Supercritical water reactor (SCWR) is one of the most promising nuclear reactor system among generation IV reactors thanks to its high thermal efficiency and simplicity. One of the main features of supercritical water is the strong variation of thermal-physical properties in the vicinity of the pseudo-critical temperature, which makes it very hard to predict the thermal-hydraulic behavior near this point. In this paper, CFD is used to investigate heat transfer of supercritical water in a 2×2 rod bundle with SST k-ε turbulence model. Two steady-state and one transient cases are simulated. The results show that there is strong non-uniform temperature distribution around the circumferential direction. Heat transfer deterioration (HTD) is found in the front of the heated section along the axial direction when the bulk temperature is near the pseudo-critical point due to secondary flow. Comparision of transient and steady-state flow shows that when the mass flux is less than 700 kg/m2s, the temperature in transient state is smaller than that in the steady-state, especially when the mass flux is 400 kg/m2s, the temperature difference is more than 10 °C.

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