In order to demonstrate tritium confinement performance in DT (deuterium-tritium) fusion reactors at any accident, TPL/JAERI has carried out intentional tritium release experiments using Caisson Assembly for Tritium Safety study (CATS). Some data have been accumulated such as actual tritium behavior in the normal ventilated area and response of tritium monitors and emergency isolation valves. The experimental results of confinement of released tritium of about 3 GBq in 12m3 of the Caisson under ventilation of 50 m3/h were consistent with those of simulation by a three dimensional eddy flow calculation code modified from FLOW-3D. Using this simulation code, the tritium confinement performance in large rooms can be demonstrated for actual fusion reactors.