Patent Number: 043449140
Section: description

DETAILED DESCRIPTION OF THE INVENTION Reference will now be made in detail to the present preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings. Referring initially to FIG. 1, the core region 11 of a nuclear reactor 12 includes a plurality of fuel pins 13 of rod-like configuration which contain the fissionable material. Fuel pins 13 extend vertically in parallel side by side relationship with each other within a housing or wrapper 14 which is typically of hexagonal cross section. A flow 16 of fluid such as liquid sodium for example is directed upwardly through the bundle of fuel pins 13 to extract the thermal energy generated by the fission reaction. Fuel pins 13 are supported at the bottom ends through fuel pin end members 17 which engage on parallel attachment rails 18 that extend transversely within housing 14 below the fuel pin assembly 23, a row of the fuel pins being supported on each individual rail. Rails 18, in this example, are of circular cross section and are formed along the upper edges of a series of spaced apart, parallel, upstanding support plates 19 which are secured to housing 14, the plates having a thickness smaller than the diameter of the rounded surfaces 21 of the rails. To maintain a small spacing between adjacent fuel pins 13 and between the outermost pins and the wall of housing 14, a wire 22 is wrapped spirally around each pin with the exception of one or more specialized fuel pins to be hereinafter described. Aside from certain of the fuel pin end members 17, the reactor 12 may otherwise be of known construction and thus need not be further described. The fuel pin end members 17a and 17b in this example are of two different types. End members 17a of the first type are conventional and do not enable disengagement of the associated fuel pins 13a from rails 18 except by the cumbersome process of removing the entire fuel pin assembly 23 from housing 14 to enable sidward sliding of the fuel pin 13a off the end of the rail 18. The fuel pins 13a cannot be withdrawn from the fuel pin assembly 23 by a direct upward force as the rails 18 cannot pass through the relatively narrow slots at the lower portions of the end members 17a. Predetermined ones 13b of the fuel pins 13 have lower end members 17b embodying the present invention and constitute what is herein termed a retrievable fuel pin. The retrievable fuel pins 13b may be withdrawn directly upwardly from the associated rail 18 by an axially directed force of predetermined magnitude and the same pin or a replacement may be installed by being pushed axially downward into the assembly 23 with a sufficient force. The end member 17b construction which provides for a retrievable fuel pin 13b may be understood by reference to FIGS. 2, 3 and 4 in conjunction. The end member 17b in a preferred embodiment is constructed as a single integral element which, in addition to fastening the fuel pin 13b to an attachment rail 18, also functions as a bottom end closure of the fuel pin. End member 17b is formed of a resilient material, for reasons to be hereinafter discussed, that is nonreactive with the fluid of flow 16 and which is stable in the high temperature environment of the core region 11. In most instances spring steel is a suitable material for the end member 17b. End member 17b has an end plug portion 27 of circular cross section which fits into the lower end of the cylindrical cladding 28 in coaxial relationship therewith and which is weld sealed to the cladding. A circular flange 29 at the lower end of the plug portion facilitates such securing of the end member 17b to the other components of the fuel pin 13b. The lower portion of the end member 17b is divided into a pair of spaced apart blades 31 by a slot 32 which extends upward from the lower end of the end member to a point slightly below flange 29. At an intermediate position along slot 32, the facing surfaces 33 of the blades 31 which define the slot 32 have a circular profile conforming with the rounded surface 21 of rail 18 and which is of similar diameter to define a rail seat region 34 of the slot. Immediately below the seat region 34, the facing surfaces 33 are closer together to define lands 36 spaced apart a distance similar to the thickness of the support plate 19 of the rail 18 to define a land region 37 of the slot. Below the land region 37, the slot 32 has a slightly greater width. At the lowermost or outer region 38 of the slot 32, the facing blade surfaces 33a and 33b are divergent causing the outer end region of the slot to be of progressively greater width towards the lower ends 39 of the blades 31. The lower ends or tips 39 of the blades 31 also have convergent side surfaces 41 causing the tips of the blades to be pointed. To impart sufficient elasticity to the blades 31 a relatively narrower inner portion 42 of slot 32 extends upwardly from seat region 34 and terminates at a bore 43 located below flange 29. The foregoing description of the widths of various regions of the slot 24 in relation to the diameter of rail 18 and the thickness of rail support plate 19 should be understood to refer to conditions when the end member 17b is not stressed or under spring tension. As will be described in connection with operation of the invention, the blades 31 are temporarily forced apart and temporarily enlarge the various regions of the slot as the end member 17b is in