Patent Number: 043354661
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Although the following description is directed to using Cs-137 as a burnup monitor, other monitors can be used, if desired. In the methods according to the invention, the detector of the invention is used to obtain substantially instantaneously a gross gamma activity profile of any object having such a profile, for example a fuel assembly. That profile can then be used either (1) later, as is (without normalizing the curve), to identify that fuel assembly, much as a fingerprint is used to identify humans, or (2) it can be used as a relative burnup profile and calibrated in either of two ways (described below) to determine true burnup. The multielement detector used in the methods of this invention will comprise a plurality of either (1) ionization chambers or (2) proportional chambers. These are current measuring detectors, not pulse counting detectors, and take advantage of the high radiation fields associated with irradiated fuel assemblies so as to provide the fast measurement. Such chambers are well known in the art (see, for example, F. H. Attix et al., Radiation Dosimetry, Academic Press, Inc., New York (1960)) and will not be described here in detail. However, in order to minimize the necessity of mathematical corrections for any differences in the detectors, all chambers making up one multielement detector should preferably be substantially identical. Although the number of individual chambers can be varied broadly, generally at least 5 will be used to form the multielement detector. When one wishes to obtain a profile, the individual chambers will be preferably located along a substantially straight line, will be spaced apart equidistantly, and will occupy a total length (measured between the two outermost chambers) equal to or greater than the length of the fuel assembly being measured. For convenience, the individual detectors can be mounted on a base. The multielement detector, as described below, can be operated in cooperation with any suitable electronics system which (1) separately amplifies the current signal from each individual current-detecting chamber, (2) converts the signal to an amplified voltage signal, and (3) then multiplexes, digitizes and stores each individual signal in a separate channel of a multiple channel device to which a suitable output device is connectable. In FIG. 1, for example, is shown a schematic diagram of elements of an apparatus which would be suitable for use with a multielement detector. Although a microprocessor system is preferably used, it is not required. When one uses the multielement detector for either purpose described above, it is required that each of the chamber detectors be operated at a voltage such that saturation of the chamber does not occur. Such a voltage is determined by standard means that are well known in the art, as are described for example in F. H. Attix et al. (cited above). When the multielement detector is used to obtain a profile to be used for identifying a particular fuel assembly, a gross gamma activity profile will be measured at some initial time t.sub.o. This embodiment of the invention is independent of whether or not the axial gross gamma activity profile is in agreement with the true burnup profile. Thus, the measurement can be made in-core, if desired, and need not be made after waiting a particular cooling time. However, for this embodiment of the invention, if a profile is made at a particular time t.sub.o such that the cooling time is less than 9 months, this profile can be used for purposes of identifying that fuel assembly only for a limited period of time, for example up to about 2 months because the profile may vary as a function of cooling time. However, if the cooling time is greater than about 9 months when the first profile is made, the second profile can be made at any later time. After a second profile is made, using the same detector geometry, the two profiles are compared. If they are substantially identical, it is highly likely that there has been no tampering with the fuel assembly. However, if there is a significant difference in the profiles, it is highly likely that some of the fuel has been removed. In the embodiments of the invention wherein a measure or burnup is to be obtained, the following requirements in the method of using the detector must be fulfilled. The axial gross gamma activity profile must be made with the multielement detector positioned out-of-core, not in-core. Additionally, the measurement should be made only after a cooling time which is greater than about 9 months. When these requirements are met, it is believed that an accurate measure of burnup can be obtained for any type of reactor and for any amount of burnup within the range from about 0 to about 40,000 MWD/MTU. These two requirements must be followed if one wishes to get an accurate measure of burnup because, as described above, an axial gross gamma activity profile measurement will not necessarily show any agreement with burnup. It has experimentally been determined, as described in the Experimental Demonstrations and in the Example below, that when cooling times are as short as 9 months for BWR and PWR fuel assemblies, excellent agreement between Ge detector (Cs-137) profiles and multielement ionization chamber profiles is obtained. One can say with reasonable certainty that such agreement will result whenever cooling times as long as or longer than about 9 months are provided, regardless of the amount of burnup and regardless of the particular reactor involved; however, one cannot predict that such agreement will result when a cooling time much shorter than 9 months is used. In order to actually determine the true burnup by using the profile which is directly obtained using the multielement detector, one of the following calibration methods should be used. (1) The profile can be used as is (without normalization) in conjunction with (a) the cooling time and (b) an earlier determined calibration curve of (detector response/declared burnup) vs. cooling time to provide a value of burnup which is within 10% of