Fast breeder reactors

Purpose: To enhance safety of a fast breeder reactor by preventing coolant boiling and fuel melting even if the reactor has failed in scram at a loss-of-coolant accident. Constitution: The reactor uses a control rod guide tube of such a constitution that a coolant at the outlet of a reactor core flows through the entire length of 4 m-long driving rods for control rods. That is, the bore of the guide tube is made larger than the outside diameter of a control rod assembly to minimize fluid resistance inside, thus guiding the coolant flowing out of the fuel assembly to the surroundings of the driving rods for control rods. Consequently, heat transmission to the driving rods for control rods is done at a fast rate, shortening the time lag of thermal expansion. If the reactor core power increases and the core outlet temperature rises because of some abnormality, negative reactivity is inserted by the thermal expansion of the driving rods for control rods, and therefore the coolant boiling will never take place, thus insuring high safety in case of such an accident as fuel melting. (Kawakami, Y.)