The concept for a plasma shutdown system has been studied for fusion experimental reactors. An impurity pellet injection system is proposed as the plasma shutdown system. The density limit disruption is employed by using impurity pellet injection. The pellet size and its injection velocity are obtained with an evaluation model of the tearing mode and a model of pellet ablation. The main functions of the impurity pellet injection system are checked by the trial manufacturer. The concept for this plasma shutdown system is extended to the International Thermonuclear Experimental Reactor (ITER).