the true burnup. Or, (2) it can be normalized to have a peak value of 1, thus providing (under particular conditions) a normalized gross gamma activity profile which is substantially identical to the normalized profile obtained by using a germanium detector. This normalized profile can then be used instead of the normalized profile obtained by using a germanium detector (measuring Cs-137) for any purpose that a normalized germanium response is useful. If one wishes to establish the true value of burnup, the curve must be calibrated, however, by making one measurement (preferably at the center of the fuel assembly) with the germanium detector. EXPERIMENTAL DEMONSTRATIONS In the following demonstrations, one air-filled ionization chamber was used to measure gross gamma activity at a plurality of axial positions along several fuel assemblies, and its normalized profile response was compared with the response of at least one other detector. In Experimental Demonstration 1, the profiles of three BWR fuel assemblies were measured; and in Experimental Demonstration 2, the profiles of three PWR fuel assemblies were measured. Before these demonstrations were done, there was no incentive provided in the art for building the multielement ionization chamber detector apparatus of the invention because it could not have been expected that the response of even a single ionization chamber would give an accurate measure of burnup for cooling times as short as 9 months. The value of burnup given on each graph is the declared value provided by the reactor operator, obtained by using the proprietary computer codes of the company. By using the formulas in the Hsue article (cited above), one can (if desired) calculate close approximations to the declared values. The germanium detector and the beryllium detector were assumed to have responses proportional to burnup. (See Hsue et al. cited above). The beryllium detector measured primarily the 2.186 MeV gamma-ray from the .sup.144 Pr fission product, and the germanium detector measured the 661 keV gamma ray of Cs-137. EXPERIMENTAL DEMONSTRATION NO. 1 In this demonstration, several BWR fuel assemblies (each having a cooling time of at least 9 months) were investigated using a single element ionization chamber, a germanium detector, and a beryllium (.gamma.,n) detector to scan the fuel assemblies. The detectors were stationary and each fuel assembly was moved past the detectors on an elevator. The experimental setup which was used is shown in FIG. 2. Readings were taken by all three detectors at 16.5 cm intervals along each fuel assembly, the germanium detector and the beryllium detector counting 400-500 seconds for each measurement, whereas the ionization chamber measurement was available as soon as the fuel was in the correct position, normally within about ten seconds. A schematic of the ionization chamber detector is given in FIG. 3. It consisted of two outer parallel plates which were operated in the ionization region of the chamber at about -300 volts, and the anode was located between the parallel plates and was made up of wires which were gold-plated tungsten wire having a diameter of 20 .mu.m with a wire-to-wire spacing of 1.25 cm. The active area was 3.8 cm.times.6.25 cm, and the plate spacing (i.e., the distance between the anode and each cathode plate) was 1.25 cm. In FIG. 4, the readout electronics for the chamber are illustrated. The electronics included a current-to-voltage amplifier which converted the current signal output from the chamber to an analog voltage and the voltage was read by a digital voltmeter (DVM). The raw data obtained from each detector was subjected to the following procedure. Because the detectors were not all at the same axial position, the profiles were shifted so that the peak positions coincided. Additionally, the peak value of each profile was normalized to unity. No other changes were made in the raw data. Shown in FIG. 5 are the normalized responses (with peaks shifted) of the three types of detectors which were used to measure a BWR irradiated fuel assembly having a burnup of 18,804 MWD/MTU and a cooling time of 10 months. Next, in order to obtain a more quantitative comparison of the responses of the three detectors, axial profiles using the three detectors were numerically integrated, the region of integration being 31 cm to 208 cm (corresponding to the length of the fuel assembly). The results, which are in unexpectedly good agreement, are shown in Table I. For the short cooling times used, these results could not have been predicted. In obtaining the numbers shown in Table I, linear interpolation was performed between data points. The errors in the ionization chamber data were measured to be 0.5%, based on consecutive scans of the same element. The errors in the Be (.gamma.,n) detector (2%) and the germanium detector (1-2%) spectra were mainly due to counting statistics. All integrated means agreed to within statistics, although there appeared to be some trend for the Be (.gamma.,n) and ionization chamber areas to be slightly larger than the Ge detector area (perhaps because the germanium detector was collimated, whereas the other detectors were not). Therefore, from the results in Table I and in FIG. 5, it can clearly be seen that the normalized response of the ionization chamber detector is in excellent agreement with both the response of the germanium detector and of the beryllium detector when the cooling time of a BWR fuel assembly is as short as about 9 months. EXPERIMENTAL DEMONSTRATION NO. 2 Irradiated PWR fuel assemblies were next measured, using an ionization chamber detector and a germanium detector. The procedures and experimental setup were similar to those described in Experimental Demonstration No. 1, except that the beryllium detector was omitted and the ionization chamber here used was slightly smaller than that used in Experimental Demonstration No. 1 so that it could be inserted in a 5 cm diameter pipe and then retrieved without the problem of contamination from the water in the storage pond. TABLE I ______________________________________ Integrated Area of Normalized Response (NR) of: Ge Ionization Cooling Burnup Detector Chamber Be(.gamma.,n) Fuel Time (MWD/ (Cs-137) Detector Detector Assembly (Months) MTU) (NR.cm) (NR.cm) (NR.cm) ______________________________________ BWR-1 17 4356 266 .