TY - JOUR
AU - Potiriadis,, Constantinos
AB - Abstract This work evaluates the 137Cs and 90Sr content of wood pellets of the Greek energy market and additionally calculates worker exposure in Greek catering establishments (rotisseries). Wood pellets and ash samples were analysed through high resolution gamma-ray spectroscopy and liquid scintillation. Imported pellets had higher 137Cs concentrations, especially pellets that were imported in bulk. Greek pellets had very smaller 137Cs amounts. Despite the high variation in 137Cs content the 137Cs exemption levels were not exceeded in any case. However, if an enrichment factor of 300 is assumed, some ashes samples may exceed 13000 Bq/kg. The highest amounts of 90Sr were measured in ashes from pellets manufactured abroad in 2015, with up to 1060 Bq/kg. A linear regression model of the ratio of 137Cs to 90Sr activity followed our data well, and other sets of data partially. A dosimetry assessment of kitchen employees revealed that maximum possible dose intake reached 400 nSv/year. INTRODUCTION Domestic use of wood pellets expanded nowadays worldwide due to energy shortages and global aims to reduce CO2 emissions(1). Pellet by-products can also find usage. Their ashes, for example can be used as soil additives(2). There are three pellet quality types(3): Improved (ash content between 0.5 and 1% w/w), standard (ash between 1 and 2% w/w) and industrial (ash over 3% w/w). Quality standards for their manufacture exist, such as DIN 51 731(4). Man-made radionuclides accumulate in wood after their release during nuclear accidents and atomic weapons detonations, as well as after the Chernobyl disaster(5–7). Radionuclides in biomass can mobilise during combustion through suspension in the atmosphere(8). The issue of 137Cs in wood pellets emerges from the fact that combustion and production of ash can enrich the concentration of the radionuclides that existed—ab initio in the wood. Thus, radiological protection issues could emerge. In 2009, Italian authorities renounced 10 000 tons of Lithuanian pellets after an investigation that revealed that they contained 137Cs, up to 320 Bq/kg, and even up to 46 000 Bq/kg for an ash sample(9) (Pellets supplying Italian market are mostly manufactured abroad(10)). Following studies however, assessed dose for civilians lower than the radiological concern levels of 10 μSv/year(11, 12). Previous Lithuanian studies found up to 4 ± 1 Bq/kg(13) of 137Cs in the pellets, and up to 150 Bq/kg in the wood ash(14) for pellets manufactured within Lithuania. However, 137Cs and 90Sr is controlled in Lithuania as exception levels(15, 16) and 137Cs levels in building materials(17, 18). At present there are no radiological restrictions for pellets in Europe. The 137Cs constraint of 1000 Bq/kg, adopted by Italy(19) is applied explicitly to nuclear waste and pellet ash cannot be regarded as such. Italian pellet producers, based on the UNI/TS 11 263:2007 quality standard, trademarked a stricter standard, named ‘Pellet-Gold’(20, 21). In this, they set a limit of 4 Bq/kg for the total pellet radioactivity albeit with no reference for specific radionuclides. In Finland, the general action limit of 1 mSv/year is also set for workers handling ashes(22). In Sweden, for power plants that generate more than 30 tons of dry ash/year, deteriorating measures are established. I.e for soil fertilisation, 137Cs must be under 10 000 Bq/kg of dry mass(23). In Greece, pellets were introduced very lately and rapidly with various imported and domestic brands. Fuel consumption data for Greece exist for 2013. During this year, about 1.5% of households used pellets as their main heating fuel(24). Since then, pellet consumption has obviously increased. Another market turnaround of catering sector professionals also took place in favour of charcoal briquettes and pellets, rather than traditional wood charcoal. Based on the above, this work is an evaluation of the contamination of wood pellets that are traded in the Greek market and additionally an exposure assessment of workers of catering establishments that may be using contaminated wood as fuel. MATERIALS AND METHODS From 2015 to 2017, a radiological survey was carried out by the Greek Atomic Energy Commission. Thirty-five samples of wood pellets and ashes were collected and analysed, each one from a single bulk. Representative samples of pellet brands were taken, from a trade site of the Attic Basin greater area, where the capital city, Athens sits. The ashes of the samples were generated in a commercial pellet burner, and were collected without any further specific treatment. Caesium analysis 137Cs activity concentration was measured by high resolution gamma-ray spectroscopy. The instruments used are based on intrinsic hyper-pure Germanium (HPGe) detectors with relative efficiencies of 50% and 70%. The efficiency and energy calibrations were obtained by means of certified gamma reference standards of mixed radionuclides (Certified point sources as well