Abstract: A reactor building assembly includes a pressure vessel within a containment vessel surrounded in turn by first and second enclosures defining respective first and second chambers. A method of operation includes channeling fresh air downwardly by gravity through the second chamber and then channeling the fresh air laterally through the first enclosure adjacent to the bottom thereof and into the first chamber. The air is then channeled upwardly by natural buoyancy through the first chamber for cooling the first chamber and mixing with stale air therein. The stale air is then discharged from the first chamber upwardly through the top of the first enclosure to the environs.

Abstract: A gravity driven cooling system pool is disposed at an elevated location with respect to the locations of nuclear fuel rods in a pressure vessel. In the event of a loss of coolant in the pressure vessel, steam pressure is initially reduced by venting the steam into the containment or a closed suppression pool containing a quantity of water under a large air space. The suppression pool condenses sufficient steam to lower the steam pressure in the pressure vessel so that water can flow by gravity from gravity driven cooling system pool to flood the fuel rods in the pressure vessel. An isolation condenser is submerged in a large supply of water elevated with respect to pressure vessel. Steam is admitted to the isolation condenser, or heat exchanger, where it is cooled by boiling the water surrounding it. This steam is vented to the atmosphere. A depressurization valve vents steam fro the pressure vessel into the containment to aid pressure reduction, and thus to aid the gravity flow of coolant.