Patent Number: 046997571
Section: description

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The fuel rod 10, a fraction of which is shown in FIG. 1, has a general construction similar to that of the fuel elements currently used in pressurized water moderated and cooled reactors (PWRs). It comprises a sheath 12, generally of zirconium base alloy, closed by plugs 14 only one of which is shown. The major part of the length of the sheath is occupied by a stack of UO.sub.2 or UO.sub.2 -PuO.sub.2 pellets 16. The fuel rod comprises means for holding the stack 16 applied against the lower plug 14 during handling and transport. In conventional fuel rods, such means comprise a helical spring placed in the chamber or "plenum" receiving the fission products, above the stack of pellets 16. The spring is compressed between the stack and the upper plug of the rod and must be of material having such a resilient force that there is no appreciable modification of the holding force despite the radiation received by the spring and the length increase of the stack when in a reactor. According to the invention, the holding means are of a type immobilized in the sheath 12 when the element is cold and substantially freely movable along the sheath when at the operating temperature which prevails in the reactor. Referring to FIGS. 1 and 2, the holding means comprise a radially expandable element 18 in the form of a thimble or diabolo in which are formed longitudinal slits spaced apart evenly angularly, cut out from the both ends and leaving a continuous central ring 20. The two sets of slits (four in number as shown) define resilient fingers which tend to spread apart until they bear against the internal surface of sheath 12. In other words, the diabolo shaped element 18 has an end diameter at rest such that the finger ends are applied against the sheath when inserted therein and oppose considerable frictional force to longitudinal movement. A washer 24 between element 18 and stack 16 forms a heat shield so that the temperature of element 18 reflects that of the cooling water rather than that of the pellets. A circumferential groove 26 is formed in each set of fingers and receives a split ring 28 of shape memory material which, at atmospheric temperature, follows the movements of the fingers and, above the transition temperature of the material, exerts a radially directed shrink force sufficient for the fingers to be inscribed in a circle of smaller diameter than the inner diameter of sheath 12. When ring 28 thus takes the initial shape which was given to it, it allows elements 18 to slide either under the upward thrust of a stack (should swelling of the pellets occur) or downwards under the action of its own weight and vibrations. However, upon cooling of the reactor after it has been shut down, the temperature of the ring drops below the transition point, the fingers spread out against the force of the ring and frictionally lock element 18, thus retaining the stack of pellets 16. As an example, the following matters may be used for the components of the holding means: Expandable element 18: "INCONEL 718" or 13/8 stainless steel. PA0 Ring 28: titanium-nickel alloy having a titanium content between 51 and 53% (whose transition temperature is about 100.degree. C.). PA0 Washer 24: alumina. Referring to FIGS. 3 and 4, the radially expandable element consists of a bellows 30 having a rotational symetry about the rod axis, whose external folds are arranged for engaging the sheath when the bellows contracts. The temperature responsive means consist of a helical spring 32 made from a shape memory material and located along the axis of the bellows 30, compressed between the end walls of the bellows. As shown in FIG. 3, the bellows is at a temperature lower than the transition point of the spring material, the external folds of the bellows are in frictional contact with the internal surface of sheath 12 and hold the stack of pellets 16 in position. During operation in a reactor, when the temperature of spring 32 is higher than the transition point, the spring expands until it resumes the shape which was initially given to it and the external folds of bellows 13 no longer engage the sheath, which leaves the stack of pellets 16 free to expand. The shape memory element 32 may as well consist of a twisted washer or any element for retracting the folds of the bellows 30 above the transition temperature. In the ebodiment shown in FIGS. 5 and 6 (where the elements identical to those if FIGS. 1 and 2 are again designated by the same reference numeral), the radially expandable element consists of a ring 34 with a C-shaped cross-section. A central groove in the ring receives a continuous ring 36 made of a shape memory material. Referring to FIG. 7, a holding device has a radially expandable element 18 and two rings 28 identical to those shown in FIGS. 1 and 2. However, element 18 is not directly in contact with washer 24. It is separated therefrom by means for accomodating the variations in length of the stack while the temperature is below the transition temperature of the material forming rings 28. The accomodation means comprise two abutment washers 38 and 40 connected together by a spring 42 extending a resilient force tending to spread apart the washers 38 and 40. A second spring 44, opf shape memory alloy, has its ends connected to the washers. The transition temperature of the alloy forming the second spring 44 is lower than that of the alloy of rings 28. For example, rings 28 may be of the abovedefined alloy while spring 44 is of titanium-nickel alloy with 45-45.5% at. of titanium. Thus, when the temperature initially increases after the fuel rod has been placed in the reactor, spring 44 exerts a tractive force which moves washers 38 and 40 towards one another, and retracts spring 42. Then, as the temperature increases further and reaches the transition temperature, rings 28 unlock the radially expandable element 18. Conversely, when the temperature decreases, after shut down of the reactor, rings 28 allow the expandable element 18 to lock onto the sheath. Then spring 44 relaxes and spring 42 applies a force on the stack of pellets and takes up the axial clearance with a resilience which compensates for the variations in length. The embodiment shown in FIGS. 8 to 12 comprises a radially expandable element formed by a split ring 50 which may again be considered as a ring having a C-shaped cross-section. Ring 50 has a diameter at rest larger than the inner diameter of the sheath and is applied resiliently against the internal wall of the sheath of the fuel element when inserted. Then it holds the fuel pellets in position due to the frictional force exerted by the sheath. The temperature responsive means comprise two coiled springs 52 and 54. The two springs are mutually coaxial and have opposite winding directions and different diameters. Each end of each spring has a lug anchored to the ring and the two ends of a spring are anchored close to the edges of the slit. Referring to FIG. 10, the end lugs 56 and 58 of the large diameter spring 52 are illustrated. When the temperature of springs 52 and 54 increases from the ambient temperature, a relative rotation of the end lugs of the springs may occur eve if they are of a material exhibiting normal thermal expansion characteristics. Additionally, the memory effect of the material, if they are of shape memory material, results in a winding action beyond the transition temperature. The material may typically be a titanium-nickel alloy having a titanium atomic content of 51 to 53%. The winding movement of the end lugs anchored in ring 50 tends to "close" this latter and to remove friction between the ring and the sheath. Ring 50 them becomes free to move along the sheath and the stack of pellets may freely expand. During cooling, the lower mechanical resistance of the memory alloy or, if the alloy has a reversible memory, cooling down below the transition temperature, allows ring 50 to resume its initial shape and to engage the sheath, thus retaining the stack of pellets.