as certified volume sources of standard density). The samples were crushed, homogenised and put in standard 260 ml beakers of predefined geometry (the standard box of the Department of Environmental Radioactivity Monitoring of the Greek Atomic Energy Commission). All the measurements were performed with a low background configuration. After sample preparation, the samples were measured in two HPGe detectors. The measured concentrations of the natural radionuclides for the major natural radioactive series, as well as the concentration of the detected artificial radionuclides are given. Measurements took place for at least 8 hours each. The minimum detectable activity (MDA) was calculated using the Curie method at 95% confidence interval(25). The detection limit was estimated as 0.5 Bq/kg for 137Cs. 137Cs was measured by the activity of the peak at 661.6 keV. Strontium analysis—pellets—sample preparation About 2.5 g of each sample was analysed. Thirty mg of strontium nitrate were added to gravimetrically calculate the chemical recovery at the end of the process. The samples were wet-ashed on crucibles by consecutive addition of HNO3 65% w/w and HCl acid 37% w/w and finally they were dry-ashed at 500° C for 5 hours to remove the organic materials. Radionuclide separation Two radioactive isotopes of strontium (90Sr and 89Sr) are of significance in radiological measurements in the environment. 90Sr (half-life 28.1 y) is the more significant in terms of environmental impact. It is a beta emitter with only one decay mode leading to 90Y. 89Sr (half-life 52 d) also decays as a beta emitter to stable 89Y. The only radioactive strontium isotope that should be expected within the final solution is 90Sr, which originates from the Chernobyl fallout as well as atomic weapons tests(26). A small amount of each sample was analysed. An aliquot of strontium carrier was added to calculate the chemical recovery. After wet and dry ashing, the residue is diluted into 3 N HNO3 and 90Sr was isolated via extraction chromatography by the use of a resin (SrSpec resin from Triskem)(27). The samples were measured by liquid scintillation counting (Quantulus) after 18 d when the equilibrium between 90Sr/90Y is achieved. The counting time was 600 min. Measurement by liquid scintillation The samples were measured by liquid scintillation counting (Quantulus) after 18 d when the equilibrium between 90Sr/90Y is achieved. The counting time was 600 min. RESULTS AND DISCUSSION Caesium results and calculations Table 1 lists the samples analysed, the indicative country of origin (The list of countries is at the disposal of the authors) and any other available details (year of production, exact region of production etc.). A specimen of imported wood charcoal from an African country was included in the analysis as a reference material. The results, thereof, are summarised in Figure 1. Table 1. Codification of samples, country of origin and other details. Sample code Sample origin Sample characteristics P1 Country A Pellets of 2014 production P2 Country A Pellets of 2014 production P3 Country A Pellets of later 2016 P4 Country A Pellets of 2015 production P5 Country A Imported in Greece in Bulk by a Greek company P6 Country A Pellets of early 2016—higher quality P7 Country A Pellets of early 2016—higher quality P8 Country A Pellets of later 2016 P9 Country A Pellets of later 2016 P10 Country A Pellets of later 2016—higher quality P11 Country A Pellets of 2015 production P12 Country A Pellets of early 2016 P13 Country A Pellets of early 2016 P14 Country A Beech Wood Pellets P15 Country A Pellets of early 2016 P16 Country A Pellets of later 2016 P17 Country A Pellets of later 2016—higher quality P18 Country A Pellets of later 2016 P19 Country A Pellets of later 2016 P20 Country B Mountainous region in SW Of Country B P21 Country B Mountainous region in SW Of Country B P22 Country C Imported in bulk by a Greek company P23 Country C Produced in SW region of Country C P24 Greece Wood pellets P25 Greece Pellets from north East Region of Greece P26 Greece Pellets from Central Greece P27 Control sample Carbonized pellets of African origin Sample code Sample origin Sample characteristics P1 Country A Pellets of 2014 production P2 Country A Pellets of 2014 production P3 Country A Pellets of later 2016 P4 Country A Pellets of 2015 production P5 Country A Imported in Greece in Bulk by a Greek company P6 Country A Pellets of early 2016—higher quality P7 Country A Pellets of early 2016—higher quality P8 Country A Pellets of later 2016 P9 Country A Pellets of later 2016 P10 Country A Pellets of later 2016—higher quality P11 Country A Pellets of 2015 production P12 Country A Pellets of early 2016 P13 Country A Pellets of early 2016 P14 Country A Beech Wood Pellets P15 Country A Pellets of early 2016 P16 Country A Pellets of later 2016 P17 Country A Pellets of later 2016—higher quality P18 Country A Pellets of later 2016 P19 Country A Pellets of later 2016 P20 Country B Mountainous region