Monitoring of natural radioactivity in working places

Transcription

1 . Microchemical Journal Monitoring of natural radioactivity in working places S. Righi a,, M. Betti b, L. Bruzzi a, G. Mazzotti c a Uniersity of Bologna, Department of Applied Chemistry and Science of Materials, Viale Risorgimento 2, Bologna, Italy b European Commission, Joint Research Centre, Institute for Transuranium Elements, P.O. Box 2340, Karlsruhe, Germany c Health Physics Serice, Raenna Hospital, Viale Randi 5, Raenna, Italy Abstract Radioactive materials are treated and transformed mainly in nuclear industries. However, non-nuclear industries use raw materials containing significant levels of natural radionuclides; the processing of these materials can expose workers and people living near such sites to radiation levels well above the natural background. This radioactivity is due to nuclides belonging to the 238 U and 232 Th decay chains and to 40 K. Inductively coupled plasma mass spectrometry ICP-MS. and gamma-ray spectrometry have been used together to determine uranium and thorium concentration and the conditions of secular equilibrium in their decay chains for samples from ceramic and phosphate-fertiliser plants. In fact, the knowledge of the secular equilibrium conditions is necessary in order to make correct assumptions for the dose assessment. The results indicate that the secular equilibrium is verified for the samples from ceramic plants, whereas a situation of disequilibrium between parents and daughters of the natural chains is detected in phosphate fertilisers Elsevier Science B.V. All rights reserved. Keywords: Gamma-ray spectrometry; ICP-MS; Natural radioactivity; Phosphate-fertiliser; Zircon mineral 1. Introduction Over the last several decades the population in most countries has been growing in urban areas, increasingly aggravating the typical environmental problems of cities air and soil quality, supply of drinkable water, wastewater treatment, solid waste Corresponding author. disposal, noise, etc... In 1900, less than 14% of the world s people lived in cities; now approximately 47% do. On the threshold of third millennium almost 3 billion people live in big cities 1. The strong industrialisation that normally occurs with intense urbanisation heightens health and environmental problems of cities. In this way the impacts of urbanisation and industrialisation are merged. The health effects of noise, air and water pollution, occupational exposure, etc., are not X00$ - see front matter 2000 Elsevier Science B.V. All rights reserved.. PII: S X

2 120 ( ) S. Righi et al. Microchemical Journal easily quantified. Many of these effects are not particularly harmful in isolated contacts, but continued exposure to low-level environmental pollution may be a much more serious problem. Consequently there has been increasing attention to the many non-nuclear industries which have the capability for low-level but consistent exposure to ionising radiation. Such industries are capable of generating significant critical group or collective exposures or both; doses can be delivered both to plant workers and to populations living in the neighbourhoods of plants. Thorium-, zirconiumand titanium-mineral processing plants treating monazite, zircon, rutile, ilmenite, etc.., phosphate-ore industries producing fertilisers, detergents and acids., coal power plants, oil and gas extraction facilities and to some extent the ceramic industry are examples of these types of industries. The ubiquity of radioactivity in natural materials e.g. air, water, minerals, fossil fuels, and wood. has been recognised for many years 2. The levels of radioactivity, higher than natural background, which can occur in raw materials, products and wastes of above indicated industries, is due to enhancement of natural radionuclides by industrial processes. Table 1 shows the specific activity of 238 U and 232 Th in the Earth s crust and in some raw materials used by the industries above-mentioned; the values are based on bibliographic data 36. The comparison among the specific activities of various materials indicates that the specific activities of uranium-238 and thorium-232 are, in raw materials, up to two orders of magnitude higher than those of the earth crust. To distinguish between NORM naturally occurring radioactive materials. exposures that are strictly natural terrestrial and cosmic radiation. and NORM exposures that result from man s industrial activity, Gesell and Prichard in suggested the term TENR technologically enhanced natural radioactivity.. Since 1975 the TENR exposure has been investigated, and there has been a growing awareness of the problem. The EC legislation regarding radiation protection devotes an entire section of one of its Directives Dir. 9629Euratom, Title Table 1 Specific activity in the earth s crust and raw materials Material Activity Bq kg. U Th Earth crust Ilmenite Bauxite ore Copper ore Coal Zircon sand Phosphate rock VII. to treat the exposure to natural radiation sources and the EC has recently issued recommendations and reference levels for the implementation of Title VII of EC Directive. For radiological protection, all the nuclides of uranium and thorium series are relevant. However, industrial processing often disturbs the secular equilibrium that characterises the original sources of radioactivity with substantial negative consequences. This indicates that a simple activity determination of a limited number of radionuclides belonging to a radioactive decay chain would not suffice. For effective assessment it is necessary to ascertain the secular-equilibrium conditions and the entire spectrum of radionuclides. High-resolution gamma-ray spectrometry is widely used to studying natural radioactivity because provides a fast, multielemental and non-destructive method of radioactivity measurement 81. Although widely utilised, this technique does not permit direct determination of 238 U and 232 Th because both are very weak gammaemitters; usually, 238 U and 232 Th concentrations are determined from their decay products assuming secular equilibrium between parents and daughters 25. In this study, inductively coupled plasma mass spectrometry ICP-MS. has been used as a complementary technique to gamma-spectrometry to determine the conditions of secular equilibrium. The activities of U and Th daughters mea- sured by gamma-spectrometry. have been compared with those of the parents as determined by ICP-MS. This paper describes the application of

