Included in a project on a filtered and vented system for mitigating reactor meltdown accident consequences, FILTRA, a computational model, was developed for the investigation of iodine adsorption and desorption in a granular bed. The model considers physical adsorption, chemisorption, and desorption of free molecules carried by a gas through the bed. The calculational results show good agreement with experiments. A simplified version of the model has been used for calculating condensation and vaporization of volatile fission products in the primary circuit. The radioactive decay was ignored.