Nuclear fuel assembly hold down spring

A nuclear fuel assembly having a plurality of multi-leaf hold down spring sets extending from a top nozzle. Each spring set consists of a multiple number of springs leafs in order to provide a large working range of spring deflection. Each spring leaf has a straight, flat base section followed by a straight, flat tapered beam with a secondary spring set having a curvature at its peripheral end.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains generally to a nuclear reactor fuel assembly and more particularly to an improved hold down spring on the top nozzle of the fuel assembly.

2. Related Art

The primary side of nuclear reactor power generating systems which are cooled with water under pressure comprises a closed circuit which is isolated and in heat exchange relationship with the secondary side for the production of useful energy. The primary side comprises the reactor vessel enclosing a core internal structure that supports a plurality of fuel assemblies containing fissile material, the primary circuit within heat exchange steam generators, the inner volume of a pressurizer, pumps and pipes for circulating pressurized water; the pipes connecting each of the steam generators and pumps to the reactor vessel independently. Each of the parts of the primary side comprising a steam generator, a pump and a system of pipes which are connected to the vessel form a loop of the primary side.

For the purpose of illustration,FIG. 1shows a simplified nuclear reactor primary system, including a generally cylindrical reactor pressure vessel10having a closure head12(also shown inFIG. 2), enclosing a nuclear core14. A liquid reactor coolant, such as water is pumped into the vessel10by pump16through the core14where heat energy is absorbed and is discharged to a heat exchanger18, typically referred to as a steam generator in which heat is transferred to a utilization circuit (not shown), such as a steam driven turbine generator. The reactor coolant is then returned to the pump16, completing the primary loop. Typically, a plurality of the above described loops are connected to a single reactor vessel10by reactor coolant piping20.

An exemplary reactor design is shown in more detail inFIG. 2. In addition to the core14comprised of a plurality of parallel, vertical, co-extending fuel assemblies22, for purposes of this description, the other vessel internal structure can be divided into the lower internals24and the upper internals26. In conventional designs, the lower internals' function is to support, align and guide core components and instrumentation as well as direct flow within the vessel. The upper internals restrain or provide a secondary restraint for the fuel assemblies22(only two of which are shown for simplicity in this figure), and support and guide instrumentation and components, such as control rods28. In the exemplary reactor shown inFIG. 2, coolant enters the reactor vessel10through one or more inlet nozzles30, flows down through an annulus between the vessel and the core barrel32, is turned 180° in a lower plenum34, passes upwardly through a lower support plate37and a lower core plate36upon which the fuel assemblies22are seated and through and about the assemblies. In some designs, the lower support plate37and the lower core plate36are replaced by a single structure, the lower core support plate, at the same elevation as37. The coolant flow through the core and surrounding area38is typically large on the order of 400,000 gallons per minute at a velocity of approximately 20 feet per second. The resulting pressure drop and frictional forces tend to cause the fuel assemblies to rise, which movement is restrained by the upper internals, including a circular upper core plate40. Coolant exiting core14flows along the underside of the upper core plate40and upwardly through a plurality of perforations42. The coolant then flows upwardly and radially to one or more outlet nozzles44.

The upper internals26can be supported from the vessel or the vessel head and include an upper support assembly46. Loads are transmitted between the upper support assembly46and the upper core plate40, primarily by a plurality of support columns48. A support column is aligned above a selected fuel assembly22and perforations42in the upper core plates40.

The rectilinearly movable control rods28typically include a drive shaft50and a spider assembly52of neutron poison rods28that are guided through the upper internals26and into aligned fuel assemblies22by control rod guide tubes54. The guide tubes are fixedly joined to the upper support assembly46and connected to the top of the upper core plate40.

FIG. 3is an elevational view, represented in vertically shortened form, of a typical fuel assembly being generally designated by reference character22. The fuel assembly22is of the type used in a pressured water reactor and has a structural skeleton which, at its lower end, includes a bottom nozzle58sometimes referred to as the lower end fitting. The bottom nozzle58supports the fuel assembly22on a lower core support plate60in the core region of the nuclear reactor (the lower core support plate60is represented by reference character36inFIG. 2). In addition to the bottom nozzle58, the structural skeleton of the fuel assembly22also includes a top nozzle62(sometimes referred to as the upper end fitting or top end fitting) at its upper end and a number of guide tubes or thimbles54(also referred to as guide tubes), which extend longitudinally between the bottom and top nozzles58and62and at opposite ends are rigidly attached thereto.

