Control rod assembly

Purpose: To greatly attenuate the reactor core power from that upon critical state when the coolant temperature abnormally rises due to scram failure or the like in FBR type reactors. Constitution: A heat-sensitive extendable portion is disposed to an extension tube for suspending a control rod main body. The extendable portion expands to extend the length of the extension tube if the coolant temperature rises abnormally and inserts a control rod main body deeply in the reactor core. The heat-sensitive extendable portion is constituted as a bellows structure in which liquid metal is sealed. When the temperature of the coolant is increased by about 200 deg C, the extension tube, for example, made of stainless steels with 6 m entire length is extended by about 1.5 cm due to the thermal expansion. Assuming that the natural length of the bellows in the heat-sensitive extendable portion as 30 cm and liquid metal NaK is sealed therein, the bellows portion are extended by about 1.5 cm. In this way, the extension tube is extended by 3 cm in total, by which the control rod main body is inserted more deeply to the reactor core by 1.5 cm than usual. A shape memory alloy may be used instead of the bellows. (Ikeda, J.)