liquid sodium a quantity of inert gas 20 known generally as cover gas occupies the space 76 within the reactor pressure vessel 10 above the level of reactor coolant 18 at a positive differential pressure above atmospheric pressure typically this cover gas 20 may be comprised of helium nitrogen argon or some other like inert gas which does not adversely react with the reactor coolant or interfere with various aspects of reactor operation such as detection of leaks in reactor apparatus a confinement means for the pressure vessel 10 such as a pressure vessel closure head 22 is utilized to seal the reactor pressure vessel 10 the heat generated by the reactor core 12 is conveyed from the core 12 by coolant flow entering through inlet means 14 and exiting through outlet means 16 the hot reactor coolant flow exiting through outlet means 16 is conveyed through a portion of interconnecting piping 24 and into heat exchanger 26 the hot reactor coolant transfers the heat it previously acquired from the nuclear core 12 to a fluid of another system not shown in heat exchanger 26 cooled reactor coolant exits from heat exchanger 26 and enters the inlet of a primary coolant circulating pump 28 of a construction well known in the art the pump 28 illustrated in fig1 is classified as a cold leg pump in that it pumps cooled reactor coolant without modifying the practice of this invention the pump 28 could.