Nuclear reactor, operation method thereof and steam generation method thereof

A pressurization means for primary coolants is disposed in a light water type nuclear reactor so that primary coolants from a pressure vessel provide a state of single-phase flow or a two-phase flow. Further, there is disposed a second means connected to a primary coolant supply flow channel for sending steams obtained by depressurizing primary coolants in the state of the single-phase flow or two-phase flow from the pressure vessel, to cause boiling, under a reduced pressure to a main steam pipe. Further, a means is disposed for returning primary coolants remaining after boiling under the reduced pressure and feed water from a water feed pump by using a primary coolant recycling pump. Reactor primary coolants heated in the reactor core are sent to a steam generator and released to the space in the steam generator by a nozzle or the like. Since primary coolants remaining in the steam generator are returned to the pressure vessel without sending to a turbine, it is possible to substantially reduce energy loss due to boiling under reduced steam pressure, thereby enabling to improve reactor economy. (N.H.)