RADIOMETRIC METHODS OF MEASURING THE SPECIFIC ACTIVITY OF RADIONUCLIDES OF THE URANIUM SERIES

Transcription

1 Atomic Energy, Vol. 107, No. 6, 2009 RADIOMETRIC METHODS OF MEASURING THE SPECIFIC ACTIVITY OF RADIONUCLIDES OF THE URANIUM SERIES V. G. Volkov, V. E. Stepanov, L. I. Urutskoev, and A. V. Chesnokov UDC A radiometric method of measuring the specific activity of 226 Ra in uranium ore tailings and tailings storage sites at integrated plants reprocessing uranium ores is presented. The results are compared with data from laboratory analysis of samples and measurements, performed by means of field spectrometry in dumps and tailings storage sites at the Vismut Industrial Association are compared. The radiometric methods are applicable for measuring 238 U contamination of equipment in production rooms using in the reprocessing of depleted uranium. Measurements performed in the ventilation systems of shops producing uranium tetrafluoride at the Kirovo-Chepetsk Chemical Works showed spectra of photonic ionizing radiation. It is shown that 1001 kev line can be used to determine the specific activity of 238 U under these conditions. These methods can be used in other uranium-cycle enterprises. The solution of problems within the framework of the special-purpose federal program Ensuring the nuclear and radiological safety of Russia for 2008 and in the period to 2015 revealed a need to develop rapid methods and means for measuring the specific activity of radionuclides of the uranium series. This need is due to the scale of the radiation examination of the tailings of uranium mines and uranium reprocessing and enrichment enterprises. The development of projects for rehabilitation of such objects relies on the results of radiological surveys, and the more detailed and thorough the data, the easier it is to estimate the volume and periods of rehabilitation work. Such problems were encountered in the development of projects for rehabilitation of the enterprises of Tvel company, contaminated objects and grounds of the Kirovo-Chepetsk Chemical Works, and the Vismut Industrial Association on the territory of Germany. Specialists at the Russian Science Center Kurchatov Institute proposed as a possible solution radiometric methods of measuring the specific activity of radionuclides of the uranium series. Possibilities of Field Radiometry for Measuring the Surface Activity of 226 Ra. The conventional method of determining the specific activity of radionuclides of the uranium series is the extraction and laboratory analysis of samples. This method is quite informative, since it permits full radiochemical evaluation of samples and determination of radionuclides from measurements of the γ-, β-, and α-spectra. However, because of the low representativeness and the physical and in most cases chemical action on the sample materials and the substantial expenditures on materials and labor for sample preparation, in order to maintain the reliability of the results of analysis great care is needed in performing chemical and physical processes in the preparation of counting samples and in performing spectrometric measurements. Moreover, the removal of a sample destroys the structure of the environmental object itself, and in consequence the depth distribution of the radionuclides can be determined only once and with large expenditures. These drawbacks of the sampling method do not diminish its merits; sampling is an internationally recognized method of evaluating the degree contamination of objects. Russian Science Center Kurchatov Institute, Moscow, Russia. Translated from Atomnaya Énergiya, Vol. 107, No. 6, pp , December, Original article submitted June 9, /09/ Springer Science+Business Media, Inc.