3 ( ) S. Righi et al. Microchemical Journal this procedure to the analysis of materials involved in two types of non-nuclear industries for which TENR is an issue: zirconium minerals used in the ceramic industry and the products of phosphate-fertiliser plants. 2. Materials and methods 2.1. Material samples The analyses were carried out on six phosphate fertiliser and six zirconium mineral samples. The selected zirconium minerals are utilised in the Italian ceramic industry for the production of ceramic colours, glazes, tiles and dinnerware. The analysed phosphate fertilisers are marketed in Italy and used for agriculture. Table 2 lists several characteristics of the examined samples. The samples represented two different types of zircon minerals zircon and badelleyite. and three different types of phosphate fertilisers ammonium phosphate, triple superphosphate and a blend containing 14% of PO Sample preparation Samples were dried, milled and homogenised 6. Aliquots for measurement by gamma-ray spectrometry were placed in 1000-ml Marinelli beakers and sealed for approximately 30 days before counting to re-establish the radioactive equilibrium between 226 Ra and its daughters products due to possible escape of 222 Rn during handling. Aliquots 0.1 g. for ICP-MS analysis were transferred to polytetrafluoroethhylene vessels. A mixture of HNO HFH O 10:3:3 ml. 3 2 for phosphate-containing samples and a mixture of HFHClO 8:2 ml. 4 with the subsequent addition of HCl 2 ml., for zirconium mineral samples, were added. All acid used were concentrated. The vessels were closed and placed in autoclave at 120C for 48 h and then cooled. The solutions were transferred into a perfluoralkoxy beaker, evaporated to dryness and the residue redissolved in 10 ml 1% vv HNO solution. H BO 0.9 M was added to the solution to complex any F. All chemicals used were suprapure Gamma-ray spectrometry A high-purity germanium HPGe. detector di- ameter 54 mm, length 48.5 mm and volume 109 cm 3., connected to a multichannel analyser, was used for the measurement of the gamma-ray activity of each sample. The FWHM full width at half maximum. of the detector is 1.9 kev at 1.33 MeV 60 Co. with 22.6% efficiency. The detector is placed in a well consisting of a 10-cm-thick layer of lead lined with cadmium-copper coating at the inner side. The spectrometer was calibrated using as source a water solution of 57 Co, Table 2 Characteristics of examined samples Sample Description Product Origin 1 Zircon Zirconium mineral Australia 2 Zircon Zirconium mineral Australia 3 Zircon Zirconium mineral Australia 4 Badelleyite Zirconium mineral Russia 5 Badelleyite Zirconium mineral South Africa 6 Badelleyite Zirconium mineral South Africa 7 Diammonium phosphate Fertiliser 8 Ammonium phosphate Fertiliser 9 Triple superphosphate Fertiliser Turkey 10 Triple superphosphate Fertiliser Turkey 11 Triple superphosphate Fertiliser 12 Blend 14% P O, 24% K O, 13% Ca. Fertiliser Italy 2 5 2

