Safety analysis of a supercritical water cooled fast reactor with all-upward two-pass flow

• Safety of Super FR with all-upward two-pass flow scheme has been analyzed.
• Current core design exhibits unique flow behavior with regard to two-pass.
• Depressurization strategy is effective to induce flow for once-through SCWR.

The supercritical water cooled fast reactor with all-upward two-pass flow scheme has been designed for simplifying the upper core structure. It is also possible to avoid the stagnation effect of the downward flow channel in loss of coolant flow events. To evaluate the safety performance and correspondingly improve the core and safety design for current flow scheme with different characteristics, safety analysis with regard to accidents and abnormal transients has been conducted. Safety analysis results show that the safety criteria are satisfied with large margins for all the selected accidents and transients. Furthermore, current all-upward two-pass flow core design with a downward gap space exhibits unique flow behavior of feedwater-dominant first-pass flow and valve-dominant second-pass flow because of the large volume of gap space between the first and second pass. Actuating depressurization valves at total loss of flow events is effective to induce flow for once-through supercritical water cooled reactor (SCWR) and therefore improves safety performance.