Catalytic oxidation and adsorption comprise the most conventional and reliable method for removing tritium that is accidentally released into the working area of fusion power plants. Based on both hot and cold experimental databases, a numerical calculation code with the temporary name TRITON QUEST is being developed to support the design of the air cleanup system. The code has been considerably improved in terms of mass balance equations and calculation method. It has also become possible to conduct larger-scale computations in which catalyst or adsorption beds of actual scale are used. The behavior of tritium in the case of an accidental release of 100 g of tritium in the International Thermonuclear Experimental Reactor (ITER) test plant was predicted using this numerical computation code. The results of numerical computation indicate that the tritium concentration in a room with a volume of 10 000 m3 can be reduced to the regulatory level within 24 h when an air cleanup system with 1000 kg of Pt/alumina catalyst and 2500 kg of MS5A is operated with a ventilation rate of 10 000 m3/h. The decontamination efficiency for new arrangements of the air cleanup system was also investigated. The results suggest that the new arrangements have an advantage in the regeneration of the air cleanup system.