Patent Number: 052788806
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a support for the pressurizer tank of a pressurized water nuclear reactor or the like, which maintains the tank relative to its housing against seismic shocks and other dynamic loads, while permitting certain variations in dimensions resulting from temperature and pressure variations in the reactor coolant system. 2. Prior Art In a pressurized water nuclear reactor, the reactor vessel containing nuclear fuel is coupled in a circulating coolant loop with a steam generator for extracting power from the coolant. A pressurizer tank is coupled to the coolant outlet of the reactor vessel, also known as the hot leg of the primary coolant circuit, for maintaining the required pressure in the reactor coolant system. Under operational conditions, the coolant in the hot leg may be at about 600.degree. F. (315.degree. C.) and 2,250 psi (150 bar). The reactor vessel, one or more steam generators and the pressurizer tank typically are disposed in a containment building. The pressurizer tank is a vertically elongated, normally cylindrical tank located at a higher elevation than the reactor vessel. The bottom head of the pressurizer tank is coupled to the hot leg via a conduit, and top head of the pressurizer tank is coupled to the reactor inlet or cold leg through a check valve. The pressurizer tank may be associated with a passive cooling system, for example as shown in U.S. Pat. No. 4,753,771 - Conway et al. The pressurizer is further coupled by depressurizer valves to spargers in a refueling water storage tank. By opening the depressurizer valves the reactor coolant circuit is brought down to atmospheric pressure in the containment building so that water from the refueling water storage tank can be added by gravity feed, without the need for pumps capable of high output pressure. The depressurizer valves and the conduits associated with them are coupled to the pressurizer at the top. If the depressurizing system is staged (i.e., having a plurality of separate valved conduits opening at successively lower pressures), additional weight is placed on the pressurizer supporting structures. The pressurizer tank can be placed on a foundation structure and housed in a compartment extending along the sides of the tank. Inasmuch as the pressurizer tank is vertically elongated, it is useful to support the tank against lateral forces at a point above the foundation structure, such as forces which may occur during a seismic shock. The conventional pressurizer upper support, shown in FIG. 3, comprises four individual struts attached at one end to the walls of the pressurizer compartment and extending radially inwardly. The inner end of the struts have vertical slots which engage over radially protruding lugs on the outer walls of the pressurizer tank. This conventional mounting has a low overall stiffness, which contributes to seismic loading, and a limited load capacity. Seismic loading problems are compounded if the pressurizer compartment also has a low stiffness, as characteristic of modular building structures. Moreover, mounting a number of valves and conduits at the top of the pressurizer tank for staged depressurization increases the expected loading due to the size, elevation and weight of the discharge piping and valves. It would be possible with a static tank to simply increase the size and weight of the structures supporting the tank, thereby holding the tank more intimately and securely. However, a pressurizer tank is not static. There are extreme variations in coolant system pressure and temperature between the operational and shutdown states of the reactor. The pressurizer tank can be expected to vary in dimensions due to both thermal expansion and hydraulic pressure. There is a need to provide an improved supporting structure for the pressurizer tank, which can adequately support the tank as well as the discharge piping and valves coupled high on the tank, transferring loads to the compartment walls in a favorable fashion while providing high stiffness and loading capacity. SUMMARY OF THE INVENTION It is an object of the invention to provide improved lateral support of a vertically elongated pressurizer tank, to protect against seismic loads. It is another object of the invention to provide a mounting point for relief/discharge valves and piping associated with the pressurizer tank, without causing lateral loads resulting from such items to be borne by the pressurizer tank. It is a further object to provide a durable high load capacity lateral restraint for a pressurizer tank, which accommodates the dimensional variations and temperature variations occurring during startup and cooldown of the reactor. These and other objects are accomplished for a pressurizer tank in a pressurized water nuclear reactor, mounted between structural walls of the containment interior, such as interior concrete walls, and rests on a substructure of the containment, the tank having a vertically elongated tank wall extending upwardly from the substructure. For bearing lateral loads such as seismic shocks, a girder substantial encircles the pressurizer tank at a space above the substructure and is coupled to the structural walls via opposed sway struts. Each sway strut is attached at one end to the girder and at an opposite end to one of the structural walls, and the sway struts are oriented substantially horizontally in pairs aligned substantially along tangents to the circular tank wall. Preferably, eight sway struts attach to the girder at 90.degree. intervals. A compartment encloses the pressurizer tank and forms the structural wall. The sway struts attach to corners of the compartment for maximum stiffness and load bearing capacity. A valve support structural frame carrying the relief/discharge piping of an automatic depressurization arrangement is fixed to the girder, whereby lateral loads on the relief/discharge piping are coupled directly to the compartment rather than through any portion of the pressurizer tank. The structural frame can be welded to the girder, and the girder can be pinned vertically relative to the pressurizer tank by interfitting plates on the girder and tank, fixed by horizontal pins. Thermal insulation for a lower portion of the valve support frame minimizes thermal loading of the piping. Strip shims are placed between the pressurizer tank outside surface and the support girder inside diameter to provide a lateral load path while minimizing heat transfer to the girder. The girder is free to move vertically relative to the pressurizer tank compartment walls, for accommodating dimensional variation of the pressurizer tank with changes in temperature and pressure.