Patent Number: 052157059
Section: summary

BACKGROUND OF THE INVENTION Nuclear power reactors are a well known source of energy. In one type of nuclear reactor the nuclear fuel is comprised of elongated rods formed of sealed cladding tubes of suitable material, such as a zirconium alloy, containing uranium oxide and/or plutonium oxide as the nuclear fuel. A number of these fuel rods are grouped together and contained in an open-ended tubular flow channel to form a separately removable fuel assembly or bundle. A sufficient number of these fuel bundles are arranged in a matrix, approximating a right circular cylinder, to form the nuclear reactor core capable of self-sustaining a fission reaction. The core is submerged in a fluid, such as light water, which serves both as a coolant and as a neutron moderator. A typical fuel bundle is formed by an array of spaced fuel rods supported between upper and lower tie plates; the rods typically being in excess of ten feet in length, on the order of one-half inch in diameter and spaced from one another by a fraction of an inch. To provide proper coolant flow past the fuel rods it is important to maintain the rods in precisely controlled, spaced relation such to prevent bowing and vibration during reactor operation. A plurality of fuel rod spacers are thus utilized at spaced intervals along the length of the fuel bundle for this purpose. Design considerations of such fuel rod bundle spacers include the following: retention of rod-to-rod spacing, retention of fuel bundle shape, allowance for fuel rod thermal expansion, restriction of fuel rod vibration, ease of fuel bundle assembly, minimization of contact areas between spacer and fuel rods, maintenance of structural integrity of the spacer under normal and abnormal (such as seismic) loads, minimization of reactor coolant flow distortion and restriction, maximization of thermal limits, minimization of parasitic neutron absorption, and minimization of manufacturing costs including adaptation to automated production. Commonly assigned Matzner et al. U.S. Pat. No. 4,508,679 discloses and claims a nuclear fuel rod bundle spacer uniquely constructed to address these design concerns. As disclosed therein, a spacer is formed of an array of conjoined tubular cells or ferrules surrounded by a peripheral support band, each ferrule bore thus providing a passage through which a fuel rod or other elongated element of the fuel bundle is inserted. The ferrules are spot welded together and to the peripheral support band to provide an assembly of high structural strength. The rods or elements extending through the ferrules are centered and laterally supported therein between rigid projections or stops and resilient members. The rigid projections or stops are inwardly formed as fluted or dimpled portions of the ferrule wall at locations near the upper and lower ferrule edges to maximize the axial distance therebetween and thus enhance fuel rod support. The resilient members take the form of slender continuous loop springs of generally elliptical shape held captive by oppositely directed tabs formed by C-shaped cutouts in the walls of a pair of adjacent ferrules, whereby the two sides of each spring member project into the bores of its ferrule pair. Thus, a single spring serves two ferrules in biasing the fuel rods into contact with the two axially spaced pairs of stops pursuant to centering them in the ferrule bores. To meet many of the spaces design concerns, it is important that these spacer springs exert forces within a specified range on the fuel rods, so as to maintain their contact with the stops and thus centered in the ferrule bores under all reactor operating conditions. To little spring force and the fuel rods are susceptible to excessive vibration. Excessive spring forces will impede fuel rod insertion and blemish the rod surfaces during insertion; such blemishes having the potential of fostering future corrosion. SUMMARY OF THE INVENTION It is accordingly an objective of the present invention to provide a gauge for measuring the forces exerted by nuclear fuel bundle spacer springs on the fuel rods once assembled in a fuel bundle. That is, the gauge of the invention is adapted to measure the spacer spring forces at the site of spacer manufacture before insertion of the fuel rods through their retaining ferrules. Thus the spring forces can be determined to be within engineering tolerance limits before the spacer is committed to use in a fuel bundle assembly. The measuring device is of compact size to accommodate the tight measurement-taking quarters involved and to facilitate handheld utilization. To these ends, the spacer spring force gauge of the present invention includes an elongated probe and an alignment rod mounted in parallel, appropriately spaced relation by a handle. The probe and alignment rod have effective diameters equal to the nominal diameter of a fuel rod. Thus, when the probe and alignment rod are inserted through a pair of ferrules in which the double-acting spacer spring is held captive, they simulate the presence of fuel rods. An elongated arm is pivotally mounted intermediate its ends internally of the probe in parallel relation with the probe axis. One end of the arm is acted upon by a plunger mounted for reciprocation transversely of the probe axis, while the upper arm end acts on a load cell. The face of the plunger bears against one side of the spacer spring to communicate the spring force to the load cell via the arm as the alignment rod preloads the other side of the spring from the adjacent ferrule. The electrical response of the load cell is brought out via leads routed through the handle to equipment for registering the magnitude of the fuel rod-centering force exerted by the spring. The invention accordingly comprises the features of construction, combination of elements and arrangement of parts, all as described hereinafter, and the scope of the invention will be indicated in the claims.