Patent Number: 042004918
Section: summary

This invention was made under contract with or supported by the Electric Power Research Institute, Inc. of Palo Alto, Calif. BACKGROUND OF THE INVENTION I. Field of the Invention This invention relates to measuring the power distribution of a nuclear reactor fuel element. In particular, the invention relates to a method and passive apparatus for analysis of power distribution history through the measurement of residual radiation along a fuel element. In order to verify the accuracy of nuclear calculations and to determine if anomalies occurred in the power production of the nuclear fuel, it is desirable to map the recent power distribution of a reactor and of the individual nuclear fuel elements. It has been found that the deposition of certain fission products in a fuel element or rod, specifically radioactive lanthanum-140 which is a fission product of barium, does not migrate within the fuel element and thus is representative of the recent reaction history of the nuclear fuel. The reaction history is directly correlated to the most recent power production, a consequence of the relatively short half-life of the parent reactant, barium-140. Thus, measurement of the radiation emission of lanthanum-140--which must be discriminated from other residual radiation--can be used to construct an accurate map of reactor power distribution. II. Description of the Prior Art In the past, gamma radiation from fuel elements has been measured by scanning each fuel element with collimated radiation detector capable of sensing radiation and of discriminating levels of energy. Typically, a sodium iodide scintillation type sensor or a germanium-lithium solid state sensor is used to detect gamma radiation, together with appropriate electronic and signal processing apparatus. There is, however, a substantial danger of exposure of the detector operator to dangerous levels of gamma radiation during measurement of the residual radiation of spent, yet radiating, fuel elements, since the detectors have in the past been manipulated by the operator in close proximity to the fuel elements. Therefore, a shielded detector system has been a requirement of prior art systems. Water has been the usual means of shielding the fuel element to be measured, so the prior art sensors were generally adapted to operate underwater. In summary, according to prior art methods, to determine a distribution history of a fuel element, each element was removed from the reactor, immersed in a shielding medium and slowly scanned by a sensor which generated exposure data one point at a time along one fuel element at a time. This technique has required extensive correction for radiation decay to account for scan time differences at different spatial locations along the fuel element. SUMMARY OF THE INVENTION In order to overcome the disadvantages of prior art detectors, a radiation detector of the present invention spatially discriminates in situ the distributed radiation of a spent nuclear reactor fuel element. A sensor according to the invention comprises a cylindrical casing or wand which enshrouds a material that converts incident gamma radiation having an energy level exceeding a predetermined threshold to a correspondingly distributed neutron radiation field. The incident gamma radiation is identifiable with a particular short lifetime reaction product representing recent power distribution, such as lanthanum-140. The converter material, which may be beryllium or deuterium or a compound thereof, sheaths a neutron field-sensitive activant, such as gold, in the form of a filament longitudinally disposed in the casing. The activant receives, and in essence stores along its length, information on the level and distribution of incident neutron radiation by creation of radioactive isotope characterized by a long lifetime of low level radiation which can later be measured by conventional low level and relatively safe radiation detection techniques. In operation, a wand according to the invention is placed alongside a spent fuel element within a reactor core and left for several hours to a few days to fully expose the activant. The wand is thereafter removed for analysis. One of the purposes of the present invention is to provide means for analyzing the power distribution history of a nuclear reactor fuel element in order to monitor the proper and efficient operation of a nuclear reactor. The present invention also provides means for measuring fission product power distribution history without danger of exposure of operators to radiation. According to the invention, the residual radiation of spent fuel elements can be measured and recorded by an in situ detector. Thus, measurements can be made with virtually no danger of exposure to dangerous levels of radiation. Another purpose of the invention is to provide means and a method for measuring the spatial distribution of residual short lifetime gamma radiation without compensation for time-related residual radiation decay. An advantage of the present invention in this instance is that distributed short lifetime residual radiation is simultaneously measured in a manner creating a relatively persistent spatial image more readily analyzed by conventional lower level radiation detection methods. Thus, the need to correct for time-related decay differences is eliminated. Other purposes and advantages of this invention will become apparent upon reference to the following detailed description and accompanying drawing.