GEM device incorporating floating type neutron absorber

The present invention concerns an improvement of a GEM (Gas Expansion Module) device for enhancing an automatic reactor shut down performance of an FBR type reactor. The device comprises a sealed outer cylindrical tube having an upper gas space in the upper portion, and an entrance nozzle for reactor core loading and a coolant flow inlet at a lower end thereof. Upon loading to the reactor core, coolants are charged from the coolant flow inlet into the outer cylindrical tube to compress the upper gas space thereby setting the coolant liquid level in the outer cylindrical tube to a level at which the pressure is balanced. A vessel incorporated with a neutron absorber is floated on a coolant liquid surface in the outer cylindrical tube, and when the flow rate of the coolants is reduced, the amount of leaked neutrons is increased automatically along with the reduction of the liquid level in the GEM device. Further, a neutron absorbing effect can be enhanced due to the vessel incorporated with neutron absorbers lowered to a level of the reactor core portion. As a result, negative reactivity can be applied to the reactor. Since a large negative reactivity can be applied automatically, upon occurrence of an accident of reduction of coolant flow rate, safety of the reactor core can be improved. (N.H.)