Patent Number: 043476228
Section: summary

BACKGROUND OF THE INVENTION This invention relates to a method for surveillance of the nuclear fuel in fuel elements and fuel bundles to detect any diversion of the nuclear material therefrom. Typically nuclear fuel, such as uranium or plutonium oxide, is in the form of pellets or powder contained in a suitable container such as an elongated cladding tube sealed by end plugs to form a fuel element as shown, for example, in U.S. Pat. No. 3,378,458. As typically used in a nuclear reactor core, a number of fuel elements are supported in spaced array between upper and lower tie plates to form a separately replaceable fuel assembly or bundle as shown, for example, in U.S. Pat. No. 3,689,358. A sufficient number of such fuel bundles are arranged in a matrix, approximating a right circular cylinder, to form the nuclear reactor core capable of self-sustained fission reaction. Periodically the core is refueled by replacement of some of the fuel bundles to restore the necessary reactivity. Thus the fuel bundle is the normal unit of fuel material transfer and use throughout the fuel cycle. That is, the fuel elements are assembled into bundles at the fuel fabrication factory. The bundles are shipped to the reactor and placed in the core. Eventually the bundles are removed from the core and stored as such or are shipped to a reprocessing plant. An object of this invention is to provide a method and apparatus for obtaining a unique indication or signature of individual fuel elements and individual fuel bundles at any point in the fuel cycle to assure that the fuel material therein has not been removed or otherwise tampered with. Another object is to provide nondestructive surveillance of nuclear fuel elements and bundles. SUMMARY This invention is based upon the known fact that nuclear fuel contains varying amounts (typically in the order of several hundred parts per million) of tramp ferromagnetic particles, particularly particulate iron, primarily from oxidative corrosion and abrasion of the fuel processing equipment. This invention is based further on the recognition that the ferromagnetic particle distribution (that is, the sizes of the particles, the amount or number of particles and the location of the particles) is random. Hence the ferromagnetic particle distribution is unique for each fuel element and for each bundle of fuel elements. These randomly distributed ferromagnetic particles cause changes in magnetic susceptibility proportional to the changes in the ferromagnetic particle content as the fuel element or fuel bundle is passed through a constant or direct current magnetic field. If desired, known amounts of ferro or paramagnetic material could be added at random or at known positions in the fuel material. This added magnetic material could be used to augment the tramp magnetic material and, especially if placed in known positions could be used to provide, for example, type identification of the fuel material. Thus in accordance with the invention the fuel element or fuel bundle is passed through a sensing coil in a constant magnetic field and the signals produced by the sensing coil, due to changes in magnetic susceptibility caused by the changing ferromagnetic particle content, are recorded to provide a unique signature of the particular fuel element or fuel bundle. At any subsequent time the particular fuel element or fuel bundle similarly can be scanned again whereby the subsequent signature thus obtained can be compared to the originally recorded signature to determine whether or not any fuel material in such fuel element or fuel bundle has been removed or otherwise tampered with. If the fuel material contains an additive, such as a burnable neutron absorber, with high paramagnetic susceptibility and in varying amounts along the length of the fuel element or fuel bundle, the contribution of such paramagnetic material to the signature signal can be determined and separated from the contribution of the ferromagnetic material. The fuel element or fuel bundle is passed through two different constant magnetic fields of different strengths and the differential susceptibility changes in the two different magnetic fields is determined. For some applications it may be desirable to obtain and record the signature signals of both the ferromagnetic and paramagnetic material. The fuel surveillance method of the invention provides the outstanding advantage of requiring no changes in the fuel element or fuel bundle design or composition.