Supercritical-water-cooled reactor system (SCWR) System Safety Assessment - 2018

Conceptual Super-Critical Water-cooled Reactor (SCWR) designs have been completed in Canada, EU and Japan. The thermodynamic expression “supercritical” describes the state of the substance where there is no clear distinction between the liquid and the gaseous phases. The SCWR design was one among six different Generation IV system designs selected on a fourteen-country initiative expressing a strong interest in collaborative Research and Development (R&D) to develop future generation of nuclear energy systems for deployment beyond 2030.
The SCWR concepts follow two main types, namely the reactor pressure vessel (PV) analogous to conventional Light Water Reactors (LWRs), and distributed pressure tubes (PTs) or pressure channels analogous to conventional Heavy Water Reactors (HWRs). The safety analysis of the Canadian SCWR concept and the EU High Performance Light Water Reactor concept covered key accident scenarios expected during operation. The large-break loss-of-coolant accident (LOCA) is assumed to be the limiting scenario for the Canadian SCWR with a maximum predicted cladding temperature of 1075°C. A limiting accident scenario is defined as a scenario that gives rise to maximum cladding temperature. The peak cladding temperature calculated for an HPLWR accident, initiated by an inadvertent isolation of all main feedwater and main steam valves with a delayed low-pressure injection to the core, is about 910°C. On the other hand, the limiting accident scenario for HPLWR is the small break (15%) loss-of-coolant accident, where the peak cladding temperature is about 1000°C (350°C above the steady-state temperature).
The safety assessments concluded that there are no major impediments to further developing SCWR design concepts. However, there are gaps in understanding how the reactor core will behave under SCWR pressures and temperatures. The proposed core concepts require a number of engineering assessments to evaluate the structural integrity and heat transfer characteristics. The results of these analyses will provide feedback to further refine the core design concepts.