Safety Analysis of a Super Fast Reactor With Upward Flow Cooling in Two Pass at Supercritical Pressure

Safety characteristics of the supercritical-pressure light water-cooled fast reactor (Super FR) with upward flow core cooling in two pass is investigated for the abnormal transients and accidents at supercritical pressure. Upward flow cooling has advantage of simplifying the upper core structure in comparison with the downward flow scheme that part of the coolant flows downward in the blanket fuel assemblies from the top dome of reactor pressure vessel. It also has advantages that flow stagnation does not occur at loss of coolant flow events due to the buoyancy of the coolant. The coolant flow scheme of this design is the all blanket fuel assemblies and part of the seed fuel assemblies are cooled with upward flow first, the coolant flows radially above the core and flow downward in the gap between the core and the shroud to the lower mixing plenum and cools the rest of seed fuel assemblies with upward flow till the upper mixing plenum before core outlet. To evaluate the safety performance, eleven transients and four accidents at supercritical-pressure are analyzed. Safety analysis results show that the safety criteria are satisfied with large margins for all the selected transients and accidents. But in the total loss of coolant flow accident the MCST (maximum cladding surface temperature) is still high. Because of this flow scheme, it is found that the MCST is sensitive to the volume of the gap between two pass. Actuating depressurization valves with low flow single at total loss of flow events is effective to induce flow for once-through SCWR and therefore improves safety performance.