Non-destructive testing of nuclear fuel rods

#The present invention relates to non-destructively testing nuclear fuel rods and particularly to determining that nuclear fuel rods have been manufactured in strict accordance with engineering specifications.

#A nuclear fuel rod is comprised of a column of fuel pellets sealed in an elongated cladding tube formed of a zirconium alloy. The several hundred fuel pellets are typically in the form of sintered ceramic tablets approximately one centimeter in both diameter and length. A typical nuclear fuel for use in water cooled and moderated power reactors comprises uranium dioxide (UO.sub.2) in which from about 0.7 to 5.0 percent by weight is fissile U-235 mixed with fertile U-238. The pellet column is maintained compacted by a spring acting in a tube plenum chamber which also contains a suitable getter. The fuel rods are arranged in groups and supported as separately replaceable fuel assemblies in the reactor core. A typical fuel assembly is shown in U.S. Pat. No. 3,689,358.

#To decrease local power peaking and to improve power distribution throughout the fuel assembly, it is common practice to vary the U-235 pellet enrichment concentration along the length of the rod. This is typically done with respect to groups or zones of pellets of various lengths, and a fuel rod may have as many as seven such pellet zones of varying enrichment concentrations within the range of 0.7 to 5 percent. It is also common practice to include in a fuel assembly a number of fuel rods containing a strong neutron absorbing material or "burnable poison", such as gadolinium. By appropriately locating predetermined concentrations of this burnable poison within a fuel assembly, enhanced control of the nuclear reaction is afforded, and thus less reactor control by the mechanical positionings of neutron absorbing control rods is required. Reactor shutdown safety margin is also improved. Moreover, the power distribution profile of a fuel assembly can be more efficiently optimized and maintained over a longer operating life. Typically, a neutron absorbing poison, such as gadolinium oxide (gadolinia) is mixed in with uranium oxide (urania) in concentrations up to 5 percent by weight prior to sintering into pellet form. As with enrichment, gadolinia concentration varies from pellet zone to pellet zone within a fuel rod.

#For a fuller understanding of the nature and objects of the invention, reference may be had to the following detailed description taken in conjunction with the accompanying drawings, in which:

#FIG. 1 is a schematic diagram of a system for performing non-destructive inspections of gadolinia and pure urania nuclear fuel rods constructed in accordance with the present invention;

#FIG. 2 is a diagram of a computer program for processing test data results obtained by the system of FIG. 1 in accordance with the method of the present invention; and

#An exemplary non-destructive testing system for performing the method of the present invention is illustrated in FIG. 1 as including an input queue 10 on which a plurality of nuclear fuel rods 12 are loaded. Each rod is comprised of a column of fuel pellets sealed in a zirconium alloy cladding tube which may be 1.27 centimeters in outside diameter and 4.2 meters long. Each fuel pellet is one centimeter in both diameter and length and comprises uranium dioxide or urania (UO.sub.2) compressed and sintered into a ceramic pellet shape. The urania consists of fertile U-235 and currently from about 0.7 to 5.0 percent by weight fissile U-235. However, in some of the nuclear fuel rods, the pellets also include additive amounts of a strong neutron absorbing material or burnable poison, such an gadolinia, in varying concentrations currently up to 5.0 percent by weight. The pellet column is maintained compacted by a spring (not shown) acting in a tube plenum section which also contains a suitable getter. In the case of pure urania or non-poisoned fuel rods, the pellets are arranged along the tubes length in groups or zones of specified varying lengths, with each zone typically comprised of pellets containing uniform concentrations of U-235 enrichment. The enrichment varies from zone to zone in accordance with engineering specifications. In the case of poisoned or gadolinia-bearing fuel rods, the pellets are also arranged in zones of uniform enrichment and gadolinia content, with either or both varying from zone to zone, all according to specification.

#Since the system must handle gadolinia-containing fuel rods and pure urania fuel rods separately, the rods 12 are advanced on input queue 10 to a optical reader 14 where a unique serial number engraved on the head end plug of each rod is read in succession. This serial number identifies the particular fuel rod and also the engineering specifications to which it should have been fabricated. The serial numbers are read out via leads 14a to a computer 16. Since these serial numbers distinguish gadolinia rods from urania rods, the computer can control a sorter 18 to successively transfer the latter 12a to an input queue 20 and the former 12b to input queue 22. Alternatively, gadolinia rods and urania rods, which typically are fabricated at different locations, are loaded into their respective queues. However, the serial number of each rod must be entered into computer 16 in a manner such that it can be identified and tracked through its inspection procedure so that its test results can be analyzed with respect to the applicable engineering specifications stored in the computer in the form of "rod maps".

#Still referring to FIG. 1, gadolinia rods 12b are successively unloaded from their input queue 22 and fed by pinch rollers 24 at a closely regulated, uniform speed through a magnetometer 26 which is preferably of the construction disclosed in commonly assigned Grossman et al. U.S. Pat. No. 4,243,939, the disclosure of which is specifically incorporated herein by reference. Thus, this magnetometer includes a pair of superconductive electromagnets persistently energized to develop constant magnetic fields of different high field strengths (sufficient to saturate any iron impurities in the fuel pellets) through which each gadolinia rod 12b is passed in succession. The differential paramagnetic susceptibility of the material of each pellet is detected during rod movement through these two magnetic fields and is read out to computer 16 over lead 26a. The susceptibility signals, respectively corresponding to a multiplicity of data points uniformly distributed along the length of the pellet column in each rod 12b, e.g., every 0.125 inches, are compared to known standards stored in the computer to determine and retain the gadolinia concentration data at each data point.