the process of being engaged on rail 18 or is being disengaged therefrom. In operation, with reference to all figures in conjunction, the retrievable fuel pin 13b may be initially installed in the fuel pin assembly 23 in either of two ways. As the nonretrievable fuel pins 13a must be fitted onto the ends of the rails 18 and then be slid sidewardly into position before the assembly 23 as a whole is installed in housing 14, it may be convenient to install the retrievable fuel pins 13b initially in the same manner. Alternately and unlike the nonretrievable fuel pins 13a, the retrievable pins 13b may be engaged on rails 18 by a strictly downward axial movement. Provided that sufficient force is exerted, the rail 18 wedges lands 36 apart and is then received in the conforming seat region 34 of slot 32 at which point the resiliency of the material of the end member 17b snaps the blades 31 back towards each other. The retrievable fuel pin 13b is thus clamped to rail 18 by a spring clip action since withdrawal of the end member 17b requires a sizable upward force sufficient to again wedge the lands 36 apart against the resistance created by the resiliency of the end member material. To assure that the fuel pin 13b remains fastened to rail 18 during reactor operation, the blades 31 are proportioned in relation to the spring constant of the end member material and to the thickness of lands 36 to prevent upward motion of the fuel pin 13b until the upward force on the fuel pin exceeds a predetermined magnitude which is greater than the combined upward forces which the fuel pin may experience in the course of reactor operation. Such upward forces which may be experienced during reactor operation may arise from several causes including the hydraulic pressure differential between the upper and lower ends of the fuel pin 13b that results from the upward flow 16 of fluid, floatation force on the fuel pin from the fluid environment, and possible momentum forces arising from vibration or motion of the reactor as a whole. It is preferable that a sizable safety margin be provided for in fixing the pull free force required to detach the end member 17b from the rail 18. In one specific embodiment of the invention in which the maximum lifting forces that may be experienced by the pin 13b during operation of the reactor, from the causes described above, is about 1.1 lb (0.15 N), the blades 31 are proportioned to require an upward force of 10 lbs (1.4 N) before the end member 17b disengages from rail 18, this particular value being given for purposes of example only as other pull free forces may be appropriate in other embodiments of the invention. The fuel pin 13b may be retrieved, after a period of reactor operation, by gripping the upper end with grappling mechanism of known construction and pulling directly upward with sufficient force to spread the blades 31 sufficiently to allow rail 18 to pass between lands 36. The same pin 13b may be reinstalled or a replacement pin may be substituted by a downward axial movement of sufficient force. The divergent configuration of the lower end of slot 32 together with the pointed configuration of the tips 39 of blades 31 automatically corrects for angular misorientation and/or for misalignment between end member 17b and rail 18 during the process of installation of guiding the end member into the correct position, relative to the rail, for engagement. A rare exception occurs if the pointed tips 39 contact the rail 18 while in exact alignment with the rail centerline in which case the fuel pin may be backed off and turned slightly after which the installation process may proceed. It has been pointed out that the nonretrievable fuel pins 13a have a spiral wire 22 winding to assure the desired fuel pin spacing is maintained. If such a wire is present on the retrievable fuel pin 13b, it must be turned in the manner of a screw while being withdrawn and must be turned in an opposite direction while being installed to avoid interference with the wires 22 of adjacent fuel pins. To avoid this complication in instances where the retrievable fuel pin 13b is surrounded by nonretrievable fuel pins 13a, the wire 22 may simply be omitted from the retrievable fuel pin. The wires 22 of the surrounding fuel pins 13a establish the desired spacing by contact with the sides of the retrievable pin 13b. Only a small number of the fuel pins 13b are of the retrievable form in this particular reactor 12 since the purpose of the retrievability in this example is to facilitate analysis of the physical and chemical changes that occur in fuel pins over a period of time. Analysis of only a representative sample of the fuel pins is necessary for this purpose. As is apparent, any or all of the other fuel pins may be of the retrievable form in instances where that is advantageous. For example in reactor systems in which it is possible to identify a malfunctioning fuel rod from among the others without removing the entire assembly 23, the capability of removing and replacing any of the fuel pins will simplify maintenance procedures. The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously modifications and variations are possible in light of the above teaching. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modification as are suited to the particular use contemplated. It is intended that the scope of the invention is defined by the claims appended hereto.