4 122 ( ) S. Righi et al. Microchemical Journal Co, 85 Sr, 88 Y, 109 Cd, 113 Sn, 139 Ce, 137 Cs, 241 Am emitting -rays with energy ranging between 59.5 and kev. The solution was prepared and certified by LMRI France.. The gamma-ray spectrometer is equipped with software Silena GammaPlus. for data acquisition and analysis. The measured count-rates were corrected by a factor that takes into account the self-attenuation of -rays in the sample 7. Nuclides 234 Th, 214 Bi, 228 Ac and 212 Pb were chosen for the radioactivity determination. The evaluation is based on mean of the most significant photopeaks of each radionuclide: 234 Th 63.29, 92.8 and kev., 214 Bi , and kev., 228 Ac , and kev., 212 Pb , kev.. The means were calculated from the photopeak amplitudes taking into account their relative percent yield. The counting time for each sample was s ap- prox. 3 h Inductiely coupled plasma mass spectrometry An Elan 5000 Perkin-Elmer SCIEX. Thornhill, Table 3 Instrumental conditions for ICP-MS Rf power W Argon outer gas flow 15 rate l min. Argon intermediate gas flow 0.8 rate l min. Argon nebuliser gas flow 1.1 rate l min. Nebuliser type Cross flow Spray chamber Scott type, double pass, Rayon Load coil-sampler 15 fixed. distance mm. 3 Quadrupole working pressure Pa. Sampler and skimmer Platinum cones. Setting % of range Ion lens Actinides Bessel box B 55 Bessel box P 50 Einzel lens E1 30 Photon stop S2 35 Table 4 Data obtained by ICP-MS Sample Product U Th ppm. ppm. 1 Zircon % 2 Zircon % 3 Zircon % 4 Badelleyite % 5 Badelleyite % 6 Badelleyite % 7 Diammonium phosphate % % 8 Ammonium phosphate % % 9 Triple superphosphate % 3.9.5% 10 Triple superphosphate % % 11 Triple superphosphate % 6.3.3% 12 Blend % % Ontario, Canada. ICP-MS instrument, modified for installation in a glove-box as described previ- ously 18, has been used for the U and Th determinations. The only departure from the protocols described in 8is that the samples were not introduced into the glove-box, since the radioactivity of the samples was low-level Bqkg.. The ICP-MS operating conditions are summarised in Table Results and discussion The 238 U concentrations in zirconium-mineral samples and the 238 U and 232 Th concentrations in phosphate-fertiliser samples determined by ICP- MS are listed in Table 4. The 232 Th concentrations in zirconium-mineral samples are not reported because in some cases the measurements did not show repeatability, probably because of a matrix effect which was not possible to eliminate. Table 5 shows the 238 U and 232 Th concentrations converted to specific activities. The specific activities of 234 Th, 214 Bi, 228 Ac and 212 Pb in zirconium-mineral and phosphate-fertiliser samples determined by gamma-ray spectrometry are given in Table 6. It appears that the specific activities of the materials vary considerably, probably depending on their origin particularly for zirconium miner-

5 ( ) S. Righi et al. Microchemical Journal Table 5 Specific activities obtained by concentrations measured by ICP-MS Sample Product U Th Bq kg. Bq kg. 1 Zircon % 2 Zircon % 3 Zircon % 4 Badelleyite % 5 Badelleyite % 6 Badelleyite % 7 Diammonium phosphate % 750.6% 8 Ammonium phosphate % 440.9% 9 Triple superphosphate % 16.5% 10 Triple superphosphate % 460.8% 11 Triple superphosphate % 24.3% 12 Blend % % als. and the type of processing employed particu- larly for fertilisers.. Among the same types of material zirconium minerals or fertilisers. the activities of the nuclides belonging to the 232 Th decay chain range over approximately one order of magnitude whereas those ones of 238 U decay chain are included in a more limited range 24 times.. The 238 U concentrations in zirconium minerals ppm., converted to specific activities Bqkg., are in good agreement with the specific activities of 234 Th and 214 Bi, ranging between Bqkg and Bqkg, respectively see Fig. 1.. There is good agreement between the specific activities of 228 Ac Bqkg and Pb Bqkg. too. From the above results, it is apparent that the secular equilibrium has been preserved in 238 U decay chain from 238 Uto 214 Bi and in 232 Th decay chain from 228 Ac to 212 Pb. In spite of the fact that the entire chains have not been investigated, it is reasonable to hypothesise that secular equilibrium has been preserved in both chains. The situation for the fertiliser samples is de finitely different see Fig. 2. The U concentrations in phosphate fertilisers 6045 ppm., converted to specific activities Bqkg., show values slightly higher than specific activities of Th Bqkg; ratio Th U and remarkably higher than Bi specific activities Bqkg; ratio Bi U The Th concentrations in the fertilisers 458 ppm., converted to specific activity Bqkg., show values remarkably higher 228 than Ac specific activities 390 Bqkg; ratio Ac Th but in good agreement 212 with Pb specific activities Bqkg.. It is clear that during the fertiliser-production processing secular equilibrium is broken. The industrial processing induces a separation between U and Ra the last one is selectively removed from fertiliser., whereas it appears that Th behaves like U The long half-life of Ra 1600 y. does not permit the re-establishment of secular equilibrium in 238 U chain. In the 232 Th chain the re- Table 6 Data obtained by gamma-ray spectrometry Sample Product Th Bi Ac Pb Bq kg. Bq kg. Bq kg. Bq kg. 1 Zircon % % % % 2 Zircon % % % % 3 Zircon % % % % 4 Badelleyite % % % % 5 Badelleyite % % % % 6 Badelleyite % % % % 7 Diammonium phosphate % % % 620.6% 8 Ammonium phosphate % % 118.6% 387.2% 9 Triple superphosphate % % % 126.6% 10 Triple superphosphate % % 176.5% 556.2% 11 Triple superphosphate % % % 227.3% 12 Blend % % 909.6% %

