Patent Number: 043354661
Section: summary

In order to comply with various safeguards agreements, inspection organizations such as NRC (Nuclear Regulatory Commission) and IAEA (International Atomic Energy Agency) need a capability of very quickly and accurately monitoring in a non-destructive manner the fissile content of spent fuel assemblies in storage pools. Presently, measurements of the content of residual and produced fissile material are not directly measured but rather are inferred by measuring particular data which is correlated to burnup (which is a measure of nuclear reactor fuel consumption, expressed either as a percent of fuel atoms that have undergone fission or as the amount of energy produced per unit weight of fuel). It is known in the art that the amounts of certain fission products which are present in a fuel assembly, such as Cs-137, .sup.144 Ce-Pr, and Ru-106, are proportional to burnup and can be used as burnup monitors. See, for example, S. T. Hsue et al., Los Alamos Scientific Report LA-6923 (ISPO-9) (1978). It is known in the art that the gross gamma activity of a spent fuel assembly depends both upon the cooling time (i.e., time measured from discharge from the reactor) and upon the intensity of various fission product gamma rays. The gamma rays from a spent fuel assembly can be divided into two categories, (1) gamma rays from direct fission products and (2) gamma rays from isotopes resulting from neutron capture on direct fission products. The number of the type (1) gamma rays is known to be proportional to the reactor neutron flux; and the number of the type (2) gamma rays is known to be approximately proportional to the square of the reactor neutron flux. However, only type (1) gamma rays have been found to be proportional to burnup. Thus, a gross gamma activity measurement of a spent fuel assembly will not in general be expected to give an accurate measurement of burnup due to the possible interference of the type (2) gamma rays, described above. In all detectors in which gross gamma activity is measured, the detector response is proportional to the sum of the gamma rays emitted, which depends upon cooling time and intensities of the fission products (which depend upon burnup and operating history). However, corrections to the data for these factors is not generally possible because the relative contribution of each factor is not known. Thus, it generally cannot be known prior to experimental determination or complicated calculations when, if ever, a gross measurement of total emitted gamma rays will agree with the true burnup. Typically, the preferred method for measuring relative burnup has been to use high resolution gamma ray spectroscopy (HRGRS) and to perform a series of measurements of the intensity of gamma rays having a particular energy at various points along the length of the fuel assembly and then to use the integrated area of that profile and an established calibration curve of calculated burnup vs. integrated Ge detector response (measuring, for example, the 661 keV gamma ray of Cs-137) to provide the corresponding burnup value. The use of a germanium detector to monitor the intensity of the 661 keV gamma ray of Cs-137 as a function of axial position along a fuel assembly provides a very accurate (2-6%) measure of relative burnup but takes a long period of time and requires a multichannel analyzer system, a mechanical scanning system, and a collimator assembly. An alternative scanning technique is to employ a cadmium telluride detector for the profile measurements and then to calibrate the profile by use of a germanium detector for a gamma-ray absolute intensity measurement, normally at one point in the center of the profile. Although both of these techniques provide statistically satisfactory data, both require a quite long period of time for the measurements, often one hour or longer per assembly, and both require collimators. A 1965 publication entitled Richard J. Nodvik, "Evaluation of Gamma Scanning as a Tool for Determining Fuel-Burnup Distribution in Large Power-Reactor Cores," Transactions, 1965 Annual Meeting, American Nuclear Society, described the use of a miniature ion chamber inserted in-core during reactor operation for gross gamma scanning, (a technique which was being evaluated as a tool for determining burnup distributions within large power reactor cores). However, although that reference initially mentioned the term "distribution," there was no further discussion of the subject. And it was found that the gamma activity generally overestimated the burnup in assemblies that occupied the central region of the core (where higher burnup normally occurs) and underestimated the burnup in assemblies that formed the periphery of the core (where lower burnup normally occurs), implying that a measured profile would be flatter than the true burnup profile. The agreement between the gross gamma intensity and burnup was not good, deviations having ranged from -16 to +13%. The agreement must be good at every point in the profile in order to get a good measure of burnup. Therefore, in view of the above, a single ionization chamber would probably not be expected to be very useful in a method of accurately measuring burnup. Furthermore, the uses of the apparatus of this invention in rapidly measuring burnup and rapidly measuring an identifying characteristic which is used to determine whether a fuel assembly has been tampered with would be unobvious. And although two ionization chambers (each anode having a plurality of wires) have been used in measuring profiles in two coordinates of particle beams, (as described in C. K. Hargrove et al., "A Multiwire Proportional Chamber System for Monitoring the Position and Profile of a Charged Particle Beam," Nuclear Instruments and Methods, 113 (1973), pp. 141-145), the more versatile and less cumbersome apparatus of this invention has not previously been known. SUMMARY OF THE INVENTION An object of this invention is an apparatus for and another object is a method for measuring data directly correlatable with the burnup profile of a reactor fuel assembly in a period of measurement time which is less than 10 seconds, rather than nearly an hour or more as is required in the prior art apparatus described above. Other objects of this invention are a method and apparatus for determining within 10 seconds whether a fuel assembly has been tampered with. Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be utilized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. To achieve the foregoing and other objects and in accordance with the purposes of the present invention, as embodied and broadly described herein, the apparatus of this invention may comprise: a multiplicity of spaced apart substantially identical ionization chambers or proportional chambers, the individual chambers being operably connected so as to provide a multielement detector having a capability of substantially instantaneously obtaining a profile of data which is directly correlatable with burnup as a function of axial position. Further according to the invention in another embodiment, the multielement detector of the invention is used to substantially instantaneously and nondestructively measure a profile directly correlatable with the burnup profile of an object, for example a spent fuel assembly, with an accuracy equivalent to that of a germanium detector by measuring the gross gamma activity profile with the detector located outside the core of the reactor after a cooling time as short as 9 months and at a voltage such that saturation of the chambers does not occur. In yet another embodiment, the profile substantially instantaneously obtained by using the multielement detector of the invention is used to determine whether a particular object, for example a fuel assembly, has been tampered with. The apparatus according to the invention exhibits the following combination of advantages. It has the capability of being used to obtain a relative gross gamma activity profile measurement (which can be used to identify a particular fuel assembly, much like a fingerprint) in a very short period of time, less than 10 seconds. And unexpectedly, it has been found that the integrated area of the normalized gross gamma activity profile obtained with the multielement detector agrees to within the statistics of the normalized profile obtained by employing a germanium detector to measure the intensity of the 661 keV gamma ray of Cs-137 at a multiplicity of axial positions, using a cooling time as short as 9 months, provided that the detector is used out-of-core and provided that saturation of the detector does not occur. And, if desired, an absolute burnup profile can be obtained in a few minutes using the multielement detector if a germanium detector is additionally used to make one measurement for calibration of the normalized profile (referred to above). The apparatus of the invention, furthermore, can operate in both the ionization range and in the proportional range. And, furthermore, no problems which are intrinsic in mechanical scanning are encountered with the apparatus of the invention. The multielement detector is quite versatile, allowing one to measure long and short fuel assemblies with one convenient device, adjustable by varying the number of individual chambers and by varying the spacings between chambers. This device is less cumbersome than a large fixed-size detector employing one chamber with an anode made from a multiplicity of wires. Unlike multiwire detectors, no sophisticated construction techniques are required; and because individual detectors are used, repair is made easier. Additionally, the electronics setup which is used in cooperation with the multielement detector is much simpler than that needed with HRGRS, can be made portable, and may even be battery powered.