Natural circulation cooling analysis in PWR-1000's core and spent fuel storage

The analysis of natural circulation cooling in PWR-1000's core and spent fuel storage was done. Referring to Fukushima accident, an analysis is required for knowing the capability of natural circulation cooling in steady state condition of PWR-1000 core after operated 1 year and shutdown in 0.1 second. The analysis was done using step calculate the decay heat from fuel assemblies, and calculate the meat and coolant temperature distribution using COBRA-EN code. Furthermore, the analysis is also for knowing the capability of natural circulation in steady state condition of fuel storage after fuel has been used in reactor operation for 1 year and shutdown time for 10 days, using COOLOD-N2 code. The calculation result of natural circulation with high pressure of 15 MPa in core channel showed that averaged decay heat is still 236.28 MW after shutdown 0.1 second. Decay heat with natural circulation resulted that the coolant saturation temperature is 342.82 °C, coolant outlet temperature is 342.82 °C, cladding maximum temperature is 343.65 °C, and meat maximum temperature in the middle fuel is 375.45 °C. Furthermore boiling was occurred for fuel length of 1.39 m or 38.1 % on upper side of fuel rod. The calculation result for fuel storage after fuel has been used in reactor operation for 1 year and shutdown time for 10 day, showed that decay heat of 8.36 MW can not be cooled by natural circulation and boiling will be happened for 75-80 % fuel rod length. Both analysis results recommend that the fuel cooling either in core or in fuel storage must be done by forced convection after reactor shutdown, to avoid boiling. (author)