Natural convection type reactor

This invention concerns a natrual convection type BWR reactor capable of spectral shift operation due to the control for the reactor core void coefficient by a simple structure. A plurality of annular water feeding spurgers are disposed each at a different height below the level of reactor water in a downcomer between the shroud and a pressure vessel. Further, the feedwater pipeways are branched into a plurality of lines, which are connected with the plurality of water feeding spurgers respectively. Further, feedwater conrol means is disposed to each of the plurality of feedwater pipeways. In this constitution, when the feedwater ratio to the feedwater spurger above the downcomer and that to the feedwater spurger below the downcomer is changed interlockingly, the recycling flow rate is controlled depending on the ratio of feedwater. This enables to increase the burn-up degree of the fuels and improve the reactor fuel economy. (I.J.)