in SW Of Country B P21 Country B Mountainous region in SW Of Country B P22 Country C Imported in bulk by a Greek company P23 Country C Produced in SW region of Country C P24 Greece Wood pellets P25 Greece Pellets from north East Region of Greece P26 Greece Pellets from Central Greece P27 Control sample Carbonized pellets of African origin Table 1. Codification of samples, country of origin and other details. Sample code Sample origin Sample characteristics P1 Country A Pellets of 2014 production P2 Country A Pellets of 2014 production P3 Country A Pellets of later 2016 P4 Country A Pellets of 2015 production P5 Country A Imported in Greece in Bulk by a Greek company P6 Country A Pellets of early 2016—higher quality P7 Country A Pellets of early 2016—higher quality P8 Country A Pellets of later 2016 P9 Country A Pellets of later 2016 P10 Country A Pellets of later 2016—higher quality P11 Country A Pellets of 2015 production P12 Country A Pellets of early 2016 P13 Country A Pellets of early 2016 P14 Country A Beech Wood Pellets P15 Country A Pellets of early 2016 P16 Country A Pellets of later 2016 P17 Country A Pellets of later 2016—higher quality P18 Country A Pellets of later 2016 P19 Country A Pellets of later 2016 P20 Country B Mountainous region in SW Of Country B P21 Country B Mountainous region in SW Of Country B P22 Country C Imported in bulk by a Greek company P23 Country C Produced in SW region of Country C P24 Greece Wood pellets P25 Greece Pellets from north East Region of Greece P26 Greece Pellets from Central Greece P27 Control sample Carbonized pellets of African origin Sample code Sample origin Sample characteristics P1 Country A Pellets of 2014 production P2 Country A Pellets of 2014 production P3 Country A Pellets of later 2016 P4 Country A Pellets of 2015 production P5 Country A Imported in Greece in Bulk by a Greek company P6 Country A Pellets of early 2016—higher quality P7 Country A Pellets of early 2016—higher quality P8 Country A Pellets of later 2016 P9 Country A Pellets of later 2016 P10 Country A Pellets of later 2016—higher quality P11 Country A Pellets of 2015 production P12 Country A Pellets of early 2016 P13 Country A Pellets of early 2016 P14 Country A Beech Wood Pellets P15 Country A Pellets of early 2016 P16 Country A Pellets of later 2016 P17 Country A Pellets of later 2016—higher quality P18 Country A Pellets of later 2016 P19 Country A Pellets of later 2016 P20 Country B Mountainous region in SW Of Country B P21 Country B Mountainous region in SW Of Country B P22 Country C Imported in bulk by a Greek company P23 Country C Produced in SW region of Country C P24 Greece Wood pellets P25 Greece Pellets from north East Region of Greece P26 Greece Pellets from Central Greece P27 Control sample Carbonized pellets of African origin Figure 1. View largeDownload slide Results of measurements on 137Cs in pellets. Figure 1. View largeDownload slide Results of measurements on 137Cs in pellets. Figure 1 shows the 137Cs activity values in the pellets. From the above depicted measurements, it is obvious that pellets of Country-A origin had the highest activity concentrations in 137Cs, and especially pellets that have been manufactured in the years of 2015 and 2016. Pellets of Country A of 2014 had considerably lower activity values. A very high activity concentration of 137Cs was noted for pellets of Country-C origin that were imported in bulk and traded by a Greek company (Figure 1). It is disputable on whether the raw material for the production of the pellets actually came from the stated countries as listed on the pellet labels (Country-C is a very characteristic example). This is partially due to the fact that other pellets of the same country of origin, with different production date, showed a lot lower amounts of 137Cs (1.1 Bq/kg in pellets of south west Region of Country-C for example). Additionally pellets of Country-B origin, of a mountainous region south west in particular, had very small amounts of radioactivity. This is despite the fact that this country is a northern European country, so higher amounts of radioactivity are expected. Pellets that were produced in Greece from wood that also grew in Greece had very small amounts of 137Cs as well. Assuming that during the combustion of biomass the remaining ash residues are at least 3% w/w ash (industrial type of pellets), 137Cs may appear enriched in ash as much as 33 times approximately. If it is assumed that ash is about 0.5% by weight of the total pellet mass, (improved pellet type) 137Cs may appear enriched in ash as much as 200 times. The highest potential 137Cs activity value possible in the ashes of the pellets would also be the Country C pellets. The level of 137Cs activity could potentially reach a level of 8200 Bq/kg in ash. Even in this case however, the above indicates that the 10 000 Bq/kg for 137Cs, the exemption levels for 137Cs as set by the International Atomic Energy Agency(28) are definitely not exceeded. The same also applies for