Patent Number: 041644433
Section: summary

BACKGROUND OF THE INVENTION This invention relates to nuclear reactors and in particular to an apparatus for holding down fuel assemblies within the reactor core. In pressurized water reactors the coolant flow rate and fuel assembly flow resistance are such that the hydraulic uplift force is of sufficient magnitude to cause the assemblies to jitter and even lift off the core support structure. Various approaches have been used to eliminate this detrimental movement. One suggested solution involves the use of a lock down device which attaches the lower end of the fuel assemblies to the core support structure. While this device will function properly it does introduce mechanical complexity since the device must not only lock and unlock remotely but it must release reliably after a year of operation in the reactor environment. Another approach has been to use springs located above each fuel assembly which bear against an upper alignment plate, thereby urging the fuel assemblies down. As reactors have been designed with increasingly large hydraulic uplift forces the spring force and the springs have become very large. Any component at this location limits the ability to obtain a desirable flow pattern and tends to increase the pressure drop of the coolant. In the design of nuclear reactors, a loss of coolant accident must be considered which involves a break of either the inlet or outlet line connected to the reactor vessel. In the event of a break of the outlet line the increased flow results in a substantial increase in the upward force on the fuel assemblies. A break of the inlet line on the other hand reverses the flow direction. Steam generated in the core remains behind as a steam pocket and forces the water backwardly through the core to the break at the inlet line. It would be desirable to maintain the water within the core while permitting the steam to escape through the break. SUMMARY OF THE INVENTION It is an object of the invention to hold down fuel assemblies in a simple manner which will eliminate or reduce the need for spring hold down forces and in a manner which will minimize flow restrictions due to the hold down structure. It is a further object to introduce these forces in a manner which will compensate for variations in the primary flow through the reactor with concomitant variations in upward force on the fuel assemblies. It is a further object to provide a steam release path through the reactor in the event of an inlet line break where the invention is used with the preferred embodiment. These and other objects are achieved in the invention wherein a nuclear reactor vessel is divided by a seal plate structure into a high pressure upper plenum and a low pressure plenum. A piston is in sealing and sliding relationship with the seal plate structure and it has push rods attached thereto which extend downwardly and hold down the fuel assemblies of the core. The piston in the preferred embodiment is located above the seal plate and seals against vertical extensions on the seal plate structure. The high pressure plenum is directly connection to the reactor vessel inlet and, therefore, the pressure above the piston approximates the inlet pressure. The lower surface of the piston is in direct fluid communication with the reactor vessel outlet and, therefore, the pressure below the piston approximates the reactor vessel outlet pressure. The pressure difference acting on the piston is a function of the pressure difference through the reactor and, therefore, the hold down force inherently compensates for differences in flow through the reactor. The substantial portion of the hold down structure is out of the primary fluid flow path since it is located at or above the seal plate structure. Only the tubular push rod which surrounds the control rod extends down toward the primary flow path. This push rod also forms a flow path for downward flow of coolant which is passing from the upper plenum downwardly to cool the control rods. The close seal between the piston and the seal plate structure is effected above the seal plate whereby adjustments may be made and installation effected prior to placing the core alignment barrel on the fuel assemblies. Assembly of the reactor is thereby simplified since the close fitting tolerances need not be handled during this operation. In the event of a loss of coolant accident due to a break of the outlet line, the inherent compensation characteristic of the invention increases the hold down force at the time that the high flow is tending to lift the assemblies. In the event of an inlet line loss of coolant accident, the reverse force on the piston causes the piston to disengage from the seal plate structure, thereby providing a flow path for the steam to pass to the broken inlet connection without passing downwardly through the core and forcing water out.