Fuel assembly for use in BWR type reactors

Purpose: To enable to increase the reactor shutdown margin, shorten the time required for the power-up upon reactor start-up, as well as eliminate the requirement for the controlling operation during load following operation. Constitution: Short poison member of a length less than that of the upper plenum portion is the fuel rod is displaceably arranged in the upper position within the fuel reaction range with respect to the vertical direction of the reactor core in a water rod, such that the short poison member is displaced out of the upper fuel reaction range due to the increase in the flow rate of coolants flowing in the water rod when the flow rate of the reactor core coolants is increased to above a predetermined value. Accordingly, since the poison is inserted in the upper portion of the reactor core where the reactivity is high at normal temperature, the shutdown margin can be increased. Further since the poison is removed, from the reactor core upon reactor start-up along with the increase in the coolant flow rate and whereby the power flowrate property can be improved, and the power loss upon start-up is reduced to improve the efficiency and eliminate the requirement for the control rod operation in the load following operation. (Yoshihara, H.)