Patent Number: 
Section: claims

1. A natural circulation reactor comprising: a prestressed concrete reactor vessel having an inner concrete surface defining a vessel chamber, said inner concrete surface having a surface area;  a steel liner positioned within said vessel chamber and spaced from said prestressed concrete reactor vessel inner concrete surface;  an insulating chamber defined by an outer surface of said liner and said inner concrete surface of said prestressed concrete reactor vessel, said outer surface of said liner having a surface area; and  insulating material positioned in and substantially filling said insulating chamber, said insulating material extending from said inner concrete surface of said prestressed concrete reactor vessel to said outer surface of said steel liner, said insulating material in intimate contact with substantially all of said surface area of said inner concrete surface of said prestressed concrete reactor vessel and with substantially all of said surface area of said outer surface of said steel liner. 2. A natural circulation reactor in accordance with  claim 1  wherein said insulating material is fabricated from High Aluminate Cement Concrete. claim 1 3. A natural circulation reactor in accordance with  claim 1  wherein said insulating material is fabricated from heat resistant concrete, refractory castable concrete, or fire bricks. claim 1 4. A natural circulation reactor in accordance with  claim 1  wherein an inner surface of said steel liner is not insulated. claim 1 5. A natural circulation reactor in accordance with  claim 1  further comprising an insulating chamber extending between said steel liner and said prestressed concrete reactor vessel inner surface, and wherein said insulating chamber is at least partially filled with High Aluminate Cement Concrete. claim 1 6. A natural circulation reactor in accordance with  claim 1  further comprising an insulating chamber extending between said steel liner and said prestressed concrete reactor vessel inner surface, and wherein said insulating chamber is at least partially filled with fire bricks, heat resistant concrete, or refractory castable concrete. claim 1 7. A natural circulation reactor in accordance with  claim 1  wherein said steel liner is substantially coaxially aligned within said vessel chamber. claim 1 8. A prestressed concrete reactor vessel for a nuclear reactor, said prestressed concrete reactor vessel comprising: a prestressed concrete shell having an inner concrete surface defining a vessel chamber therein, said inner concrete surface having a surface area;  a steel liner positioned within said vessel chamber and spaced from said shell inner concrete surface;  an insulating chamber defined by an outer surface of said steel liner and said inner concrete surface of said prestressed concrete shell, said outer surface of said steel liner having a surface area; and  insulating material positioned in and substantially filling said insulating chamber, said insulating material extending from said inner concrete surface of said shell to said outer surface of said steel liner, said insulating material in intimate contact with substantially all of said surface area of said inner concrete surface of said prestressed concrete shell and with substantially all of said surface area of said outer surface of said steel liner. 9. A prestressed concrete reactor vessel in accordance with  claim 8  wherein said insulating material comprises High Aluminate Cement Concrete. claim 8 10. A prestressed concrete reactor vessel in accordance with  claim 8  wherein said insulating material comprises fire bricks, heat resistant concrete, or refractory castable concrete. claim 8 11. A prestressed concrete reactor vessel in accordance with  claim 8  wherein an inner surface of said steel liner is not insulated. claim 8 12. A prestressed concrete reactor vessel in accordance with  claim 8  wherein said steel liner is substantially coaxially aligned within said vessel chamber. claim 8