The reactor consists of a pressurized vessel with inner and outer walls. They form an annular part and a central room. The heat exchange tubes lie in the annular part and is mounted on a tube-shaped thin plate. The thin plate seals the upper end of the annular part which stretches from its lower portion. A compartment is formed in the lower portion of the empty space of the annular part by a halving partition and channel components. The reactor core is placed in the central room. The cooling medium circulation pump makes the medium flow upward through the reactor core and then through the heat excharge tube in the annular part. In the latter, heat turus secondary cooling medium into steam. The secondary cooling medium is discharged through the exit which leads to the steam pot.