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 modeling 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 transcritical 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.