IAEA-SM-367/14/01 UTILIZATION OF FLUORESCENT URANIUM X-RAYS AS VERIFICATION TOOL FOR IRRADIATED CANDU FUEL BUNDLES Iqbal Ahmed 1, Rolf Arlt 2, Victor Ivanov 3 and K G. Qasim 4 [1] Institute for Nuclear Power, Islamabad, Pakistan [2] International Atomic Energy Agency, Vienna, Austria [3] RITEC Ltd., Riga, Latvia and [4] Karachi Nuclear Power Plant, Karachi, Pakistan. Abstract The use of fluorescent uranium x-rays for in-situ safeguards verification of irradiated CANDU fuel bundles is described. Room temperature CdZnTe (supergrade) semiconductor detector of low sensitivity coupled to charge sensitive pre-amplifier is used. This detector is characterized by moderate resolving power in the low energy region around 100 kev. It as such allows the separation of uranium x-rays in the close proximity of tungsten x-rays emanating from the shielding/collimator assembly. On account of strong attenuation, the detection of low energy x-rays requires the shielding to be of an optimized thickness. Further, in view of high intensity of this radiation the use of small volume detector is warranted. In dealing with the subject, this paper therefore presents an assessment, not only of the detector but also the shield-collimator assembly for the required verification of short cooling time fuel bundles. Results of the associated optimization measurements with respect to collimator aperture and detector sensitivity are consequently included. The future course of work from the viewpoint of development of a suitable x-ray spectrometer specifically for the purpose of verifying extremely short (< 1 month old) cooling time fuel bundles is moreover identified. 1. INTRODUCTION The verification of irradiated CANDU fuel bundles making use of Cs-137 photo-peak (661.6 kev) works well when the cooling time of bundles is not too short. With decreasing cooling time and higher gamma field, the Cs-137 peak gets lost in the heavy Compton background of gamma rays emitting from short and intermediate half-live fission products. At cooling times as short as a few months, this situation can still be handled making use of a good resolution, small volume CdZnTe detector in conjunction with tungsten collimator, provided the verification signal is derived from Nb-95/Zr-95 gamma rays instead of Cs-137 [1]. The tungsten shield/collimator for such an application needs to be at least a few centimeters thick. In some cases, the use of thick shielding is, however, neither convenient nor practically possible. An example of such a situation is the verification of closely lying irradiated fuel bundles on densely packed storage trays with not enough access from the sides. It has been suggested to derive the verification signal in such an eventuality from the fluorescent uranium x-rays. These x-rays have the energy of about 100 kev and are absorbed within a few millimeters of lead or tungsten. Limited space and high gamma field impose restrictions not only on the thickness of shield but also on efficiency of the detector. The demand on significantly low sensitivity requires the detectors with small sensitive volume - but still large enough to produce a photo peak at 100 kev to make use of intense fluorescent uranium x-rays. The application of small detectors should in particular be useful for the detection of low energy x-rays (e.g., UK = and UK > lines) in the presence of high-energy gamma radiation. The decrease in detector thickness together with reduction of its volume leads to lowering of registration efficiency in the high-energy region. The intensity of background Compton distribution as a consequence is also decreased. This results in higher peak-to background ratio for the low energy lines. The verification of irradiated fuel bundles on storage trays making use of the fluorescent uranium x-rays is not possible in the normal scanning direction facing the bundle end plate. Large attenuation of x-rays through thick zircaloy element end-cap (4.7 mm) and bundle end-plate (1.6 mm) makes it an extremely difficult proposition not withstanding the fact that the use of heavy tungsten shield introduces its own characteristic fluorescent x-rays which reduces the possibility of monitoring the uranium x-rays further still. Having realized that it may not be possible to detect the uranium x-rays by placing the detector in front of the bundle end plate, an attempt was made to monitor them in a side view arrangement along the bundle length. In such geometry the x-rays have to traverse a minimum of water gap and zircaloy sheath thickness of only 0.42 mm. As will be seen, the availability of high-resolution miniature CdZnTe (supergrade) detector has made this initially very difficult task a practically surmountable reality.