their ashes. In the example nevertheless where the apparent activity of 137Cs exceeds 8 000 Bq/kg, it should be investigated whether and to what extent, this constitutes a risk to human health. Even in this case however, the above indicates that the 10 000 Bq/kg for 137Cs, the exemption levels for 137Cs as set by the International Atomic Energy Agency(29) are definitely not exceeded. The same also applies for their ashes. In the example nevertheless where the apparent activity of 137Cs exceeds 8000 Bq/kg, it should be investigated whether and to what extent, this constitutes a risk to human health. However if an enrichment factor of 300 is assumed (a very high enrichment of 330 has also been calculated, however further validation of this result is still needed), then there are examples of pellets that would produce ashes with 137Cs content, higher than 10 000, such as the Country A pellets of early 2016 (P15 sample) with activity level of 33 ± 2.5 Bg/kg, or the example of the pellets that are produced in Country C and imported in bulk and packaged by a Greek company (P22 sample). The latter ones could exceed 13 000 Bq/kg. Strontium results and calculations As far as strontium is concerned, the analysis of the ashes of the pellets and the equivalent information about the pellet origin are presented in Table 2. Table 2 shows a codification of the origin of the pellets, the caesium measured in the pellets and exactly below in each row, the caesium and strontium measured in the ash of the pellets, and the enrichment factor of 137Cs (the ratio of caesium content of ash to pellets). The highest amounts of strontium are measured in ashes from pellets of Country A that were produced in 2015 and the values reach up to 1060 Bq/kg. The lowest amounts are found in pellets produced in Country C and reach about 100 Bq/kg. The enrichment factor of 137Cs was found ranging from 50 to 250, with an exception of a beech wood pellets sample where it reached 330. The enrichment factor is comparable with the enrichment factor that was assumed by the equivalent ash content of the three categories of pellet quality as explained before (50–200). Table 2. 137Cs measured in the pellets, 137Cs and 90Sr measured in the ash of the pellets, and the enrichment factor for 137Cs. Csash/Cspellet 137Cs 90Sr Enrichment (Bq/kg) (Bq/kg) factor Country C Produced in South Region of Country C (industrial quality) 1.1 ± 0.4 Ash 40 ± 4 100 ± 4 36 Greece Pellets made in North Eastern Greece (standard quality) 0.9 ± 0.2 Ash 80 ± 6 202 ± 6 90 Country A Beech WoodPellets (improved quality) 0.3 ± 0.2 Ash 100 ± 12 175 ± 6 330 Country A Pellets of early 2016 (standard quality) 5.5 ± 0.7 Ash 380 ± 25 340 ± 10 70 Greece Pellets made in Central Greece (improved quality) 4 ± 1 Ash 956 ± 120 670 ± 18 239 Country A Pellets of 2015 (improved quality) 13.5 ± 1.1 Ash 3355 ± 945 715 ± 20 250 Country A Pellets of2015 (improved quality) 20 ± 2 Ash 5060 ± 435 1030 ± 30 253 Csash/Cspellet 137Cs 90Sr Enrichment (Bq/kg) (Bq/kg) factor Country C Produced in South Region of Country C (industrial quality) 1.1 ± 0.4 Ash 40 ± 4 100 ± 4 36 Greece Pellets made in North Eastern Greece (standard quality) 0.9 ± 0.2 Ash 80 ± 6 202 ± 6 90 Country A Beech WoodPellets (improved quality) 0.3 ± 0.2 Ash 100 ± 12 175 ± 6 330 Country A Pellets of early 2016 (standard quality) 5.5 ± 0.7 Ash 380 ± 25 340 ± 10 70 Greece Pellets made in Central Greece (improved quality) 4 ± 1 Ash 956 ± 120 670 ± 18 239 Country A Pellets of 2015 (improved quality) 13.5 ± 1.1 Ash 3355 ± 945 715 ± 20 250 Country A Pellets of2015 (improved quality) 20 ± 2 Ash 5060 ± 435 1030 ± 30 253 Table 2. 137Cs measured in the pellets, 137Cs and 90Sr measured in the ash of the pellets, and the enrichment factor for 137Cs. Csash/Cspellet 137Cs 90Sr Enrichment (Bq/kg) (Bq/kg) factor Country C Produced in South Region of Country C (industrial quality) 1.1 ± 0.4 Ash 40 ± 4 100 ± 4 36 Greece Pellets made in North Eastern Greece (standard quality) 0.9 ± 0.2 Ash 80 ± 6 202 ± 6 90 Country A Beech WoodPellets (improved quality) 0.3 ± 0.2 Ash 100 ± 12 175 ± 6 330 Country A Pellets of early 2016 (standard quality) 5.5 ± 0.7 Ash 380 ± 25 340 ± 10 70 Greece Pellets made in Central Greece (improved quality) 4 ± 1 Ash 956 ± 120 670 ± 18 239 Country A Pellets of 2015 (improved quality) 13.5 ± 1.1 Ash 3355 ± 945 715 ± 20 250 Country A Pellets of2015 (improved quality) 20 ± 2 Ash 5060 ± 435 1030 ± 30 253 Csash/Cspellet 137Cs 90Sr Enrichment (Bq/kg) (Bq/kg) factor Country C Produced in South Region of Country C (industrial quality) 1.1 ± 0.4 Ash 40 ± 4 100 ± 4 36 Greece Pellets made in North Eastern Greece (standard quality) 0.9 ± 0.2 Ash 80 ± 6 202 ± 6 90 Country A Beech WoodPellets (improved quality) 0.3 ± 0.2 Ash 100 ± 12 175 ± 6 330 Country A Pellets of early 2016 (standard quality) 5.5 ± 0.7 Ash 380 ± 25 340 ± 10 70 Greece Pellets made in Central Greece (improved quality) 4 ± 1 Ash 956 ± 120 670 ± 18 239 Country A Pellets of 2015 (improved quality) 13.5 ± 1.1 Ash 3355 ± 945 715 ± 20 250 Country A Pellets of2015 (improved quality) 20 ± 2 Ash 5060 ± 435 1030 ± 30 253 Relation of Caesium to Strontium The relation of caesium to strontium concentration was further explored. In this graph (Figure 2) the ratio of 137Cs to 90Sr activity is plotted against 137Cs activity, and together with the results of our work (bottom left) we included the results of Desideri et al.,(30) (top left) and Ladygiene et al.