The fuel assembly22further includes a plurality of transverse grids64axially spaced along and mounted to the guide thimbles54and an organized array of elongated fuel rods66transversely spaced and supported by the grids64. Although it cannot be seen inFIG. 3, the grids64are conventionally formed from orthogonal straps that are interleaved in an egg-crate pattern with the adjacent interface of four straps defining approximately square support cells through which the fuel rods66are supported in a transversely spaced relationship with each other. In many conventional designs, springs and dimples are stamped into the opposing walls of the straps that form the support cells. The springs and dimples extend radially into the support cells and capture the fuel rods therebetween; exerting pressure on the fuel rods cladding to hold the rods in position. Also, the assembly22has an instrumentation tube68located in the center thereof that extends between and is mounted to the bottom and top nozzles58and62. With such an arrangement of parts, fuel assembly22forms an integral unit capable of being conveniently handled without damaging the assembly of parts.

As mentioned above, the fuel rods66in the array thereof in the assembly22are held in spaced relationship with one another by the grids64spaced along the fuel assembly length. Each fuel rod66includes a plurality of nuclear fuel pellets70and is closed at its opposite ends by upper and lower end plugs72and74. The pellets70are maintained in a stack by a plenum spring76disposed between the upper end plug72and the top of the pellet stack. The fuel pellets70, composed of fissile material, are responsible for creating the reactive power of the reactor. The cladding which surrounds the pellets functions as a barrier to prevent the fission by-products from entering the coolant and further contaminating the reactor system.

To control the fission process, a number of control rods78are reciprocably movable in the guide thimbles54located at predetermined positions in the fuel assembly22. Specifically, a rod cluster control mechanism80positioned above the top nozzle62supports the control rods78. The control mechanism has an internally threaded cylindrical hub member82with a plurality of radially extending flukes or arms52. Each arm52is interconnected to the control rods78such that the control rod mechanism80is operable to move the control rods vertically in the guide thimbles54to thereby control the fission process in the fuel assembly22, under the motive power of control rod drive shafts50which are coupled to the control rod hubs80, all in a well known manner.

As previously mentioned, the fuel assemblies are subject to hydraulic forces that exceed the weight of the fuel rods and thereby exert significant forces on the fuel rods and the fuel assemblies. These forces are countered by a combination of the weight of the fuel assemblies22and a plurality of hold down spring assemblies56on the top nozzles62which push against the upper core plate40(FIG. 2) of the reactor. The hold down spring assemblies56thereby prevent the force of the upward coolant flow from lifting the fuel assemblies into damaging contact with the upper core plate, while allowing for changes in fuel assembly length due to core-induced thermal expansion and radiation growth. Operating experience has shown that these hold down springs can be subject to stress corrosion cracking which can reduce their effectiveness.

Accordingly, a new hold down arrangement is desired that will maintain its resiliency over extended fuel cycles. Furthermore, a new hold down assembly is desired that will be more resistant to stress corrosion cracking.

SUMMARY OF THE INVENTION

These and other objects are achieved by an improved fuel assembly having a top end fitting and a bottom end fitting connected together by a structural assembly having an axial dimension that extends from the bottom end fitting to the top end fitting, with the top end fitting having a hold down spring assembly projecting above an upper surface of the top end fitting. The hold down spring has a primary spring member extending above the top end fitting, that includes a first straight leg portion having one end attached to a frame of the top end fitting at an acute angle to a plane orthogonal to the axial dimension of the fuel assembly, with the acute angle being greater than 0°. An arcuate transition portion extends at the other end of the first straight leg with a straight second leg portion extending from the transition portion toward the frame at an acute included angle with the first leg. The primary spring member is oriented on the top end fitting so that the transition portion is at the vertical highest elevation, whereby movement of the end fitting and an upper plate of the reactor that the nuclear fuel assembly is designed to operate in, relatively towards each other, primarily loads the transition portion and deflects the first leg portion about the attachment to the end fitting frame. The hold down spring assembly also includes at least one secondary spring that has a first and second end. The first end is attached to the top end fitting adjacent the first end of the primary spring first leg. The second end terminates adjacent the transition portion and includes means for interacting with the transition portion to resist downward movement of the transition portion as the primary spring member deflects in a cantilever fashion.

Preferably, the end of the primary spring that is attached to the frame of the top end fitting is supported in a slot in the frame that extends substantially at the acute angle. Desirably, the spring is clamped on a first portion of the surface on the frame that extends substantially at the acute angle where a periphery of the first portion of the surface of the frame under the primary spring is radiused to transition to a second portion of the surface of the frame under the primary spring that extends substantially parallel to the plane orthogonal to the axial dimension.

