[en] In an LMFBR type reactor, integrated type fuel assemblies are contained in a pot which has an inlet port and a discharge port disposed at the upper portion thereof for liquid metal coolants allowing the coolants to pass therethrough. A high pressure plenum having a connection pipe connecting the inlet port and a primary coolant pipeline is disposed. The pot and the primary coolant pipeline are made separable. Further, the spent fuels can be taken out of the reactor together with the pot while being immersed in the coolants as they are. Then, the liquid metal coolants inflow from the inlet port by way of the connection pipe and cool the integrated type fuels while they pass through the pot. Upon fuel exchange, the pot is separated and raised from the connection pipe of the high pressure plenum, and the spent integrated type fuels are taken out to the outside of the reactor together with the pot while being immersed in the coolants as they are. The stand-by time for the attenuation of the after-heat from the reactor shutdown to the start of the fuel exchange is greatly shortened to improve the plant operation efficiency. (N.H.)