2 Obviously, for radiation examination of tailings and tailings repositories sampling must be used for verify the results and monitor radionuclides but sampling cannot ensure representativeness and reliability, which are required for the development of projects of liquidation and rehabilitation of contaminated objects. In a radiation survey of objects such as temporary storage sites for radioactive wastes at the Kurchatov Institute, the radioactively contaminated objects and grounds of the Kirovo-Chepetsk works, specialists at the Kurchatov Institute and the Moscow Scientific-Industrial Association Radon used together with sampling radiometric methods and means for measuring the specific activity of radionuclides [1 3]. The survey yielded representative data which formed the basis of the development of a project for rehabilitation of radioactively contaminated grounds and objects at the Kurchatov Institute [4]. A similar approach would also be useful in radiation examination of objects contaminated by radionuclides of the uranium series. The advantage of such an approach is the large volume of measured, using the radiometric method, information on the contamination of the grounds and objects that is verified by the results of laboratory analysis of samples. Obtaining the entire volume of information only according to the results of sampling would require a multiyear effort and large materials expenditures, while radiometric measurements alone would not make it possible to obtain complete data on the radionuclide composition and its variation in time and space. On the one hand, the radiometric method is geared toward large-scale spatial measurements in locations where the nuclide composition of the main pollutants varies very little or not at all. Examples of such sites are grounds contaminated as a result of the Chernobyl accident, the flood plain of the Techa River, and others [5 7]. Tailings at uranium mines and tailings repositories at works reprocessing uranium raw materials show such a distribution of radioactive contamination. On the territory contaminated with 137 Cs as a result of the Chernobyl accident, aside from surface activity, the radiometric method was used to evaluate the thickness of the layer of soil containing more than 80% of this radionuclide [8, 9]. On the territory where the contamination is not of surface character (the contamination is concentrated in the upper soil layers no more than 3λ thick, where λ is the free-path length of a photon emitted in the soil) but rather volume character, for example, temporary repositories for wastes, sludge dump at the Kirovo-Chepetsk works, or sludge dump at the Podolsk Nonferrous Metals Plant the radiochemical method was used in the logging geometry [2]. All these features of the contamination of uranium tailings and tailings repositories had to be re-examined in the course of the development of the means of performing radiometric measurements. Radiometric Method of Measuring 226 Ra at the Tailings Repository of the Vismut Industrial Association. The problem of determining the specific activity of uranium-series radionuclides was first encountered in measurements on the tailings at the Vismut association, which were performed together with German specialists. To choose the optimal energy range for determining the specific activity of 226 Ra, the spectrum of the ionizing photonic radiation of the surface of wastes was measured with Korad apparatus [9]. It is evident in Fig. 1 that the 609 kev emission line of 226 Ra can be reliably identified. For this reason the counting rate in this energy range can be chosen to measure its specific activity. It is evident that the distribution of the radionuclides in the tailings of the uranium mines is of the volume type. It is known that the counting rate N of a detector in the total absorption peak is due to the detection of only unscattered radiation and is determined by the photon flux striking the detector. The photon flux Φ(r, Ω, E), recorded by the detector with collimator solid angle Ω ( Ω = πα 2,where α is the aperture angle of the detector) in the direction r and with energy E, which corresponds to the photopeak of the emitted photon, will be related with the specific activity σ m and volume activity σ v of the radionuclide by the relations N = Φ(r, Ω, E)ε Ω = Aqε Ω = Aσ v λ i ε Ωn i /4π = Aσ m ε Ωn i /4πµ i, (1) where ε is the average efficiency of detection of photon radiation in the solid angle Ω (the solid angle of the detector s collimator); A is the coefficient of proportionality determined during calibration; q is the average activity along the surface falling within the solid angle of the collimator; n i is the quantum yield per decay; λ i and µ i are the free-path length of the photons in the material of the tailings and the mass coefficient of absorption (µ i = 0.08 cm 2 /g in the tailings material for 609 kev radiation), respectively. It follows from expression (1) that to within a constant and a geometric factor the specific activity σ m is determined by the counting rate in the total absorption peak. The background component due to the passage of photons through the lateral walls of the lead shielding is taken into account using the standard procedure of covering the openings of 401

3 3000 Counting rate 352 kev Channel Fig. 1. Radiation spectrum measured with the Korad radiometer at the surface of tailings of the uranium mines of the Vismut association. the collimator with a lead cap and then measuring the background component of the counting rate N c,which is then subtracted from N 0. Thus, by performing at each point two measurements N 0 and N c without and with a cap, respectively, the counting rate in the photopeak can be determined as N = N 0 N c and the specific activity σ m,averaged over the angle of vie of the collimator of the detector, of a radionuclide emitting the recorded photonic ionizing radiation can be determined according to the counting rate in the total absorption peak of the detected radiation. Another method of measuring the specific activity of 226 Ra according to the 609 kev line is similar to determining the 137 Cs content in soil [8, 9]. The measurements were performed with the Korad radiometer, consisting of a 256 channel gamma spectrometer, making it possible to measure and store in memory the surface activity and the content (concentration) of a nuclide at 1000 points as well as record 44 measured radiation spectra. The apparatus possessed a photon collection area of about 1.5 m 2 ; the energy resolution of the detector on the 662 kev line was 7.6%. The following intervals in the instrumental spectrum of the collimated scintillation detector were chosen on the basis of numerical Monte Carlo calculations according to the method for measuring the 226 Ra content on the basis of the counting rate in individual energy ranges: kev the Compton range, kev the photopeak range, and kev additional range for taking account of the contribution of other lines, arising as a result of the decay of uranium, thorium, and 40 K, to the main ranges [8, 9]. The calibration of the Korad radiometer and the determination of its sensitivity according to the energy of the radiation were performed using a certified planar 137 Cs source with uniformity not worse than 6%. The Korad radiometer is programmed for measuring the surface activity and content of the radionuclide of interest, i.e., the measurements are stored in the values of the surface activity and content, so that these data must be converted to the specific activity of 226 Ra. To convert the surface activity, it was assumed that 226 Ra is distributed uniformly at depth greater than three times the free-path length of a 609 kev photon in the tailings material h 3λ i = 3/µ i ρ,where ρ is the density of the matter in the tailings. In this case, the specific activity is related with the surface activity q by the relation σ m = qµ i. To determine the sensitivity of the Korad radiometer from the measurements of the 226 Ra content and convert to the specific activity, calibration measurements were performed on special sites organized by specialists at the Berlin Institute for Monitoring the Environment at the tailings of the Vismut. These sites were used to calibrate the field spectrometers with semiconductor detectors, with whose help the specific activity of 226 Ra was measured according to the counting rate in the photopeaks of individual lines. Specifically, to determine the specific activity of 226 Ra field semiconductor detectors were calibrated according to the 2204, 1847, 1766, 1509, 1407, 1378, 1238, 1121, 786, 609, 352, 295, and 242 kev lines. Aside from 226 Ra, 40 K was identified according to the 1460 kev line and 232 Th according to the 2614, 911, and 583 kev lines. The sites were m flat surfaces on which tailings samples were obtained with step 2 2 m. One hundred samples were obtained 402

