Abstract: A pressurized water nuclear reactor has a reactor vessel arranged in a pool, which is filled with a neutron absorbing liquid, for example borated water. The reactor vessel is closed except for tubes connecting it with a tray above it. The coolant in the circuit rises from the vessel to the tray, gives up its heat by flashing, and flows back to the bottom of the vessel, driven by natural circulation. The tray is separated from the pool by a vapor-filled bell, which surrounds it. In the bell the vapor gives up its useful heat to a condenser. The relatively low boron content of the cooling circuit, compared to the pool, is achieved by continuous dilution of the condensate from vapor additionally generated out of the pool water. The dilution process is an equilibrium with continuous inflow of the pool water. The inflow is automatically controlled by the pool level, which rises when the pool water is pressed out from below the bell by overproduction of vapor.