a reactor vessel including a hot pool for containing a high-temperature fluid discharged from a reactor core and a cold pool which is separated from the hot pool by a partition and contains a low-temperature fluid;

an intermediate heat exchanger (IHX) for transferring heat from the hot pool to an external steam generation system and positioned in the hot pool, the IHX having an upper portion communicating via a guide pipe with the hot pool and a bottom portion communicating with the cold pool for discharging the fluid from the hot pool into the cold pool after extracting heat from said fluid;

a cylinder surrounding the IHX and defining an annular space therearound, and having an open top portion extending above a level of the fluid in the hot pool, and a bottom portion communicating with the cold pool;

the guide pipe extending through said annular space of the cylinder and communicating at a first end with the hot pool and communicating at a second end with the upper portion of the IHX for allowing passage of the fluid from the hot pool into the IHX;

a decay heat exchanger (DHX) positioned in the annular space of the cylinder surrounding said IHX and spaced from the IHX by a designated distance for transferring reactor core decay heat to external air;

a switch valve installed on an outer wall of the guide pipe in the cylinder and having a buoy floatable on the fluid by buoyancy to switch a flow path from the guide pipe into the annular space of the cylinder; and

a pump arranged in the cold pool for pumping the fluid from the cold pool to the reactor core and to the hot pool and evacuating the annular space of the cylinder through the bottom portion of the cylinder to lower a level of fluid in the cylinder under the level of the fluid in the hot pool and under the DHX by virtue of a pressure differential between the hot pool and the cold pool caused by normal operation of the pump, whereby, upon pump failure, the level of fluid in the cylinder rises to contact the DHX and the switch valve is opened so that the fluid flows from the hot pool into the annular space of the cylinder, thereby allowing conduction heat transfer from the IHX to the DHX and from the fluid flowed into the annular space to the DHX, and thus transferring the reactor core decay heat to external air by the DHX is enabled.