Patent Application
20130114780
1. Field
This invention pertains generally to a pressurized water nuclear reactor fuel assembly and, more particularly, to hermetically sealed rods housing a reactive material that are employed with such a fuel assembly.
2. Description of 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 from and in heat exchange relationship with a secondary circuit 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,
An exemplary reactor design is shown in more detail in
The upper internals 26 can be supported from the vessel or the vessel head and include an upper support assembly 46. Loads are transmitted between the upper support assembly 46 and the upper core plate 40, primarily by a plurality of support columns 48. A support column is aligned above a selected fuel assembly 22 and perforations 42 in the upper core plate 40.
Rectilinearly moveable control rods 28, which typically include a drive shaft 50 and a spider assembly 52 of neutron poison rods, are guided through the upper internals 26 and into aligned fuel assemblies 22 by control rod guide tubes 54. The guide tubes are fixedly joined to the upper support assembly 46 and the top of the upper core plate 40. The support column 48 arrangement assists in retarding guide tube deformation under accident conditions which could detrimentally affect control rod insertion capability.
The fuel assembly 22 further includes a plurality of transverse grids 64 axially spaced along and mounted to the guide thimbles 84 and an organized array of elongated fuel rods 66 transversely spaced and supported by the grids 64. Also, the fuel assembly 22, as shown in
As mentioned above, the fuel rods 66 in the array thereof in the assembly 22 are held in spaced relationship with one another by the grids 64 spaced along the fuel assembly length. Each fuel rod 66 includes a plurality of nuclear fuel pellets 70 and is closed at its opposite ends by upper and lower end plugs 72 and 74. The pellets 70 are maintained in a stack by a plenum spring 76 disposed between the upper end plug 72 and the top of the pellet stack. The fuel pellets 70, composed of fissile material, are responsible for creating the reactive power of the nuclear 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 rods 78 are reciprocably moveable in the guide thimbles 84 located at predetermined positions in the fuel assembly 22. Specifically, a rod cluster control mechanism 80, positioned above the top nozzle 62 in selected fuel assemblies, supports a plurality of control rods 78. The control mechanism has an internally threaded cylindrical hub member 82 with a plurality of radially extending flukes or arms 52 that form the spider previously noted with regard to
Accordingly, a new rod design is desired that will overcome this manufacturing difficulty.
The foregoing object is achieved employing a new rod design formed from an elongated tubular cladding having a hollow interior with an axial dimension. A lower end plug closes off one end of the tubular cladding and an upper end plug closes off another end of the tubular cladding. A column of reactive elements occupies a lower portion of the hollow interior of the tubular cladding above the lower end plug and the upper portion of the hollow interior of the tubular cladding forms a gas plenum. A spring extends substantially between a top of the column of reactive elements and the upper end plug and a spacer is situated between the spring and the upper end plug. Preferably, an upper end of the spring that rests against a lower portion of a surface of the spacer is beveled and the thickness of the spacer above the lower portion of the surface of the spacer is greater than the thickness of the beveled upper end of the spring. In one embodiment, the spacer is a sleeve that is encircled by at least a portion of the spring with the sleeve having a radially outwardly extending lip just above an upper end of the spring. Desirably, the lip is approximately between 0.015 inch (0.038 cm) and 0.030 inch (0.076 cm) thick. In still another embodiment, a lower portion of the sleeve is bulged out against the spring and preferably the spring is a coil spring having approximately four closed coils encircling the sleeve. In one embodiment, the rod is a nuclear fuel element and the reactive element is a fissile material. In another embodiment, the rod is a nuclear control rod and the reactive element is a neutron absorbing material. In each case, the spacer insulates the spring against at least a portion of heat applied to the upper end plug. Preferably, the spacer is constructed from a metal selected from the group of materials consisting of zirconium, zirconium alloys, titanium alloys, niobium alloys and chromium alloys and more preferably from one or more of the latter three alloys. The invention also contemplates a nuclear fuel assembly having such a rod.
A further understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which:
The inventions claimed hereafter provide a new core component for a nuclear reactor and, more particularly, an improved fuel rod or control rod. As previously mentioned with respect to
The fuel rod cladding and end plugs are constructed from zirconium alloys and the plenum spring is typically made from 300 stainless steel. The end plugs are sealed to the cladding with a tungsten inert gas girth weld. Experience has shown that if the welding parameters are not set to a low enough temperature in welding the upper end plug some melting of the end of the spring that rests against the upper end plug will occur. If the spring starts to melt it can form a zirconium/iron and zirconium/nickel eutectics that have lower melting points and an affinity for hydrogen. The presence of eutectic and hydriding can adversely affect clad integrity which has to be avoided so that the fission gases are not released into the surrounding environment.
In accordance with one embodiment of the inventions claimed hereafter an upper surface 86 of the plenum spring 76 is feathered or beveled to spread the contact area on the underside of a radially extending lip 90 of a spacer sleeve 88 around which an upper portion 92 of the plenum spring is closely wound. By being closely wound it's meant that the spacing between the coils of the spring around the spacer sleeve 88 is more closely packed than the spacing of the spring coils below the spacer sleeve 88. The spacer sleeve 88 is preferably constructed from a material preferably from the group of metals comprising zirconium, zirconium alloys, titanium alloys, niobium alloys and chromium alloys and most preferably from one or more of the latter three alloys. The spacer sleeve 88 is also preferably thin walled, e.g., between 0.015 inch (0.038 cm) and 0.030 inch (0.076 cm) thick and more preferably between 0.015 inch (0.038 cm) and 0.020 inch (0.051 cm) thick. The sleeve is thin walled to maximize the volume available for the collection of fission by-products and makes it easier to bulge the spacer sleeve into the coils of the plenum spring to secure the coils in close proximity on the spacer sleeve 88. The spacer sleeve 88 presents a heat transfer barrier and reduces the risk of spring melting with the hot end plug and formation of eutectic which can give rise to fuel rod failures. The radially extending lip 90 is inserted between the top of the spring and upper end plug 72 and prevents contact between the top end plug and the spring and the tubular portion of the spacer sleeve helps to dissipate the heat transmitted by the end plug. The lip 90 is thicker than the spring coil feathered end and will resist melting better than the spring, in that the feathered end of the coil tapers down to between approximately 0.002 inches (0.005 cm) and 0.005 inches (0.013 cm) and melts very easily. Without the spacer sleeve insert 88, the weld heat input and temperatures have to be limited which reduces the weld parameter range which does not allow the weld process to be as robust as otherwise possible. If the spacer sleeve 88 is made from a zirconium alloy, some protection against eutectic formation is provided. If the spacer sleeve is made of a titanium alloy, niobium alloy or chromium alloy, no eutectic formation should occur even at high weld heat.
While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular embodiments disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.