2. DESCRIPTION AND CHARACTERISTICS OF DETECTORS USED The detection probe was manufactured by the RITEC Ltd. It consisted of a CdZnTe (CZT) hemispherical detector coupled to a charge sensitive preamplifier in an integral assembly. It could be placed in a watertight housing and connected to pulse processing/data acquisition equipment through a watertight connector and a submarine cable. For irradiated fuel gamma ray spectrometry, these detectors are available in 10 60 mm 3 volume. In an environment of extremely high radiation field characteristic of x-rays, these as mentioned above are required to have low registration efficiency while preserving their high spectroscopic performance (i.e., energy resolution 10 kev at 661.6 kev; peak-to-compton ratio 2.5). Therefore detectors with very small volume were needed. Until recently the minimum reachable detector volume was ~1 mm 3. The technological process for the fabrication of extra small hemispherical detector demanded a high accuracy in planting the central and outer electrodes in a small area with very high precision. The fabrication of a detector of still smaller volume (~0.44 mm 3 ) that is used for the reported work is therefore a step forward [2]. The main aim being to optimize the detector sensitivity for the detection of high intensity uranium x-rays, two detectors of widely different response characteristics were tested and utilized for the measurements. Both these CdZnTe detectors were of super grade quality and had exceptionally good resolution (5.5-8.0 kev at 662 kev of Cs 137 photo-peak). They respectively had a volume of 1.88 mm 3 and 0.44 mm 3. As their volumes were in the ratio 5:1, they respectively were labeled as (1) SDP 311/Z/20/S/5 and (2) SDP 311/Z/20/S/1. Their respective characteristics as certified by the manufacturer are given hereunder in Table-I Table I Characteristics of X-Ray Detectors Used Parameters SDP 311/Z/20 S/5 SDP 311/Z/20 S/1 Basic Dimensions of 45 24 4 45 24 4 Detector/preamplifier Assembly (mm) Detector Volume (mm 3 ) 1.88 0.44 Operating Voltage (V) 250 100 FWHM (kev at 662 kev) 5.5 8 FWTM ( kev at 662 kev) 31.4 20.0 Peak to Compton Ratio 3.6 1.9 Peak to Valley Ratio 60.8 23.6 Recording Sensitivity (mm 2 ) 0.0054 0.0007 Output Signal Polarity Positive Positive The detector plate with CZT crystal mounted on it was located below the pre-amplifier plate. The entire assembly comprising the detector and the pre-amplifier was enclosed in a rectangular (45 mm 24 mm 4mm) brass housing. The pre-amplifier coupled to the detector was provided with a connector at the top for detector bias/pre-amplifier supply and signal transmission (Fig. 1). Detector 24 5 White dot 4 (4.3 with paint) Detector's Plate 3.5 45 Brass housing Preamplifier's plate Connector 6 5 4 3 2 1 Fig. 1 Design Features of CdZnTe (SDP311/Z/20S/1) Detector
3. TEST SHIELD-COLLIMATOR ASSEMBLY The detector plate was placed between two blocks of tungsten shield of dimensions (65 mm 45 mm 12 mm) each of which was lined on the interior with 2 mm thick copper. The block in front was provided with a 5 mm diameter collimator. The collimator diameter could be further reduced to 2.5 mm with the help of a tungsten insert tube. The detector-preamplifier assembly sandwiched between the blocks of tungsten was positioned inside a stainless steel container with the help of a lead cover plate secured by long screws such that the collimator was directly in-front the window (0.5 mm thick) provided for in the stainless steel container (Fig. 2). Fig. 2 Shield-Collimator Assembly With Embeded Detector Inside Stainless Steel Housing 500 Lem o-circuit-connector 5mm Lead 450 W indow 0.5mm St Steel Detector 150 15 Lead 13 Tungsten 17 4. MEASUREMENT EQUIPMENT The measurement equipment comprised the same electronics and mechanical scanning system that has been customized for safeguards verification by the IAEA. 4.1 Pulse Processing and Data Acquisition Equipment Spectroscopy Amplifier (TC-244) NIM/BIN Unit MMCA model MCA-166 Palmtop Computer (HP-200LX) / Laptop Computer With WinMCA or WinSCAN Software 4.2 Mechanical Scanning Equipment CANDU Bundle Verifier for Stacks (CBVS) consisting of a mechanical hoist with stepping motor and a motor controller. 5. IN-SITU MEASUREMENT GEOMETRY The reported test measurements were carried out in the available empty space adjacent to the 11 th bundle on the spent fuel storage tray in a given stack. The available space was 100 mm wide allowing the 95 mm wide test