4 TABLE 1. Results of Laboratory Analysis of Samples and Data Obtained Using the Korad Radiometer on the Calibration Site No. 1 of the Vismut Industrial Association Measurements performed with the Korad radiometer (±20%) Laboratory analysis of samples Specific activity of 226 Ra determined from 40 K, Bq/kg (±23%) 224 Ra, Bq/kg (±25%) 226 Ra, Bq/kg (±20%) rate in the photopeak, Bq/kg 226 Ra content, 10 5 Bq/m q con q act Fig. 2. Specific activity q of 226 Ra as measured from the surface activity and content. from each area and measured in the laboratory; this made it possible to judge the uniformity of the radionuclide distribution over the surface. In Table 1, the sampling data are compared with the measurements performed with the Korad spectrometer on area No. 1, which consists of tailings from a mine shaft. The calibration measurements showed that the specific activity and content C of 226 Ra are related as σ m = αc, where α = (Bq/kg)/(Bq/m 2 ). The same relations were obtained on the calibration area No. 2, which consists of tailings from the chemical separation of uranium. Comparison of the Results of Field Spectrometry and Field Radiometry. Aside from the calibration sites, measurements were also performed on tailings with unknown specific activity. On sections with unknown activity, the specific activity of 226 Ra was measured with a field spectrometer with a semiconductor detector, placed 1 m above the surface of the ground, in order to verify the results obtained with the Korad radiometer. A 1 cm thick lead collimator was used to protect the detector from scattered radiation. About 50 measurements were performed with the Korad radiometer at five locations; the exposure time varied from 100 to 1800 sec depending on the specific activity and the equivalent dose rate at the measurement location, which varied from 0.18 to 0.7 µsv/h. The exposure time in the measurements was 1800 sec. 403

5 TABLE 2. Comparison of the Measurements from Sampling and Performed with the Korad Radiometer on the Background Site of the Vismut Industrial Association, Bq/kg 40 K 226 Ra Sampling (±35%) Korad radiometer (±25%) A comparison shows that, as expected, the specific activity determined according to the surface activity q act is less than that determined from the content q con (Fig. 2). This is explained by the absorption of unscattered radiation in the volume of the tailings material with increasing penetration depth of the radionuclides. The specific activity determined from the content measured with the Korad radiometer correlates well with the results of the semiconductor spectrometry. This is explained by the fact that the instruments were calibrated on the same sites, while the data obtained from measurements of the surface activity during calibration were somewhat too low at the outset. In additions, most of the data agree well within the measurement error, which shows the possibility of using the collimated radiometry for measuring the specific activity of 226 Ra in the tailings from uranium production. The quite wide dynamic range of the measurements, reaching two orders of magnitude, should be noted. The results of the field semiconductor spectrometry were obtained with a longer exposure time than for field collimated spectrometry. The semiconductor detector is ten times more expensive than the Korad radiometer and it requires the use of liquid nitrogen and means for delivery to the measurement site. At the present time, the mass of the collimator for the Korad radiometer has been decreased to 8 9 kg by using a scintillator photodiode assembly, which made the apparatus easily transportable. The spatial resolution of the Korad radiometer is about 1.5 m 2,which is at least 10 times less than the spatial resolution of the field semiconductor detector. The high productivity of the measurements, the possibility of using several instruments at the same time because of their relatively low cost, and the large volume of the measurements make the data much more reliable. For this reason, field radiometry can serve as an alternative to field spectrometry for radiation surveys of the tailings from uranium production. The measurements performed against the background of site at the Vismut association made it possible to determine the sensitivity limit of the method, which was ~200 and ~100 Bq/kg for 40 K and 226 Ra, respectively, which is adequate for mapping mine tailings and measuring the specific activity of natural radionuclides in building materials (Table 2). Methods of Performing Measurements on Radiologically Dangerous Objects of the Kirovo-Chepetsk Chemical Works. A fundamentally different situation was encountered in radiation surveying of radioactively contaminated territories and objects of the Kirovo-Chepetsk works. During the last few years of their existence, the shops that produced uranium tetraand hexafluoride operated on depleted uranium raw material. It is obvious that of the decay products of the uranium series only 234m Pa could reach equilibrium over such a short time; this made it possible to determine the specific activity of 238 U according to the 765 and 1001 kev lines. Even though the intensity of these lines is at least 20 times less than that of the 226 Ra lines, in its absence they were well identified in the spectrum of the radiation of contaminated objects. The specific activity of 238 U was measured according to the counting rate of the radiometer in the photopeaks of these lines after calibration on the standard sources [10]. As some conclusion it can be noted that the latest modification of the Korad radiometer, called Korad-M, where a Kolibri spectrometer is used as a detection block, make it possible to store hundreds of measured spectra in memory. Processing of laboratory measurements makes it possible to use modern numerical methods and to determine the specific activity for a known collection of radionuclides according to the counting rate in selected spectral ranges. Specifically, the energy ranges of the radiation spectrum, for which the inverse matrix of the calculations of the specific activity of 226 Ra, 232 Th, and 40 K was found according to their contribution to the counting rate in these ranges, were determined on the basis 404

