Patent Number: 040574660
Section: summary

BACKGROUND In well-known commercial nuclear power reactors, the reactor core is of the heterogeneous type, that is, the nuclear fuel is in the form of elongated, cladded rods. These rods or elements are grouped together and supported between upper and lower tie plates to form separately removable fuel assemblies or bundles. A sufficient number of such fuel assemblies are arranged in a matrix, approximately a right circular cylinder, to form the nuclear reactor core capable of self-sustained fission reaction. The core is submersed in a fluid, such as light water, which serves both as a coolant and as a neutron moderator. A plurality of control rods, containing neutron absorbing material, are selectively insertable among the fuel assemblies to control the reactivity, and hence the power level of operation, of the core. In some reactors, for example, the boiling water type, the power level also can be adjusted by changing the rate of coolant flow through the nuclear core. Typically, the above-mentioned fuel rods or elements comprise a sealed tube, formed of a suitable metal such as a zirconium alloy, containing a plurality of sintered pellets of an oxide of a suitable fuel, such as uranium oxide, as shown, for example, by J. L. Lass et al in U.S. Pat. No. 3,365,371. The tube, sealed by end plugs, thus serves as a cladding to isolate the nuclear fuel from the moderator-coolant and to prevent the release of fission products. The tubular fuel rod cladding, which may be of the order of 0.032 inches in thickness, is subjected to severe service because of the high pressure, high temperature and nuclear radiation in the environment of the nuclear reactor core. Fuel elements of the type under discussion, in general, have given reliable performance. However, some fuel elements failures have occurred for several reasons. (The term "failure" is herein meant to indicate that the fuel rod cladding has developed one or more openings, cracks or holes which permit escape of fission products from the fuel element into the surrounding coolant.) One type of failure that has been observed is characterized by longitudinal, brittle splits or cracks in the cladding generally occurring adjacent fuel pellet interfaces or adjacent cracks in the pellets. It is presently believed that such failures are primarily caused by mechanical interaction between the fuel pellets and the cladding during certain conditions of fuel operation!. Thus, this type of failure is designated pellet-cladding interaction failure. More specifically and as presently understood, the circumstances of likely pellet-cladding interaction failure are briefly as follows: During exposure (burnup) in the fuel, the fuel pellets expand or swell. The pellets also become distorted in shape. In particular the pellets tend to take on an "hour-glass" shape as opposed to their original cylindrical shape. In other words, the pellet tend to expand more at their ends than at their centers. Additionally the end surfaces of the pellets tend to become convex with the result that adjacent pellet edges move away from one another. Irradiation also lowers the ductility of the cladding. Thus a sudden large change in the power level of irradiated fuel can cause relatively rapid swelling of the fuel pellets against the cladding. If the expanding, separating edges of adjacent pellets (or adjacent sides of a pellet crack) lock against the cladding, the resulting localized strain may exceed the ultimate strain of the embrittled cladding with resulting cracking to produce the pellet-cladding interaction failure. It is obviously highly desirable to eliminate or at least to minimize the incidence of such pellet-cladding interaction failures. SUMMARY An object of the present invention is to provide a method of conditioning the fuel in a reactor core to a predetermined maximum power level of operation so that subsequent relatively rapid changes in power level (particularly power increases) below and up to this maximum power level can be made with minimum risk of pellet-cladding interaction fuel rod failures. This and other objectives are accomplished by taking advantage of the discovery that reactor fuel can be conditioned for subsequent high power operating level changes by a method of systematically increasing the local power of the fuel in the high power range (that is, within the power range of fuel pellet-cladding interaction) at or below a discovered critical rate. It is found that such power increase, at or below the discovered critical rate, provides a gradual, long-term, accommodation between the cladding and the fuel pellets, in response to the stresses created by expanding fuel pellets, without cladding failure. By "long-term" is meant that the accommodation persists for a significant period of time, perhaps not indefinitely-depending on operating conditions, but for at least a period of time sufficient for practical application of the conditioning method in commercial nuclear reactor core operation.