detector housing to be conveniently inserted into it, leaving a water gap of ~ 5 mm in between the nearest element of a bundle and stainless steel window of the container (Fig. 3). In this scanning direction, the x-ray attenuation was affected only by the following: Zircaloy Sheath Thickness: 0.419 mm Water Gap: 5 mm Stainless Steel Window Thickness: 0.5 mm Fig. 3 Test Collimator Loaded In-Situ For Measurements on KANUPP Irradiated Fuel 6. OPTIMIZATION OF COLLIMATOR APERTURE The tungsten collimator was of 5 mm diameter. There was however a possibility as stated above, to reduce it further to 2.5 mm. Measurements were carried out making use of both the collimator sizes to determine the suitability of one over the other. A 5 mm thick lead absorber disk was employed between the Cs-137 test source and the detector to simulate the response of the detector to uranium x-rays in the presence of tungsten x-rays It was seen that by reducing the diameter of collimator by half (5 mm to 2.5 mm) the integrated count rate decreased by 70%. The 2.5 mm diameter collimator also did not allow the low intensity lead x-rays to be appreciated in the background of much more prominent tungsten x-rays. Moreover the larger diameter (5mm) collimator offered better peak to Compton ratio for the Cs-137 photo-peak, a feature that could serve as an advantage from the viewpoint of the detection of uranium x-rays. The 5 mm diameter collimator was accordingly employed for the presented verification measurements. 7. OPTIMIZATION OF DETECTOR SENSITIVITY 7.1 Measurements With Detector No. 1 (SDP 311/Z/20S/5) Originally, test measurements were carried out making use of the detector no.1, labeled as SDP 311/Z/20S/5. Extensive tests were performed to determine its response to the low energy x-rays, utilizing a strong Cs-137
source with a lead absorber insert between the source and tungsten shield/collimator to produce their respective fluorescent x-rays. The capability of the detector was finally assessed through the actual in-situ measurements on irradiated fuel bundles of varying cooling times. 7.11 Response of Detector (SDP 311/Z/20S/5) to Cs-137 Source with U-Pellet Insert To determine response of the detector to uranium x-rays in the presence of tungsten x-rays, test measurements were carried out with front tungsten shield / collimator removed and a uranium pellet inserted between the Cs-137 source and detector. The amplifier (TC-244) pulse shaping time was 1000 ns and counting time 200 s. Based on the calibration using 661.62 kev Cs-137 gamma ray. The uranium x-rays were seen at 94.3, 98.2 and 111.1 kev representing the known x-ray structures as follows: Table II Response of Detector SDP 311/Z/20S/5 to Uranium X-Rays (Test Measurements) Uran X-Rays Energy (kev) Relative Intensity Lines Observed (kev) K= 1 98.434 45.1 98.2 K= 2 94.654 28.2 94.3 K> 1 111.298 10.7 111.1 K> 2 114.445 4.15 K> 3 110.421 5.65 K> 4 114.844 0.12 The uranium x-rays were seen to be clearly separated from the tungsten x-rays observed at 58 and 67 kev respectively. 7.12 Response of Detector (SDP 311/Z/20S/5) to Cs-137 Source With Pb Absorber Insert Although lead was not used in the main shield/collimator assembly, yet interference from its x-rays was studied vis-à-vis tungsten x-rays. Test measurements in this case too were conducted with front tungsten shield / collimator block removed and inserting a 0.25 inch thick lead disc between the Cs-137 source and the detector. The instrument settings remaining the same as in the previous test with U-pellet. It was seen that while 58 kev tungsten x-ray was well resolved, the 67 kev line got merged in 73 kev lead x-ray. In addition a lead x-ray at 84.4 kev could be identified. The two lead x-rays representing the known structure as follows: Table III Response of Detector SDP 311/Z/20S/5 to Lead X-Rays (Test Measurements) Pb X-Rays Energy (kev) Relative Intensity Lines Observed (kev) K= 1 74.969 46.2 73 K= 2 72.805 27.7 K> 1 84.938 10.7 84.4 K> 2 87.300 3.91 K> 3 84.950 5.58 K> 4 87.580 0.09 K> 5 84.470 0.312 7.13 Test to Determine Response of the Detector to an Irradiated Fuel Bundle Optimization of collimator aperture and response of the detector to different interfering x-rays having been carried out, the detector along with its shield/collimator assembly (with 2.5 mm diameter collimator insert removed) was loaded into the stainless steel housing. Tests were subsequently carried out in-situ on irradiated
