The UK Atomic Energy Authority is developing a process unit to test emerging technologies for nuclear fusion (H3AT). Part of that process includes demonstrating the performance of equipment items which will be used in a full scale fusion reactor's fuel cycle. A big challenge is that the fusion reactor is expected to run in pulses, which means the design has to account for process dynamics from the very start. APS was used to model the process in parallel to the design work, to provide information on the operating ranges for the equipment, as well as insight into the total inventory, distribution, and concentration of tritium throughout the cycle. This was instrumental to the sizing of equipment, and identified key challengesfor the control scheme design.