Patent Number: 043449140
Section: summary

BACKGROUND OF THE INVENTION This invention relates to nuclear reactors and more particularly to apparatus for fastening fuel pins to the attachment rails which support the fuel elements of a reactor. The upright parallel fuel pins which contain the fissionable material at the core of a reactor are commonly supported through bottom end members which are engaged on transverse attachment rails. More particularly, the attachment rails are typically of circular cross section and extend along the upper edges of upstanding support plates of less thickness. The fuel pin lower end member has an upwardly extending slot with a width similar to the thickness of the upstanding plate and which terminates at a transverse passage of circular cross section through which the rail extends. The lower ends of such fuel pins are thus locked to the rails and the pins cannot be removed by vertical forces. An annular housing encircles the fuel pin bundle to prevent lateral displacement of the pins. It is not possible to disengage a particular individual fuel pin of the kind described above from the rail and then remove it from the core except through an exceedingly complex, time consuming and costly sequence of operations. Removal of such a fuel pin requires that it be moved sidewardly off the end of the associated rail. If it is one of the interior pins, this cannot be done without first removing others of the pins that are attached to the same rail. Space constrictions and the typically high level of radioactivity do not permit such operations to be performed at the core region of most reactors. As a practical matter, retrieval of a particular individual fuel pin of the above described type from a reactor requires removal of the fuel pin assembly as a whole after which the above described operations must still be performed using, at least in most cases, complex remotely controlled tools. The problems discussed above in connection with removal of a fuel pin from the reactor core are, for essentially similar reasons, also encountered where an individual fuel pin is to be installed into the reactor core. It is highly advantageous, at least in some reactor operations, if one or more specific fuel pins can be more simply and conveniently retrieved after a period of reactor operation and if the same pin or a replacement can be reinstalled without an extensive disassembly of the reactor structure. Among other advantages, this capability greatly facilitates analysis of the chemical and physical changes which occur in fuel pins in the course of reactor operation. A retrievable fuel pin cannot be locked to the attachment rail in the manner described above. To enable retrievability, a typical prior practice has been to provide a slotted lower end member which fits onto the rail but which offers no significant resistance to upward withdrawal of the fuel pin. A stop plate is situated above the fuel pin assembly. To hold the retrievable pin down, it has been made more lengthy than the other fuel pins so that the top of the retrievable pin protrudes above the level of the tops of the other pins to almost contact the stop plate. Among other disadvantages, this arrangement does not provide a desirable amount of tolerance for axial growth of the retrievable fuel pin. It would be preferable to provide for fastening of a retrievable fuel pin at the lower end rather than at the top. SUMMARY OF THE INVENTION Accordingly it is an object of this invention to provide for the fastening or retrievable fuel pins to attachment rails within nuclear reactors. It is another object of this invention to provide an end member construction for fastening fuel pins to attachment rails within a nuclear reactor which enables the withdrawal of the fuel pins, by exertion of axial forces thereon of a predetermined magnitude. It is still another object of the invention to provide for the fastening of an end of a fuel pin to an attachment rail by pushing on the pin and for the retrieval of the pin by pulling thereon including providing for self piloting of the pin onto the rail in the event of an initial misalignment or misorientation of the pin relative to the rail. It is still a further object of the invention to provide for the holding down of retrievable fuel pins without restricting axial growth of the fuel pins from thermal expansion, void swelling or other causes. It is another object of the invention to provide a retrievable fuel pin which need not necessarily extend above the level of other fuel pins in a reactor. 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. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. To achieve the foregoing and other objects and in accordance with the purpose of the present invention, as embodied and broadly described herein, a retrievable nuclear reactor fuel pin has an end member for fastening the fuel pin to a transversely extending rail, the end member having a pair of blades defining a slot for receiving the rail. The end member is formed at least in part of resilient material and the slot has a configuration which requires wedging apart of the blades by the rail when the fuel pin is pulled away from the rail. In a further aspect of the present invention, in accordance with its objects and purposes, wherein the rail has a rounded surface of predetermined diameter which is received in the slot of the fuel pin end member, the facing surfaces of the blades which define the slot have a configuration and spacing which causes one region of the slot to have a width smaller than the predetermined diameter and which causes a subsequent region of the slot to have a wider curved profile substantially conforming to the rounded surface of the rail. In another aspect of the invention, the ends of the blades are pointed and the facing surfaces of the blades which define the outer end region of the slot are angled relative to each other to cause the outer end region of the slot to be of progressively greater width towards the ends of the blades. In still another aspect of the invention, the blades extend from an end plug portion secured to the fuel pin and forming an end closure thereof, the end member including the blades and end plug portion being a single integral element formed of the resilient material. In a further aspect of the invention the resilient material establishes spring forces which prevent withdrawal of the fuel pin from the rail except by an axial force having a magnitude exceeding the maximum axial forces which may be exerted on the fuel pin during operation of the reactor. The present invention fastens a fuel pin to an attachment rail within a reactor while enabling installation and retrieval of the pin by axially directed pushing or pulling forces of at least a predetermined magnitude. More complex operations, such as removal of other pins, sideward sliding movement along the attachment rail, or the like, are not required in order to unfasten and retrieve a particular pin. The invention does not require any specialized hold down structure at the top of the fuel pin and does not obstruct axial growth of the fuel pin from thermal expansion, void swelling or other causes. During installation of the pin, angular misorientations with the rail and small lateral misalignments are self corrected. In one preferred form, the end member which engages the rail may be an economically manufactured single integral element secured to the end of the fuel pin and which also serves as an end closure therefor.