In still another embodiment, the at least one secondary spring above has a substantially flat leg that extends from the first end to an intermediate portion near the second end where the flat leg is radiused in the direction of the frame. Preferably, the radiused intermediate portion is curved at between 10° and 70°.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As previously indicated, the hold down spring assemblies56shown inFIG. 3are important structural members for a nuclear fuel assembly. Several leafs are assembled together to form a spring set in order to provide the needed hold down force to the fuel assembly to counteract the upward lift forces due to the hydraulic flow and to permit fuel assembly growth due to differential thermal expansion and irradiation dosage during normal plant operation.

Conventional hold down springs56are mounted on the top fuel nozzles62and are retained by a pin60located at diametrically opposite corners of the top plate20as shown inFIG. 3. Typically, the top nozzle62supports four spring sets56as illustrated inFIG. 4. Each spring set has a primary spring84and at least one secondary spring86with two secondary springs being shown inFIG. 3and three being shown inFIG. 4. In accordance with the prior art, spring leafs84and86have a flat, horizontal base88that is secured against the top plate20of the top nozzle62by a pin60. The leafs then curve upward away from the top nozzle20with the primary spring member84having a first flat leg90that extends at an acute angle, greater than 0°, with the top plate20to an arcuate transition portion92at the other end of the first leg90. A straight second leg portion94extends from the transition portion92toward the frame of the top nozzle62at an acute included angle with the first leg90. The secondary spring leafs86of this prior art embodiment have a short flat section that corresponds to the flat spring base88of the primary spring and then curve upward under the primary spring, extending over a straight portion under the primary spring and terminating adjacent the transition portion92at a second end, with the second ends of the secondary leafs86interacting with the transition portion92of the primary spring84to resist downward movement of the transition portion92as the primary spring84deflects downward in a cantilevered fashion. The second leg94of the primary spring extends through an opening in the secondary spring leafs86to the top nozzle frame62where it interacts with a stop that is not shown.

Fuel assemblies22are installed vertically in the reactor core14and stood upright on the lower core plate60(36). As can be appreciated fromFIG. 2, after the fuel assemblies are set in place, the upper support structure26is installed. The upper core plate40then bears down against the hold down springs56on the top nozzle62of each fuel assembly22to hold the fuel assemblies in place. The springs are generally made of nickel-chromium-iron alloy718. The retaining pin60, which holds the spring set in place, can be either threaded into the top nozzle or welded to prevent loosening while in service.

The improvement of this invention is illustrated inFIG. 5. Like reference characters are used for corresponding components of the spring56and top nozzle62, though it should be appreciated that the design of the individual components will deviate from the corresponding components of the prior art illustrated inFIGS. 3 and 4, as hereafter described. In accordance with this invention, the spring base88of each leaf, i.e., the primary spring members and secondary spring members, are formed from a short section of straight, flat beam followed by a long straight, flat beam90whose thickness is tapered, extending in a direction along the leafs away from the base88. The beams88and90thus form one continuous flat leg. Other than the top primary spring84, there is a slight bend96at the end portion of the straight secondary beams86. Since the spring set56is a cantilevered structural system, the maximum bending moment and stretch occur at the support end98. From the flexure loading of a straight beam, the absolute magnitude of strain or stress on the inner and outer fibers are equal. However, as for the curved base of a conventional spring design, the absolute magnitude of strain or stress on the inner and outer fibers are not equal due to a curvature effect which can be appreciated from the graphical representation of the strain distribution for the prior art leaf spring design shown inFIG. 7and the strain distribution of the leaf spring design of this invention illustrated inFIG. 8. This analysis assumes an elastic-plastic deflection up to the operation condition. Based on the same loading analysis, the maximum absolute strain for the straight (flat) end spring design is equally distributed on the inner and outer fibers. The maximum strains are 0.014247 and 0.010104, respectively for the curved base of the prior art and the straight base design of this invention. This means the maximum strain is reduced by approximately 29% for the straight base design.

FIG. 6provides another view of the spring design illustrated inFIG. 5taken from another angle. The sections88and90shown inFIGS. 5 and 6provide a straight, flat beam leaf spring set that extends from a slanted slot100in the top nozzle. The straight beams extend until the transition portion92in the primary spring leaf and the slightly curved sections96in the second end of the secondary spring leafs. The slanted slot100extends at an acute angle, greater than 0°, to a plane orthogonal to the longitudinal axis of the fuel assembly. The bends96are radiused at between 10° and 70°. Similarly, the lower lip102of the slot100is similarly radiused between 10° and 70°. In other respects, the top nozzle62is similar to that shown inFIG. 4.