Cooldown capability of Ringhals 3

Stricter requirements on licenses for operating nuclear power plants in Sweden have resulted in a need for more thorough analyses at Ringhals nuclear power plant. At Unit 3, it is of interest to investigate the cooling process after a reactor shutdown to find methods of verifying both the cooldown capability (i.e. cooldown time) and the performance of the heat exchangers involved in this process. Since few other analyses have considered the transient conditions during cooldown, it is important to evaluate how these conditions affect the process. It is also of interest to find out how the cooldown capability is affected by plugged tubes and fouling in the heat exchangers. The investigation was made by constructing a transient computational model of the cooling chain responsible for removing residual heat from the reactor. The model describes the process of reducing the temperature of the reactor coolant from 177°C to 60°C. The heat transfer capacities for the heat exchangers are evaluated using Kern’s method. The results show that the heat load due to the so-called sensible heat (heat stored in the water and structural material), constitutes 10 to 15% of the total heat load. The sensible heat load is too large to neglect when evaluating the cooldown capability and it is therefore required that the evaluation of the cooling chain includes transient terms. Furthermore, to meet the cooldown time requirement of maximum 36 hours, the required heat transfer capacities of the component cooling and residual heat removal heat exchangers are found to be 1306 kW/°C and 541 kW/°C, respectively. The cooling chain in its current state is over-dimensioned and well capable of meeting its requirements. Plugging of tubes (decreasing the heat transfer area) and fouling (decreasing the heat transfer) in the heat exchangers are not a concern with respect to the cooldown time requirement. However, more measurements to determine to the actual fouling values in the heat exchangers are recommended to verify these conclusions