6 of Monte Carlo instrumental spectra of the collimated detector and calibration on a standard source. Thus, 226 Ra radiation prevails in the range kev, the main range of the radiation for determining the specific activity of 232 Th is kev, and the range determining the contribution of 40 K to the total spectrum is kev. Conclusion. The measurements performed on grounds and objects contaminated with radionuclides from the uranium series showed that it is in principle possible to develop effective radiometric methods of determining their specific activity directly on the surface of the uranium mine tailings and tailings from processing uranium raw materials. The methods will be needed for radiation surveys of objects contaminated by these radionuclides. It is obvious that radiometric methods will have to be adapted to the concrete conditions of the object being surveyed but for the conditions of the tails from uranium production the difficulties arising can be solved for the standard collection of radionuclides and separate data on their depth distribution. The simultaneous use of spectrometric and radiometric methods, their comparison at individual points with data obtained by sampling will make it possible to collect reliable information required for developing projects for decommissioning and rehabilitation of uranium mines and developing capacities for processing uranium raw materials. REFERENCES 1. V. G. Volkov, V. N. Potapov, O. P. Ivanov, et al., New instruments and systems for radiation monitoring and their use in performing rehabilitation work at the grounds of temporary radwaste storage sites at the Russian Science Center Kurchatov Institute, in: 7th Int. Conf. on the Safety of Nuclear Technologies: Radwaste Handling, St. Petersburg, September 27 October 1, 2004, pp A. G. Volkovich, V. G. Volkov, A. S. Danilovich, et al., Application of new instruments for radwaste sorting in rehabilitation work at the Russian Science Center Kurchatov Institute, in: 8th Int. Conf. on the Safety of Nuclear Technologies, St. Petersburg, September 26 September 30, 2005, pp A. S. Danilovich, O. P. Ivanov, V. N. Potapov, et al., Instruments and methods for precise field measurements of soil radiocontamination, in: Proc. Radioecology and Environmental Radioactivity, Bergen Norway, June 15 20, 2008, Pt. 2, pp V. G. Volkov, G. G. Gorodetskii, Yu. A. Zverkov, et al., Project Rehabilitation status and problems, in: 6th Int. Conf. on Radiation Safety: Transport of Radioactive Materials ATOMTRANS-2003, St. Petersburg, September 22 26, 2003, pp A. G. Govorun, O. P. Ivanov, V. I. Liksonov, et al., Collimated detector technique for measuring a 137 Cs deposit in soil under a clean protected layer, Appl. Rad. And Isot., 48, No. 9, (1997). 6. A. P. Govorun, A. V. Chesnokov, and S. B. Shcherbak, 137 Cs concentration distribution of in the flood plain of the Tech River in the area of the settlement Muslyumovo, At. Énerg., 84, No. 6, (1998). 7. A. V. Chesnokov, A. P. Govorun, M. V. Ivanitskaya, et al., Cs-137 contamination of the Techa flood plain in brodokalmak settlement, Appl. Rad. and Isot., 50, (1999). 8. A. P. Govorun, O. P. Ivanov, V. I. Liksonov, et al., Method and device to measure 137 Cs soil contamination in-situ, Nucl. Instrum. Meth. Phys. Res. A, 420, No. 1 2, (1999). 9. A. P. Govorun, O. P. Ivanov, V. I. Liksonov, et al., Apparatus for measuring the surface activity of 137 Cs in soil by means of field radiometry, Kontr. Diagn., No. 4, (1999). 10. Yu. A. Zverkov, V. G. Volkov, A. G. Volkovich, et al., Radiation survey of radioactively contaminated objects and grounds of the Kirovo-Chepetsk Chemical Works, in: Nuclear and Radiological Safety of Russia, No. 4, Pt. 3, pp