6 124 ( ) S. Righi et al. Microchemical Journal Fig. 1. Specific radioactivity of 238 U, 234 Th, 214 Bi, 228 Ac and 212 Pb in zirconium-mineral samples. moval of Ra produces an interruption of secular equilibrium between 232 Th and 228 Ac because of 228 the long half-life of Ra 5.8 y., whereas the short half-life of 224 Ra permits the re-establishment of equilibrium from 228 Th to 212 Pb. It is reasonable to presume that the behaviour of 232 Th and 228 Th during the fertiliser-production process is the same. Moreover the specific activity of 232 Th and 228 Th will maintain the same values, even though the equilibrium is interrupted in between, because of their long half-life. This explains the good agreement between the specific activities of 232 Th and 212 Pb. 4. Conclusions The results indicate that, in the monitoring of natural radioactivity due to zirconium minerals in working places of ceramic applications, the specific activity values of most intense peaks of some decay chain members Bi for U chain and Ac for Th chain, measured by gamma-ray spectrometry, are representative of all the members. This is due to the secular equilibrium that probably pertains to the 238 U and 232 Th decay chains. On the other hand, for the phosphate-fertilisers, gamma-ray analysis is not able to produce values representative of all members of their chains because the industrial processing modifies the concentrations of the different radionuclides belonging to natural decay chains. In this case, it is possible to verify the 226 Ra and 224 Ra concen trations via Bi and Pb, respectively. and estimate the 232 Th concentration assuming: the same behaviour between 232 Th and 228 Th; and the re-establishment of secular equilibrium between 228 Th and 212 Pb. In short, this study indicates that the ICP-MS

7 ( ) S. Righi et al. Microchemical Journal Fig. 2. Specific radioactivity of 238 U, 234 Th, 214 Bi, 232 Th, 228 Ac and 212 Pb in phosphate-fertiliser samples. can assume an important role in the monitoring of natural radioactivity in the working place for two reasons: this technique permits the determination of the 238 U and 232 Th concentrations which are not determinable directly by gamma-ray spectroscopy because these two radionuclides are both very weak gamma-emitters; and the 238 U and 232 Th determination allows the assessment of the conditions of secular equilibrium and thus establishes whether monitoring by gamma-ray spectroscopy is adequate for determination of total activities. References 1 World Resources Institute, World Resources A guide to the Global Environment, Oxford University Press, United Nations Scientific Committee on the Effects of Atomic Radiation, United Nations, New York, NCRP, Measurement of radon and radon daughters in air, NCRP Report 97, Bethesda, Maryland, USA, United Nations Scientific Committee on the Effects of Atomic Radiation, Report of UNSCEAR to the General Assembly, United Nations, New York, C.E. Roessler, Z.A. Smith, W.E. Bolch, R.J. Prince, Health Phys M. Eisenbud, Environmental Radioactivity, Academic Press, New York, T.F. Gesell, H.M. Prichard, Health Phys D. Brajnik, M. Krizman, I. Kobal, P. Stegnar, Radiat. Prot. Dos A.S. Murray, M.J. Aitken, Appl. Radiat. Isot G.C. Kerrigan, B.H. O Connor, Health Phys F. El-Daoushy, R. Garcia-Tenorio, Nucl. Instrum. Methods A M. Bergamini, R. Borio, G. Campos Venuti et al., Sci. Total Environ G. Johnston, Health Phys

8 126 ( ) S. Righi et al. Microchemical Journal D.J. Allard, Health Phys L. Bruzzi, M. Baroni, R. Mele, E. Nanni, J. Radiol. Prot L. Bruzzi, M. Baroni, G. Mazzotti, R. Mele, S. Righi, J. Environ. Radioact K. Debertin, J. Ren, Nucl. Instrum. Methods A M. Betti, J.I. Garcia Alonso, P.h. Arbore, L. Koch, in: G. Holland, A.N. Eaton Eds.., Applications of Plasma Source Mass Spectrometry, Royal Society of Chemistry, Cambridge, 1993, pp R. Mustone, Sci. Total Environ