Fuel rods for nuclear reactors

978,737. Nuclear fuel elements. EUROPEAN ATOMIC ENERGY COMMUNITY (E URATOM). June 19,1963 [June 20,1962], No. 24464/63. Heading G6C. Fuel rods for a nuclear reactor of the kind in which the rods will be subjected to external fluid pressure (e.g. a power reactor cooled by fluid under pressure) each have fuel material in the form of a rod or a stack of slugs or pellets housed within a cylindrical sheath with end closures and a heat conductive filling material between the sheath and the fuel, the sheath having a thickness and shape such that it is flexibly and inwardly deformable under the external pressure, the filling material forming a layer between the fuel and the sheath and being viscous or liquid at the operating temperature for the rod, and the fuel and filling material layer forming an assembly within the sheath which hydrostatically supports the sheath over the whole of its internal surface whereby the sheath may deform to various shapes in accordance with changes in shape of the filling material layer caused by thermal and mechanical stresses induced by the fuel. The rods may be inserted individually into the channels of the reactor core or they may be assembled in mountings to form clusters which are inserted as units into the channels. In one embodiment (Figs. 1 and 2) the sheath 1 is of stainless steel of thickness less than 0.1 mm, e.g. of the order of 0. 05 to 0. 06 mm, and is provided with longitudinal ribs 6 along its centre portion and circumferential ribs 7 at one or each end to permit radial and axial flexible deformations. A viscous layer 8 is formed by filling the annular space between the fuel pellets 2 and the sheath 1 by a metal or alloy which is plastic at the operating temperature, preferably magnesium, the minimum thickness of which is a few tenths of a millimetre, e.g. 0. 5 mm. The pellets 2 may be uranium, either metallic and preferably lightly alloys (e.g. with molybdenum, silicon or aluminium) or ceramic such as uranium dioxide or carbide. A pellet 5 of refractory material is disposed between each end closure 3 and the fuel in order to reduce heat transfer to the ends of the rod. The rod is assembled by welding the lower end closure 3 to the sheath 1 which is quickly heated in an oven to a temperature higher than the fusion point of the filling material e.g. magnesium, a suitable quantity of this material then being placed in the sheath 1 in an argon atmosphere. Once the filling material has melted, the fuel pellets 2 are inse