Patent Number: 
Section: claims

1. An apparatus comprising:a pressurized water reactor (PWR) including:a pressure vessel configured to contain primary coolant water,a nuclear reactor core disposed in the pressure vessel and including a plurality of fuel assemblies,a control system including a plurality of control rod assemblies wherein each control rod assembly is guided by a corresponding control rod assembly guide structure, anda support element disposed above the control rod assembly guide structures,wherein the support element spans an interior of the pressure vessel; a top end of each control rod assembly guide structure is secured directly to the support element; the support element supports the control rod assembly guide structures; and the control rod assembly guide structures are anchored to each other only at their top ends. 2. The apparatus of claim 1, wherein the pressure vessel is a cylindrical pressure vessel, the support element comprises a support plate having a circular periphery supported by the cylindrical pressure vessel, and each fuel assembly includes a plurality of fuel rods. 3. The apparatus of claim 2, wherein the control rod assembly guide structures hang downward from the support plate. 4. The apparatus of claim 2, wherein the lower end of each control rod assembly guide structure includes alignment features that engage corresponding alignment features of the upper end of the corresponding fuel assembly. 5. The apparatus of claim 4, wherein there is a gap between the lower end of each control rod assembly guide structure and the upper end of the corresponding fuel assembly. 6. The apparatus of claim 5, wherein the fuel assemblies are supported from below. 7. The apparatus of claim 2, wherein the control rod assembly guide structures are not supported from below. 8. The apparatus of claim 2, wherein flow of primary coolant water in the pressure vessel in the operational state of the PWR is not sufficient to lift the fuel assemblies upward. 9. The apparatus of claim 1, wherein the control rod assembly guide structures hang downward from the support element. 10. The apparatus of claim 1, wherein the lower end of each control rod assembly guide structure includes mating structures that mate with an upper end of the corresponding fuel assembly. 11. The apparatus of claim 1, wherein there is a gap between the lower end of each control rod assembly guide structure and the upper end of the corresponding fuel assembly. 12. The apparatus of claim 1, wherein the control rod assembly guide structures are not supported from below. 13. The apparatus of claim 1, wherein flow of primary coolant water in the pressure vessel in the operational state of the PWR is not sufficient to lift the fuel assemblies upward. 14. The apparatus of claim 1, wherein there are no hold-down springs disposed on the tops of the fuel assemblies. 15. An apparatus comprising:a pressurized water reactor (PWR) including:a pressure vessel,a nuclear reactor core disposed in the pressure vessel and including a plurality of fuel assemblies,a control system including a plurality of control rod assemblies wherein each control rod assembly includes control rods selectively inserted into the nuclear reactor core and wherein each control rod assembly is guided by a corresponding control rod assembly guide structure, anda support element disposed within the pressure vessel above the control rod assembly guide structures,wherein a top of each control rod assembly guide structure is anchored directly to the support element; the control rod assembly guide structures are suspended from the support element and anchored to each other only at their top ends; and there is a gap between the bottoms of the control rod assembly guide structures and the top of the nuclear reactor core. 16. The apparatus of claim 15, wherein the control rod assembly guide structures are not supported from below the control rod assembly guide structures and each fuel assembly includes a plurality of fuel rods. 17. The apparatus of claim 15, wherein:there is a one-to-one correspondence between the control rod assembly guide structures and the fuel assemblies of the nuclear reactor core, andthe lower end of each control rod assembly guide structure includes alignment features that engage corresponding alignment features of the upper end of the corresponding fuel assembly. 18. The apparatus of claim 15, wherein flow of primary coolant water in the pressure vessel in the operational state of the PWR is not sufficient to lift the fuel assemblies upward.