fuel by lowering the housing into the vacant position adjacent to the 11th bundle (HD099C, BU: 105.7 MWh s, Discharge Date: 5 July 1974) on the top most tray (A-82) of the Stack D-9. A count rate of 121459 cps (DT=51%) was encountered. Tungsten x-rays as well as Uranium x-rays could be seen. Cs-137 peak was also clearly visible with FWHM = 13.9 kev. 7.14 Test to Determine Response of the Detector to the Gap Between Two Bundles Having observed the response of detector to 25 year old irradiated fuel bundle at its storage location, the detector housing was moved to the water gap between two fuel bundles. The count rate was found to reduce to 65918 cps. The tungsten x-rays were seen. Cs-137 peak was also clearly visible with FWHM of 10.1 kev. The uranium x-rays as expected disappeared in the water gap. 7.2 Measurements using Detector No. 2 (SDP 311 /Z/20S/1) Further tests carried out on irradiated fuel of cooling time shorter than 25 years clearly demonstrated the inadequacy of detector no. 1 (SDP 311/Z/20S/5) in coping with the high count rates involved. It accordingly became apparent that a detector of still lower sensitivity would be required for the verification of shorter cooling time fuel bundles. Further test measurements reported to, here were therefore carried out making use of the detector no. 2 (SDP/Z/20/S/1), the spectral performance characteristics of which are compared with those of the detector SDP 311/Z/20/S/5 as follows: Table IV A Comparison of Performance Characteristics of the detectors Used Detector Resolution (kev) at 661 62 kev Integral C/R Relative Efficiency Design Measured (cps) (%) SDP311/Z/20S/5 No.1 SDP311/Z/20S/1 No. 2 5.5 7.1 3800 100 8.0 7.4 460 12 The above presented comparison was carried out making use of identical source-detector geometry for both the detectors. The amplifier gain was optimized such that Cs-137 photo-peak produced the same pulse height for both the detectors. Both the detectors produced a well-defined gaussian photo-peak with a clear escape peak. In addition to the size of the CZT crystal, the detection probe no. 2 (SDP 311/Z/20S/1) also differed from the probe no. 1 (SDP 311/Z/20S/5) in certain other details. While the detector in probe No. 1 was mounted on a tungsten base plate, there was no such tungsten shielding around the detector in probe no. 2. Moreover, the detector was located 3.5 mm from the base in the probe no. 2 as opposed to a distance of 7 mm in the case of probe no. 1. This required a 3.5 mm wide spacer to be placed at the bottom end of the probe no. 2 to ensure that the CZT crystal was directly in front of the collimator. 7.21 Test In-Situ on 25 Years Old Fuel Bundles Using Detector No. 2 (SDP 311 / Z/20S/1) The measurements using the detector no. 2 (SDP 311/Z/20S/1) were carried out on 25 years old fuel bundle (same as for the previous measurements using the detector no. 1). Measurements were repeated with different pulse shaping times ranging from 1000 ns to 250 ns. Most optimum results were obtained as shown in Fig. 4 at 375 ns both from the viewpoint of throughput and resolution at Cs-137 photo-peak energy (661.6 kev). The pulse height spectrum at this shaping time clearly identified the uranium x-rays, separated from the tungsten x-rays. The encountered count rate in front of the bundle was ~ 40,000 cps. 7.22 Test In-Situ on 1 Year Old Fuel Bundle Using Detector No. 2 (SDP 311 / Z/20S/1) Following the detailed measurements on 25 years old fuel bundle, tests were conducted on one-year-old fuel. On account of very high count rates in excess of 200,000 cps, measurements were carried out with TC-244 pulse shaping time reduced to 250 ns. The pulse height spectra showed clearly separated uranium x-rays from the tungsten x-rays (Fig. 5).