(31) (top right). The combined results of all three studies are also shown in Figure 2 (bottom right). Figure 2. View largeDownload slide Ratio of 137Cs to 90Sr activity against 137Cs activity. Results of our work (bottom left), results of Desideri et al. (top left) results of Ladygiene et al. (top right), and combined results (bottom right). Figure 2. View largeDownload slide Ratio of 137Cs to 90Sr activity against 137Cs activity. Results of our work (bottom left), results of Desideri et al. (top left) results of Ladygiene et al. (top right), and combined results (bottom right). Distributions of 137Cs and 90Sr in wood are different depending on the territory, the origin of the wood type, even the tree part(32, 33). For instance, in Swedish wood 137Cs was ranging from 2 to 5 Bq/kg from global fallout in 2003(34). The ratio of 137Cs to 90Sr is also very different depending on the tree part. A study showed that activity of 137Cs was relatively uniform in trunk wood, indicating that 137Cs is more mobile in the wood xylem, while 90Sr showed a radial distribution correlated with fallout from atomic weapons tests in the trunks of some species(35). The terrestrial mobility of 137Cs and 90Sr is also very different(36). Metallic caesium has quite low melting point (29°C), however, Cs in the pellets and its ash is in an oxide form (a melting point of 490°C) or salt. The carbonate and chloride forms of Cs have even higher melting points of 610 and 646°C, respectively, quite close to calcium carbonate formation temperature(37). Therefore they are quite stable and similar or even equal distribution of 137Cs in the bottom ash and fly ash may be possible. The similar distribution of caesium in the bottom and fly ash is further substantiated within the context of this work, by the fact that caesium enrichment ratios in ash follow the ash content ratios in most of the pellets samples examined in this work. In our values (Figure 2, bottom left), a linear interdependence of 137Cs and 90Sr content is observed. In this data sequence, in particular, the caesium and strontium values appear inversely proportional, albeit not in absolute terms. A similar trend is observed in Desideri et al., however in Ladygiene et al., this trend is not followed in clarity. The linear regression model assumed is of the form of y = ax + b. In these results, the linear regression R2 (the ratio of the sum of squares of the model and the total sum of squares, times 100) reaches 0.99 in our data, and 0.89 in Desideri et al. However the results for Ladygiene et al., and consequently the combined results from all the above studies are not satisfactory. The materials that Ladygiene et al. used, however, are quite versatile, including pellets, briquettes, wood chips and raw cuttings. The distribution of radionuclides in different parts of the tree is not identical(38). Additionally, there is no information about the fuel quality standards followed by the pellet manufacturers. Tree parts, other than the bulk wood are possibly used in the pellet mix. Addition into the wood mix of parts such as the bark, the branches of even the leaves, is not clarified. Desideri et al., however, test solemnly pellets manufactured under specific standards. In conclusion, a linear correlation between caesium/strontium ratio and caesium activity is shown, but not sufficiently documented. Dosimetry assessment—construction of model This case study examines the task of charbroiling in catering establishments (restaurants, rotisseries, etc.) that use charcoal pellets as their primary fuel. The basic assumption is that workers are in direct contact with the hearth fumes during 20% of their working time. During the remaining 80%, they are present in the wider area of the kitchen compartment. Furthermore, it is assumed that 5% of the 8-hour shift is dedicated to hearth preparation, ie ignition time, and another 5% is devoted to the end of operation for hearth cleaning and ash handling. Indoor air pollutants emission from solid fuels combustion depends on fuel type and quality, dorm characteristics, cooking and heating methods, time–activity patterns, and ambient conditions(39) and may vary within the time frame of a day(40) or a week(41). Mass concentrations of particles in the flue gas from combustion of wood fuels have been reported(42–46) to be in the range of 30 mg/m3, with submicron particles (size < 1μm)(47) dominating. Research in rural communities where biomass fuels are used for cooking(48, 49) showed elevated levels of particulate matter in kitchens(50). Concentrations