Patent Number: 046997571
Section: summary

BACKGROUND OF THE INVENTION The invention relates to nuclear fuel rods for use in nuclear reactors and comprising a gas tight sheath containing a stack of fuel pellets and means for holding the stack against an end plug of the sheath. The term "fuel" should be interpreted widely as designating not only materials used for producing energy by fission, but also fertile or neutron absorbing material. The fuel rods used up to present, particularly in reactors cooled by circulation of water or molten sodium, comprise a stack of oxide fuel pellets and holding means formed by a helical spring between the stack and an end plug. This solution has considerable drawbacks. The spring requires space and reduces the volume available for receiving the fission gases released by the fuel. Under irradiation, the material forming the spring is subject to relaxation which causes them to lose a fraction of their initial properties. Often, degradation during the operation in a reactor is such that the holding means no longer provide sufficient retention during fuel handling and transportation. Attempts have been made to solve this problem. French Pat. No. 2,529,371 describes a fuel rod whose holding spring is made from a material which exerts a retaining force which decreases when the temperature increases. The only advantage of this solution is that it reduces the stresses undergone by the fuel pellets during operation in the reactor. French Pat. No. 2,018,665 describes a rod in which the spring in replaced by a member in the form of a split bucket resiliently engaging the sheath. Such a member has drawbacks: it exerts a constant retention force and remains in position when the stack of pellets contracts, so that it no longer holds the stack in position during shipping. SUMMARY OF THE INVENTION It is an object of the invention to provide a nuclear fuel rod with improved holding means. It is a more specific object to provide retaining means which require less space and need not to consist of material having high resiliency for ensuring a substantially constant holding force whatever the swelling of the pellets under irradiation. For that, it uses the fact that it is essential to hold the stack only during transport and handling of the rods, since the rods are in a vertical position during operation in a reactor and the stack of pellets then rests quite naturally on the lower plug of the sheath. According to the invention, there is provided a rod of the above-defined type in which the holding means comprise a radially expansible element, of a shape at rest such that, when in the sheath and at the atmospheric temperature, it frictionally engages the inner surface of the sheath, and further comprise means responsive to the temperature for forcibly contracting said element and reducing its friction against the sheath when the temperature exceeds a predetermined value lower than the normal operating temperature in a reactor. The means responsive to the temperature are formed from a shape memory alloy having a transformation temperature intermediate between atmospheric temperature and the normal operating temperature in a reactor. Shape memory alloys date back to twenty years or so. Below a transition temperature, they can be deformed plastically. When heated above the transition temperature, they resume the shape which was given them during a metallurgical shaping treatment. Numerous alloys are known fulfilling that condition, having different transition temperatures. A description of such alloys may be found in "Some Applications of Shape Memory Alloys", Journal of Metals, June 1980, pp. 129-137. Among the alloys which may be used for implementing the invention, titanium-nickel alloys may be of interest. They transform from the austenitic phase to the martensitic phase and inversely at a temperature less than 400.degree. C., i.e. at a value lower than that reached in the plenum chamber of the fuel rods, at least in existing pressurized water reactors. Examples of such titaniumnickel alloys may be found in British Pat. No. 2,117,001. The stack of pellets is likely to shorten when the rod cools down and the temperature is below the transition temperature. Resilient means may advantageously be inserted between the radially expandable element and the end pellet of the stack, so as to accomodate possible variations in length at that time. The resilient means may comprise a washer in abutment against the stack, connected to the radially expandable element by a compression spring and by a second spring made from a shape memory alloy, having a transition temperature higher than the atmospheric temperature range and lower than that of the first spring, the second spring exerting, above its transition temperature, a retraction force greater than the force of the first one. The holding means of the invention use the fact that their function is no longer required during operation of the reactor. On the other hand, during cooling down of the reactor before fuel unloading, the reversibility of the shape memory of the alloy allows the radially expandable element to frictionally engage the sheath and to lock on the sheath. During reactor operation, since the expandable element is substantially free, it may follow the movements of the end pellet of the stack, due for example to swelling under irradiation. Welding of the end plug during manufacture is rendered easier since there is no spring in abutting relation with the plug.