A semi-empirical model is developed that shows iodine filter behavior under normal and anomalous conditions. A two-stage accident is assumed in a 300-MW(electric) liquid-metal fast breeder reactor (LMFBR). Normal behavior is expected when there is a 10 to 30% adsorption of the inventory of 131I, released during a core meltdown of the LMFBR. No serious rise in temperature and desorption is expected in the filter under normal conditions. Under anomalous conditions, however, i.e., when >60% of the 131I that is released is adsorbed in the filter, high desorption and probably burning of the charcoal may result.