reached from 280 to 1000 μg/m3 in rural as well as suburban environments(51, 52) and even up to 3200 μg/m3(53), 50 times greater than the US ambient air quality standards for outdoor air(54). Μeat charbroiling emission rate (the ratio of aerosol mass during combustion, to the initial fuel mass) was found from 4 500 to 15 000 mg/kg(55). More versatile emission rates were also found, from 12 to even up to10000 mg/kg, depending on the charcoal type(56). Aerosol sampling, close to outdoor rotisseries that used charcoal found variations of PM2.5 concentrations of 500 to 1500 μg/m3(57). Outdoor charbroiling during fests can increase the ambient PM10 levels by approximately 5%(58). Significant concentrations of PM10 (1500 μg/m3) and PM2.5 (1200 μg/m3) were found at a Korean rotisserie in Hong Kong(59) and even higher (PM10 : 15 100 μg/m3, PM2.5 : 13 700 μg/m3) in another Korean rotisserie in Seoul(60). Other rotisserie studies measured values from up to 500 μg/m3 to 2000 μg/m3(61, 62). These levels exceed the US EPA and the European air quality standards(63) by at least two orders of magnitude. The PM levels due to indoor charbroiling may vary between 10 and 400-fold higher than the US EPA daily exposure PM limit(64) (150 μg/m3 for PM10 and 35 μg/m3 for PM2.5). Ash particles are also emitted during cleaning and handling at the end of the shift. Ash has a high propensity to become airborne, a parameter which is especially enhanced for powders of granular(65) as well elongated particle shape(66). Concentration of ash aerosols during handling is reported by IAEA(67) at around 2000 μg/m3. Additionally, wind streams in closed compartments are produced by motion of persons or objects, and temperature variations(68). Particle size, temperature and humidity also influence the above(69). These parameters were not considered in our calculations and concentration gradation was assumed as separated into two virtual compartments: The near field (NF) and far field (FF)(70). The immediate area over the hearth was considered as the NF, and the remainder of the kitchen compartment, as FF. Homogenous mixing was assumed in both compartments. The homogenous mixing approach may lead to obvious calculation errors due to convection as a result of body temperature, formation of wakes or arm movements(71–73). These phenomena can be partially simulated with computational fluid dynamics(74). However the processes determining these airflows are poorly understood(75) and only theoretical assumptions can be made about their effects(76). Furthermore, the distribution preference of 137Cs and 90Sr among bottom and fly ash may depend on a vast number of parameters such as wood species, age of trees, and combustion conditions(77–79). A study about peat combustion, showed that 137Cs was preferentially absorbed by bottom ash, and only a small fraction of it by fly ash (almost 100–15%, respectively). In this particular study however a large scale fluidised bed reactor was used. Flue gas treatment affects the 137Cs adsorption preference in the fly and bottom ash. Moisture may play a significant role in the post combustion conditions as the ash has pozzolanic properties(80) and may be beneficial as well as counter-beneficial for 137Cs sorption. Pyrogenic silica may absorb 137Cs and in alternate conditions, it may release it back again to the environment(81). The 137Cs that is bound to the cementite structures is prone to escape into the environment under humid conditions(82). As far as strontium is concerned, a study of distribution of Sr element in fly ash and bottom ash of from coal-fired power plants, revealed that strontium is preferably distributed to fly ash rather than bottom ash(83). Other studies in coal fired plants however, showed equal distribution of Sr in bottom and fly ash(84). Strontium is distributed almost equally in the various granular fractions of ash, with a slight preference to smaller fractions(85). Due to the above, equal distribution preference of 90Sr and 137Cs among bottom ash and fly ash was considered within the context of this work, for the following dosimetry assessment. Dosimetry assessment In our case, the NF and the FF were considered as two isolated compartments where aerosol is dispersed homogeneously, but with different particle concentrations. A moderate scenario of average values was considered, of 2000 μg/m3 as the assumed PM concentration in the NF, and 1000 μg/m3 in the FF, during charcoal ignition, 1500 and 500 μg/m3, respectively for main operation, and 2000 and 1000 μg/m3 respectively for ash handling. A second scenario of extreme values was considered, with the only difference that during charbroiling an extreme value of 15 000 μg/m3 for the near field and 7 500 μg/m3 for the far field was assumed. (Table 3). Table 3. Input data for inhalation exposure assessment. Task Ignition Charbroiling Ash cleanup Time (h) 0.4 7.2 0.4 Moderate scenario NF particulates concentration (10−9 × kg/m3) 2000 1500 2000 FF particulates concentration (10−9 × kg/m3) 1000 500 1000 Extreme scenario NF particulates concentration (10−9 × kg/m3) 2000 15 000 2000 FF particulates concentration (10−9 × kg/m3) 1000 7500 1000 Task Ignition Charbroiling Ash cleanup Time (h) 0.4 7.2 0.4 Moderate scenario NF particulates concentration (10−9 × kg/m3) 2000 1500 2000 FF particulates concentration (10−9 × kg/m3) 1000 500 1000 Extreme scenario NF particulates concentration (10−9 × kg/m3) 2000 15 000 2000 FF particulates concentration (10−9 × kg/m3) 1000 7500 1000 Table 3. Input data for inhalation exposure assessment. Task Ignition Charbroiling Ash cleanup Time (h) 0.4 7.2 0.4 Moderate scenario NF particulates concentration (10−9 × kg/m3) 2000 1500 2000 FF particulates concentration (10−9 × kg/m3) 1000 500 1000 Extreme scenario NF particulates concentration (10−9 × kg/m3) 2000 15 000 2000 FF particulates concentration (10−9 × kg/m3) 1000 7500 1000 Task Ignition Charbroiling Ash cleanup Time (h) 0.4 7.2 0.4 Moderate scenario NF particulates concentration (10−9 × kg/m3) 2000 1500 2000 FF particulates concentration (10−9 × kg/m3) 1000 500 1000 Extreme scenario NF particulates concentration (10−9 × kg/m3) 2000 15 000 2000 FF particulates concentration (10−9 × kg/m3) 1000 7500 1000 An important parameter of the exposure in airborne particulates is the air volume rate inhaled by an individual, or ventilation rate, or breathing or inhalation rate(86–89). In a previous study of EPA, ventilation rate was calculated directly from a person’s oxygen consumption rate(90). In our work, age, gender, and activity patterns were also considered. A mean inhalation rate was calculated for many age groups, and for sedentary/passive, light intensity, moderate intensity, and high intensity types of activity. A general expression for calculating the dose due to inhalation of air-borne dust containing long lived radionuclides is(60): Einh=Cd×DL×BR×OF×DH Where Einh is the committed effective dose due to annual inhalation (mSv), Cd is the mean annual radionuclide activity concentration in dust (Bq/kg). This formula is used by way of derogation. Kitchen employees are private persons in the radiation protection sense. In our case we will consider the maximum 90Sr and 137Cs measured values in ash. These are for 90Sr 1060 Bq/kg and for 137Cs 5500 Bq/kg. DL is the mean annual dust load in air (kg/m3), estimates of which are given in Table 3 for every task included in the calculation. BR is the breathing rate (m3/h), OF is the occupational factor, e.g. exposure duration (in our case hours within an 8 h shift). The BR × OF for every age and gender group is calculated on Table 4. Table 4. Overall dose for all the partial scenarios analysed in this context and for both 90 Sr and 137Cs, and for all breathing rates and age groups. Average annual dose (nSv/y) Age Group (years) Mean 5th 10th 25th 50th 75th 90th 95th Max 21 to 31 162 110 117 135 156 183 216 240 398 31 to 41 168 120 127 139 162 190 220 242 321 41 to 51 176 125 136 152 169 195 223 250 352 Males 51 to 61 182 125 134 156 175 206 232 255 392 61 to 71 166 125 134 145 162 180 205 223 291 21 to 31 128 87 93 106 122 145 167 182 300 31 to 41 127 94 98 109 123 138 161 173 263 41 to 51 137 98 106 116 134 153 172 187 282 Females 51 to 61 140 105 110 122 136 157 178 195 257 61 to 71 120 94 99 107 116 129 143 152 197 Average annual dose (nSv/y) Age Group (years) Mean 5th 10th 25th 50th 75th 90th 95th Max 21 to 31 162 110 117 135 156 183 216 240 398 31 to 41 168 120 127 139 162 190 220 242 321 41 to 51 176 125 136 152 169 195 223 250 352 Males 51 to 61 182 125 134 156 175 206 232 255 392 61 to 71 166 125 134 145 162 180 205 223 291 21 to 31 128 87 93 106 122 145 167 182 300 31 to 41 127 94 98 109 123 138 161 173 263 41 to 51 137 98 106 116 134 153 172 187 282 Females 51 to 61 140 105 110 122 136 157 178 195 257 61 to 71 120 94 99 107 116 129 143 152 197 Table 4. Overall dose for all the partial scenarios analysed in this context and for both 90 Sr and 137Cs, and for all breathing rates and age groups. Average annual dose (nSv/y) Age Group (years) Mean 5th 10th 25th 50th 75th 90th 95th Max 21 to 31 162 110 117 135 156 183 216 240 398 31 to 41 168 120 127 139 162 190 220 242 321 41 to 51 176 125 136 152 169 195 223 250 352 Males 51 to 61 182 125 134 156 175 206 232 255 392 61 to 71 166 125 134 145 162 180 205 223 291 21 to 31 128 87 93 106 122 145 167 182 300 31 to 41 127 94 98 109 123 138 161 173 263 41 to 51 137 98 106 116 134 153 172 187 282 Females 51 to 61 140 105 110 122 136 157 178 195 257 61 to 71 120 94 99 107 116 129 143 152 197 Average annual dose (nSv/y) Age Group (years) Mean 5th 10th 25th 50th 75th 90th 95th Max 21 to 31 162 110 117 135 156 183 216 240 398 31 to 41 168 120 127 139 162 190 220 242 321 41 to 51 176 125 136 152 169 195 223 250 352 Males 51 to 61 182 125 134 156 175 206 232 255 392 61 to 71 166 125 134 145 162 180 205 223 291 21 to 31 128 87 93 106 122 145 167 182 300 31 to 41 127 94 98 109 123 138 161 173 263 41 to 51 137 98 106 116 134 153 172 187 282 Females 