Fire tests at the German test reactor HDR were simulated using a Japanese zone model, BRI2. Eight and ten room models of the containment building were developed. Critical phenomena occurring during simulation were explored. BRI2 can be used for this type of work but care must be exercised where a side wind increases entrainment by the fire plume. Horizontal vents were described by effective vertical vents. Effect of location of the vent to ambient was found critical for cases where severely oxygen limited burning occurred.