Patent Number: 041464311
Section: description

In FIG. 1, the fuel element 10 has an outer cylindrical sheath 12, generally of thin zirconium alloy such as that, trademarked "Zircaloy", of such thickness as to deform in peripheral and axial gripping relation with the pellets 14 under operating conditions within a reactor. Referring to FIG. 2, this shows in exaggerated fashion the manner in which the pellets 14 tend to change from their original cylindrical form to the "hourglass" shape illustrated, while the sheath 12 also conforms thereto under reactor operating conditions. It will be understood that coolant pressures in the order of 1500 psi are applied to the outside of the fuel sheath 12. Referring to FIG. 3, one type of fuel failure illustrated, which usually occurs at a site S of a pellet fracture, results in failure at F of the sheath 12, with substantially no change in cross-sectional area of the sheath 12. In the type of failure illustrated in FIG. 4, there is necking at the failure site F.sub.1, resulting in a reduction of the cross-section of sheath 12 at the failure site. FIG. 5 shows a cross section of a fuel element according to the present invention, having a bond preventing layer 16 interposed between the sheath 12 and the pellet 14. The layer 16 comprises material selected from the group consisting of siloxane, silicon and graphite, and characterized by substantial intertness under irradiation of reactor intensity, together with maintenance of high stree distributing capability at reactor operating temperatures and possessing a non-bonding nature, while being fairly readily applied to the interior of the sheath 12 or the surface of fuel pellets 14, dry or in the form of liquid or emulsion which is subsequently dried. These materials do not operate in the accepted sense as lubricants, but provide an interposed non-bonding layer to effectively reduce the tendency of the pellet and sheath to effect a peripheral hang-up. Thus the creation of minimal gauge length hoop stress conditions may be effectively countered.