Patent Number: 047864613
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to hold-down springs for holding nuclear reactor internals firmly in place and more particularly to Belleville type spring assemblies for clamping upper and lower reactor internals inside of a reactor vessel while providing a coolant flow path to the reactor vessel head region. 2. Description of the Prior Art Nuclear reactor cores are usually supported within a cylindrical core barrel arranged within a reactor vessel as a liner and hung from a flange formed where the reactor vessel and reactor vessel head are joined. The core and core barrel are commonly referred to as the lower internals. Coolant flows into the reactor vessel into an inlet annulus and is directed towards the bottom of the core barrel and then up through the core. During operation, the coolant is heated by the core. The heated coolant is then discharged from the reactor vessel as working fluid. Generally, a large pressure differential exists across the core which results in a very large "sailing" or lifting force against the core. This force actually tends to displace the core and its supporting structure. Positioned above the core in the pressure vessel are components known as the upper internals through which the heated coolant may pass before exiting from the pressure vessel. The upper internals are usually contained in a second cylindrical barrel axially aligned above the core barrel. The heated coolant, when passing through the upper internals, exerts a very considerable force against those components as well. In most pressurized water reactor (PWR) constructions, the upper internals barrel is also supported from the flange formed where the reactor vessel and reactor vessel head are joined. Because of the large size of the structures involved and the significant thermal gradients which exist in the reactor vessel, axial and radial differential expansions occur at the assembly of the vessel and core components. Because of these differential expansions and in large mechanical and hydraulic forces discussed above which act on these structures, the assembly must provide a large enough force to resist displacement. In addition, it is desirable to maintain the reactor vessel head region at inlet temperatures for safety reasons and to cool the upper internals drive components. Such cooling could only be achieved with a complex system of flow passages with prior designs which utilized a single large Belleville spring to provide a spring load and deflection capability for holding core barrel and upper internals barrel against deflection. With a large Belleville spring, a clamping load is developed when the reactor vessel head is lowered onto the Belleville spring and drawn down by head studs onto the reactor vessel flange. The spring is typically deflected on the order of only about 0.150 inch resulting in about 460,000 pounds of force to clamp the upper and lower internals against a machined ledge on the inside of the reactor vessel flange. Such loading is sufficient to prevent significant upward motion of the internals during normal operation and during seismic or LOCA events. However, with large (in the range of 14 to 16 foot diameter) Belleville springs, the loading force is developed over a very short deflection and therefore requires considerable precision. Moreover, large precision machined springs are expensive, difficult to heat treat and, because of their size and shape, difficult to handle, ship and replace. Moreover, with large springs a high stress is developed in the spring over a relatively small deflection which renders its performance vulnerable to stress relaxation after which replacement may be required to maintain adequate loading forces. Replacement is difficult not only because the spring is large but also because it is typically coated with a radioactive crud. Moreover, the size of a single spring is such that the replacement spring must come through a large hatch in the reactor containment resulting in long down times for the reactor. Moreover, many of the prior large spring designs comprised a 360.degree. structure which required a complex system of flow passages in order to pass inlet coolant to the upper head region. It should be understood that flow rates of up to 16,000 GPM (or on the order of 4% of the inlet flow) have to be accommodated. Disclosed in U.S. Pat. No. 4,096,034 is a structure for clamping a core barrel and upper internals band against hydraulic displacement by using a few, large Belleville spings mounted in vertical alignment with the wall of the upper internals barrel. No provision is made in the disclosed hold down structure for a Belleville spring assembly which permits an adjustable coolant flow to the upper reactor vessel head region. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a reactor vessel having an inexpensive, testable, and easily replaceable spring assembly for holding reactor internals. It is a further object of the invention to provide a reactor vessel assembly including hold down springs having a flow passage for introducing coolant to the upper reactor vessel region. It is a further object of the present invention to provide a hold down spring assembly design for reactor vessels which is inexpensive, easily tested and easily replaced and which conservatively meets all application requirements. According to the invention, a nuclear reactor is provided which comprises a reactor pressure vessel and a lower internals assembly positioned within the reactor pressure vessel. The lower internals assembly has a core barrel with a flange having a plurality of annularly spaced coolant passages extending therethrough. An upper internals assembly is positioned within the reactor pressure vessel. The upper internals assembly includes an upper internals barrel having a flange which is axially disposed above the core barrel flange. The upper internals barrel flange has a plurality of annularly spaced core passages which, when the upper internals and core barrels are assembled, align with the coolant passages in the core barrel flange. A plurality of reactor internals hold down spring assemblies are annularly spaced about the core barrel flange between the core barrel and upper internals barrel flanges. In accordance with the invention, the hold down spring assemblies comprise a retainer having a central bore therein. The retainer carries a resilient biasing means, preferably a stack of Belleville springs. A passage means is disposed in the central bore for defining a flow passage between the coolant passages in the core barrel and upper internals barrel. Preferably, the core barrel and pressure vessel form an inlet coolant flow annulus in fluid communication with the coolant passages in the core barrel. The upper internals barrel and pressure vessel form an upper head region in fluid communication with the coolant passages in the upper internals barrel. In this manner, coolant from the inlet annulus flows through the core barrel coolant passages, the connecting flow passages, the upper internals barrel coolant passages, and to the upper head region of the reactor. Advantageously, the means defining a connecting coolant passage may comprise a bellows flange having an opening and fixed at one end of the central bore. A spring bellows is carried by the bellows flange and is disposed within the central bore. A movable plunger having a central opening is carried by the spring bellows, the plunger being adapted to be biased against one of the core barrel coolant passages by the spring bellows. Preferably, the retainer has an upper flange, adapted to seat against the upper internals flange, the upper flange being dimensioned to retain the stack of Belleville springs on the retainer when the stack is seated against the core barrel flange to resiliently support the upper internals barrel. Preferably, the plunger has a generally spherical end portion and the core barrel coolant passages have cone shaped seating surfaces. The spherical ends are biased against the seating surfaces by the bellows spring in order to effect a generally fluid tight seal therebetween. Advantageously, the upper bellows flange carries a central tube disposed within the spring bellows which extends toward but does not contact the plunger. Advantageously, the hold down spring assemblies include a locking nut adapted to cooperate with the retainer to preload the stack of Belleville springs. In another embodiment, the means defining a connecting coolant passage preferably comprises a first movable plunger which has a central opening and is movably retained within one end of the central bore and a second movable plunger having a central opening and which is movable within another end of the central bore. A spring bellows is disposed between the first and second plungers and within the central bore in order to bias the first and second plungers against the upper internals barrel and core barrel coolant passages respectively. In another advantageous embodiment of the invention, the core barrel flange is preferably formed of increased thickness and the spring assemblies are inserted in a cylindrical counter bore formed in the core barrel flange about the first plurality of coolant passages. This embodiment eliminates the need for shims or travel limiters in the pressure vessel assembly. Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.