CATHENA has been successfully employed to simulate accident and startup transients in the development of the Canadian supercritical water reactor concept. This success is attributed to the implementation of a novel method to integrate supercritical modelling elements into the code. A supercritical water properties scheme was used that does not rely on the use of pseudo phases and treats the supercritical regime as a single uninterrupted phase. The development of a robust numerical strategy manages the transitions that can occur in accident scenarios. Finally, the utilization of an expanded trans critical heat transfer look up table that smoothly transitions across the near critical and supercritical regions over a wide temperature range ensures consistent, stable simulation results.