HIGH TEMPERATURE GAS REACTOR

PROBLEM TO BE SOLVED: To provide a high temperature gas reactor capable of surely preventing overheat of a reactor core fuel by preventing leaked flow in an annular gap between an outlet pipe 82 of a cross duct 80 and a fixed reflector block 20a in a lower part of a reactor core and by sufficiently securing a flow rate of a coolant flowing to the reactor core part.SOLUTION: Between an inner circumferential surface of an opening 20b of the fixed reflector block 20a to which the outlet pipe 82 is connected and an outer circumferential surface of the outlet pipe 82, seal means 83 is provided. The seal means 83 is comprised of a plurality of uneven portions 831 formed on an outer circumferential wall surface of the outlet pipe 83 and seal ring members 833 fitted into fitting grooves 832 of the respective uneven portions 831, and is configured to block a gap that becomes a flow passage of a leaked flow of a coolant.