Patent Number: 046719237
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to an arrangement for retaining the reactor internals holddown spring forming a component installed in a nuclear reactor pressure vessel. Various types of pressurized water reactors include, within a pressure vessel, a core barrel, an inner barrel and an upper calandria, each provided with an upper, outwardly extending flange via which the respective part is supported within the pressure vessel. An annular holddown spring is associated with these components and the highest structural margins and reliability are achieved when the spring is positioned between the inner barrel flange and the core barrel flange. However, in a recently proposed new pressurized water reactor design, the region of the core barrel flange which would be covered by the holddown spring is provided with components which must be accessible at various times, particularly during refueling and inservice inspection. Individual removal of the holddown spring is extremely difficult. It might be possible to eliminate this problem by installing the holddown spring between the flanges of the inner barrel and the upper calandria, so that the spring could be lifted together with the inner barrel. However, this location presents a number of drawbacks, including the fact that it will result in lower reliability and higher spring loads. SUMMARY OF THE INVENTION It is an object of the present invention to facilitate removal of the holddown spring during the operating life of such a reactor while permitting the spring to be installed between the core barrel flange and the inner barrel flange of the reactor. Another object of the invention is to assure that the holddown spring will be properly positioned when reinstalled after removal. The above and other objects are achieved, according to the invention, by the provision of a holddown spring retention assembly for use in a nuclear reactor having a pressure vessel, a core barrel having an upper outwardly extending flange, an inner barrel having an upper outwardly extending flange and an annular holddown spring, the barrels and the spring being installed in the pressure vessel during normal operation, with the inner barrel flange disposed above the core barrel flange and the holddown spring interposed between the core barrel flange and the inner barrel flange. The holddown spring retention assembly comprises a plurality of assembly units disposed around the periphery of the holddown spring, each unit comprising: lift lug means secured to the outer periphery of the spring and having at least one outwardly radially projecting portion; and hanger means secured to the inner barrel flange and having a lower portion suspended below the inner barrel flange and below the radially projecting portion of the lift lug means; the lower portion being arranged to support the radially projecting portion during lifting of the inner barrel upwardly away from the core barrel so that the spring is lifted together with the inner barrel. The retention assembly according to the present invention thus causes the holddown spring to be automatically lifted together with the core barrel, while assuring that the spring will be properly positioned after reinstallation. At the same time, the retention assembly according to the invention does not interfere with the normal deformation which the spring must undergo during installation in order to provide the requisite loading forces and to perform its intended clamping action. The retention assembly according to the invention can be constructed to prevent possible repetitive impact damage at assembly interfaces. Differential thermal expansion between the holddown spring itself and the barrel flanges will not adversely affect assembly interface clearances under all plant operating conditions. Moreover, the retention assembly according to the invention is relatively inexpensive.