Thr oughput (Thousands) 40 39 38 37 36 35 Throughput Resolution 12 11 10 9 Resolution (FWHM at 661.62 kev of Cs-137) kev 34 250 375 500 625 750 875 1000 8 Pulse Shaping Time (ns) Fig. 4 Effect of Pulse Shaping Time on Resolution and Total Count Rate (Measurements In-Situ on 25 Year Old Fuel Bundle) 120 Counts (Thousands) 100 80 60 40 20 W X-Rays (58 & 67 kev Uran X-Rays (94.7, 98.4 & 111.3 kev) 0 0 100 200 300 400 500 600 700 800 900 1000 Channel No. Fig. 5 Response of Detector (SDP 311/Z/20S/1) to One Year Old Fuel Bundle 8. VERIFICATION MEASUREMENTS ON 1 YEAR OLD FUEL BUNDLES With the shield collimator assembly loaded in the space adjacent to the 11th fuel bundle on storage tray, the measurements were carried out with vertical top to bottom traverse. The SCANDU profile with ROI selected around the uranium x-rays clearly identified bundle locations from those of the gaps (Fig. 6)
5 4 Counts (Thousands) 3 2 1 Note:- Maxima indicate presence of fuel bundles 0 0 1 2 3 Top to Bottom Vertical Scanning Distance (m) Fig. 6 SCANDU Verification Profile of 1 Year Old Fuel Bundles (Region of Interest Around Uran X-Rays) 9. OUTLOOK It has been shown that irradiated CANDU fuel bundles can be verified using fluorescent uranium x-rays. This provides IAEA with a potentially new method, additional to the verification based on fission product gamma rays. On account of excessive attenuation suffered by the x-rays in the fuel bundle thick end-cap and end plate, this method can not however be utilized in the conventional scanning direction vertical to the bundle end plate. The alternate scanning mode vertical to the bundle length although permits x-rays to be seen, allows only a single bundle to be scanned per tray in the currently existing storage geometry. The tungsten shield collimator assembly used was quite adequate for the reported measurements. The optimum collimator diameter was found to be 5 mm. A pulse shaping time of 250 ns gave best results at count rates in excess of 200,000 cps. At higher count rates expected from bundles of cooling time less than 1 year, use of still smaller volume (< 0.44 mm 3 ) CZT detectors is imperative. The proposition of inserting detector assembly in the narrow horizontal space between adjacent CANDU bundles has its limitations as the measurement conditions would be much more difficult. Only a few millimeters of tungsten shielding could be utilized and the count rate will register an exponential increase. This would require a further reduction in the detector volume. There is an ultimate limit on that however. Presently, alternative detector options like small silicon detectors and GaAs detectors are investigated. 10. REFERENCES [1] Iqbal Ahmed, Rolf Arlt, A. Hiermann, Victor Ivanov and K. G. Qasim Safeguards Verification Of Short Cooling Time KANUPP Irradiated Fuel Bundles Using Room Temperature Semiconductor Detectors Paper No. IAEA-SM-367/A/7/03/P, International Safeguards Symposium, Vienna (29 Oct 1 Nov 2001) [2] V. Ivanov, P. Popov, A. Loutchasky, L. Aleksejeva and E. Mozchaev Further Development of Hemispherical CdZnTe Detectors for Safeguards Application Proceeding of the 21st Annual Symposium on Safeguards and Nuclear Material Management Sevilla, Spain, 4-6 May 1999, EUR 18963, ESARDA 29, p. 479-484. 11. ACKNOWLEDGEMENTS The reported work was carried out under an IAEA research contract (9983/R1). The principal author is grateful to the IAEA for the research grant and equipment support. The Pakistan Atomic Energy Commission is thanked for making available facilities for the reported investigations.