+-. 20 283 .+-. 7 277 .+-. 27 BWR-2 17 16658 253 .+-. 18 279 .+-. 7 266 .+-. 26 BWR-3 10 18804 279 .+-. 20 278 .+-. 7 271 .+-. 27 ______________________________________ The ionization chamber detector consisted of three plates with a plate separation of 1 cm and a sensitive volume of 10 cm.sup.3. The outer plates were held at about -300 volts. The electronics were identical to those used in Experimental Demonstration No. 1, described above. Several PWR fuel assemblies were measured with the two detectors, and the detector responses (after shifting the peaks to coincide and normalizing the peak value of each axial profile to unity) were in excellent agreement. The areas were numerically integrated from 60 cm to 420 cm, and the results are given in Table II. The agreement was excellent, within the statistics of the germanium detector response. Next, a fixed scanning geometry was maintained from assembly to assembly so that comparisons could be made between the declared burnup values and the response of the ionization chamber detector. In order to adequately account for different cooling times, the response of the chamber has been normalized by the operator declared burnup and plotted against cooling times in FIG. 6. The chamber response was taken from the center of the profile distribution. The distribution of data points for each cooling time suggests that the response is consistent to within approximately .+-.10%, clearly suggesting that ionization chambers can operate as stand-alone devices if cooling time information is incorporated in the data analysis. TABLE II ______________________________________ Integrated Area of NR of: Ionization Cooling Ge Detector Chamber Fuel Time Burnup (Cs-137) Detector Assembly (Months) (MWD/MTU) (NR . cm) (NR . cm) ______________________________________ PWR-1 15 17776 250 .+-. 43 249 .+-. 4 PWR-2 9 31851 272 .+-. 47 269 .+-. 5 PWR-3 9 32185 271 .+-. 47 278 .+-. 5 ______________________________________ TABLE III ______________________________________ Integrated Area of NR of: Multielement Cooling Ge Detector Ionization Fuel Time Burnup (Cs-137) Detector Assembly (Months) (atom %) (NR . in) (NR . in) ______________________________________ MTR-1 13.75 0.346 17.15 .+-. .86 16.86 .+-. .25 ______________________________________ Following the demonstrations described above, a detector according to the invention made up of a plurality of simultaneously operated individual ionization chambers was made and was tested, as described below. EXAMPLE A multielement detector was made from 15 individual and substantially identical air-filled ionization chambers, aligned along a straight line with a distance of 2.4 in. between adjacent detectors, thus providing a multielement detector with an effective sensing length of 36 in. A single high voltage supply was used to operate all of the individual ionization detectors, and the two outer cathode plates in each chamber were held at a voltage of about -300 V. The active area of each chamber was 10 cm.sup.2 and the anode was located midway between the outer cathode plates, with a total distance between the cathode plates of 1.25 cm. The anode of each detector consisted of three 20 .mu.m gold-plated tungsten wires separated by 1.25 cm, the wires permitting the capability of operating the chambers in a proportional mode. The anode output from each chamber detector was connected to an individual amplifier in the manner as shown in FIG. 1, thus enabling a profile of all 15 gross gamma intensity measurements to be obtained simultaneously and substantially instantaneously without mechanical scanning. A materials test reactor (MTR) fuel assembly (which was 36 inches long and which had a cooling time of 13.75 months) was measured using the multielement detector of the invention, and its normalized profile is shown in FIG. 7. Additionally, for purposes of comparison, a germanium detector was moved along the fuel assembly at a fixed distance from the fuel assembly. The results of these measurements are also shown in FIG. 7. In Table III, the integrated areas are given. The agreement between the normalized responses of the multielement chamber detector and the germanium detector was excellent. In FIG. 7, the profile is not symmetric about the fuel assembly because control blades were used in the MTR. From the Experimental Demonstrations and the Example above, because of the excellent agreement in the profiles, it is believed that the detector of the invention can be used to actually replace a germanium detector when MTR, PWR, and BWR fuel assemblies are measured out-of-core after a cooling time of at least 9 months, regardless of their burnup values if a calibration of ionization chamber response vs. burnup is available. However, if desired, a germanium detector can be used to calibrate the gross gamma activity profile obtained with the apparatus of the invention by measuring the response of the germanium detector at one axial position along the fuel assembly. Normally, fuel must be stored so that the critical mass is not exceeded; and unless an accurate measurement of burnup (or fissile content) is available, the fuel must be stored as if its burnup were 0. An accurate measure of burnup allows one (1) to more efficiently use the available storage space and (2) to more efficiently use the fuel itself than would be possible without this measurement. Because the present invention allows one to obtain accurate burnup measurements much more quickly than was previously possible, with reduced complexity, and with reduced interference with spent fuel storage operation, the present invention will provide for improved utilization of storage space. Additionally, improved use of the fuel itself also can result. The foregoing description of preferred embodiments of the invention has been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited for the particular uses contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.