-flow phenomenon in parallel-channel natural circulation loops will cause insufficient heat removal capacity in the PRHR system. In this study, a simplified mathematical model was established for the PRHR system of NHR200-II. The mechanism of the reverse flow phenomenon in the PRHR system was analyzed using a pressure drop-flow rate diagram, and the effects of cooler elevation and loop resistance on the number of reverse flow branches were discussed. The analysis reveals that the reverse flow phenomenon of the PRHR system can be suppressed by increasing the elevation of the air cooler. Furthermore, there exists a critical elevation of the air cooler. It is not feasible to suppress the reverse flow by changing the friction coefficients of either the PHE branches or the air cooler branch when the air cooler elevation is less than the critical value. Oppositely, the number of reverse flow branches can be reduced or even eliminated by increasing the resistance of the PHE branches or reducing the resistance of the air cooler branch when the air cooler elevation is greater than the critical value. [ABSTRACT FROM AUTHOR]