51 to 61 140 105 110 122 136 157 178 195 257 61 to 71 120 94 99 107 116 129 143 152 197 DH is the internal dose coefficient for inhalation (Sv/Bq). Effective dose coefficients for ingested and inhaled particulates (activity median aerodynamic diameters of 1 and 5 μm) for workers, as published by the ICRP Publication 119, Table A.1(91). According to ICRP 134(92), 90Sr in its most forms is considered as a fast absorption type (type F) material with the exception of strontium titanate. This refers to deposited materials that are readily absorbed into blood from the respiratory tract (fast rate of absorption). There are some indications of medium absorption type (type M) materials in cases of irradiate fuel fragments and fused aluminosilicate particles. In all other cases it is considered as a type S material with few exceptions (Table 10.2. ICRP 134). Since the type S material scenario for 90Sr is not directly applicable in the case of pellet combustion, 90Sr will be considered as type F material in the deployment of this work’s main example. As far as 137Cs is concerned, according to ICRP 137(93), it is a type F material in most it’s salt forms. Most researchers also indicate a type F material for 137Cs, even in cases such as irradiate fuel fragments and fused aluminosilicate particles. In this work an example of Type F materials is presented, both for 90Sr and 137Cs and these values are 2.4E-08 Sv/Bq and 4.8E-09 Sv/Bq respectively, for particle size less than 1μm, and 3.0E-08 and 6.7E-09 respectively, for particle size less than 5 μm. For these particulate aerosols that are generated by ash and ash dust, a default activity median aerodynamic diameter (AMAD) of 5 μm was used. The overall dose received within a year is the summation of the dose calculated in all the above scenarios. Einh=∑i,j,k(Cd×DL×BR×OF×DH)i=1,2,j=1,2,3andk=1,2 Where i refers to the two radionuclides present (137Cs or 90Sr), j refers to the three operating modes of the hearth (ignition, charbroiling and ash-cleanup) and k refers to the NF and FF, respectively. The overall dose for all the partial scenarios for both 90Sr and 137Cs, and for all breathing rates and age groups, are given in Table 4. Based on the above results the mean dose intake in all age groups and all genders is from 116 to 175, with maximum possible dose intake of almost 400 nSv/ year, for the age group of 21–31, for males. For the extreme PM values scenario, the respective maximum possible dose intake at around 4100 nSv/year. This is one order of magnitude higher compared to the average values scenario. With the help of Table 4, a calculation model of inhalation dose has been constructed (Figure 3). Figure 3 presents the results of the model in terms of 137Cs activity against the dose received. The caesium and strontium ratio was calculated from the equation resulting from linear regression of caesium and strontium results in ash and is the following formula: S90r=1119.9137CsC137s+555.96 where 90Sr and 137Cs are the activity concentrations in Bq/kg of strontium and caesium, respectively. Figure 3. View largeDownload slide A calculation of the dose received through inhalation of ash particles (due to both 137Cs and 90Sr) as a function of 137Cs activity concentration for the two scenarios examined. Figure 3. View largeDownload slide A calculation of the dose received through inhalation of ash particles (due to both 137Cs and 90Sr) as a function of 137Cs activity concentration for the two scenarios examined. According to this model in order to reach a dose of 2500 nSv/year, a value of 60 000 Bq/kg of 137Cs should be found in the ash, for the moderate PM concentrations scenario (Figure 3). For the extreme PM concentrations scenario, it is 26 500 nSv/year for 60 000 Bq/kg of 137Cs, which is almost an order of magnitude higher. Therefore, it is evident that the inhalation of ash particles from hearths does not expose kitchen staff to significant doses of radioactivity. CONCLUSIONS Pellets produced from Country A that have been manufactured in the years of 2015 and 2016 had the highest activity concentrations in 137Cs. On the contrary pellets from the same Country A of 2014 production had very small amounts of 137Cs. The highest activity of 137Cs was found for pellets of Country C origin. Country B pellets had very low levels of radioactivity. Pellets that were produced in Greece from raw materials that were also produced in Greece had also very small amounts of 137Cs. The exemption levels for 137Cs, were not exceed in any case, even if an enrichment factor of 250 is assumed. 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TI - A RADIATION DOSIM alRY ASSESSMENT OF WORKERS IN GREEK STYLE ROTISSERIES THAT USE PELLETS CONTAMINATED BY 137Cs AND 90Sr
JF - Radiation Protection Dosimetry
DO - 10.1093/rpd/ncz017
DA - 2019-02-26
UR - https://www.deepdyve.com/lp/oxford-university-press/a-radiation-dosim-alry-assessment-of-workers-in-greek-style-BBnnoRoG1S
SP - 1
VL - Advance Article
IS - 
DP - DeepDyve
ER -