Patent Number: 041644433
Section: claims

1. In a nuclear reactor having; a reactor vessel having an inlet and an outlet; a core supported within said reactor vessel, and formed of a plurality of vertically elongated fuel assemblies; a mass flow of coolant from the inlet, upwardly through said core, and to the outlet, whereby a high pressure is established at the inlet, an intermediate pressure at the core inlet, and a low pressure at the outlet; a horizontal seal plate structure within said reactor vessel dividing said reactor vessel into a high pressure plenum above said seal plate and a low pressure plenum below said seal plate, said high pressure plenum being in direct fluid communication with the reactor vessel inlet, and said low pressure plenum being in direct fluid communication with the reactor vessel outlet; a fuel assembly hold down apparatus comprising: a push rod in vertical alignment with a fuel assembly and in contact with the upper end thereof; a piston attached to the upper portion of said push rod, said push rod restraining relative downward movement of said piston; vertical surface of said seal plate structure surrounding an opening therein and in sliding sealing relationship with said piston, whereby the upper surface of said piston is exposed to the high pressure plenum and the lower surface of said piston is exposed to the low pressure plenum. 2. An apparatus as in claim 1 wherein said seal plate structure comprises a seal plate and vertical extensions on said seal plate having vertical surface thereon, said piston located above said seal plate and in sliding contact with said vertical extensions. 3. An apparatus as in claim 2 wherein said piston and said push rod are rigidly connected. 4. An apparatus as in claim 3 wherein said fuel assembly includes a control rod guide tube; said nuclear reactor having finger control rods vertically movable through said control rod guide tubes and extending upwardly into said high pressure plenum; said push rod being tubular, and surrounding one of said control rods, and in contact with the upper end of said control rod guide tube. 5. An apparatus as in claim 4 having also a horizontal alignment plate below said seal plate forming an outlet plenum therebetween, said alignment plate having openings therein engaging said control rod guide tubes for horizontal alignment of the fuel assemblies; a shroud tube surrounding said push rod and joining said seal plate and said alignment plate. 6. An apparatus as in claim 5 wherein said seal plate has an opening therethrough within the confines of said vertical extension, providing direct fluid communication between the lower surface of said piston and said outlet plenum. 7. An apparatus as in claim 5 having a plurality of control rod guide tubes and push rods for a single fuel assembly.