and a core shroud 4 surrounding the in core portion 2 a plurality of control rods 5 are inserted in the in core portion 2 the control rods 5 are operated by a control rod driving system 7 under the control of a control rod drive controller 6 the interior of the reactor pressure vessel 1 is filled up with light water as cooling water to a somewhat upper portion of the in core portion 2 when the reactor is in operation by driving a circulating pump 9 provided for a pipe 8 the cooling water in the reactor pressure vessel 1 passes through the pipe 8 and a riser tube 10 connected to the pipe 8 of a primary loop recirculation piping and reaches the interior of a jet pump 11 one end of the riser tube 10 is inserted in the reactor pressure vessel 1 to the upper end of the jet pump 11 a valve 12 provided in the pipe 8 is opened the cooling water reaching the interior of the jet pump 11 passes through a lower plenum 13 to the in core portion 2 while moving upward in the in core portion 2 the cooling water draws heat from nuclear fuel and changes into steam the steam passes the steam water separator 3 and then it is sent to a turbine 14 and condensed in a condenser 15 the turbine 14 is driven and a.