IRELAND. 2 Department of Physics, Trinity College, Dublin 2, IRELAND

Transcription

1 Occupational radiation exposures to NORM at an Irish peat-fired power station and potential implications of the use of peat fly ash by the construction industry C. Organo 1, E.M. Lee 2, G. Menezes 2 and E.C. Finch 2 1 Radiological Protection Institute of Ireland, 3 Clonskeagh Square, Clonskeagh Road, Dublin 14, IRELAND. corgano@rpii.ie 2 Department of Physics, Trinity College, Dublin 2, IRELAND ABSTRACT Annually, approximately 15% of Ireland s electricity requirement is provided through the combustion of tonnes of peat. While literature on the coal-fired power generation is quite abundant, studies on the peat-fired power generation industry from the radiological point of view are scarce. A study of the largest Irish peat-fired power plant was initiated to review the potential occupational radiation exposures arising from the occurrence of Naturally Occurring Radioactive Materials (NORM) at different stages of the industrial process and to investigate any potential radiological health consequences that may arise should peat fly ash be used as a component of building materials. Ambient gamma dose rate measurements, radon measurements, quantification of the occupational exposure from inhalation of airborne particles (personal air sampling) and gamma spectrometry analysis of peat, peat ash and effluent samples from the ash ponds were undertaken. The results indicate that the plant workers are unlikely to receive a radiation dose above 300 µsv per annum over the typical working hours. The potential use of peat fly ash as a by-product in the building industry was also found to have a negligible radiological impact for construction workers and for members of the public. 1. Introduction Around 90% of human radiation exposure arises from natural sources such as cosmic radiation, exposure to radon gas and terrestrial radiation. However, some industries processing natural resources (coal, peat, mineral sands...) may concentrate radionuclides to a degree that they may pose risk to both humans and the environment if they are not controlled. In May 2000, legal controls were introduced in Ireland covering work activities where the presence of natural radioactivity could lead to the risk of a significant increase in exposure to workers or members of the public. These controls are set out in the Radiological Protection Act, 1991 (Ionising Radiation) Order. Statutory Instrument 125 of 2000 [1] and hereafter referred to as S.I. 125 of 2000, which implements the European Union Basic Safety Standards Directive 96/29/EURATOM [2]. Article 3 of S.I. 125 of 2000 in particular provides for the regulation of naturally occurring radioactive materials in the workplace, mostly of terrestrial origin and hereafter referred to as NORM, if they are liable to give rise to a radiation dose greater than 1 msv in a year. In 2001, the Radiological Protection Institute of Ireland (RPII) initiated a programme to identify industries currently active in Ireland which, on the basis of the literature, were considered liable to involve work activities resulting in exposure to diffuse NORM sources. To date, they include the gas extracting industry, the fossil fuel (peat and coal) power production and a range of industrial processes using bulk materials with enhanced levels of natural radioactivity (e.g. bauxite refining). The Physics Department of Trinity College Dublin has led a comprehensive research programme in Ireland on building materials containing naturally occurring radionuclides with regard to the possible implementation of new EU regulations with reference to the external gamma ray dose [3]. Originally, this study stemmed from the implications of the use in the building industry of coal ash and was further extended to peat ash. The RPII and Trinity College decided to collaborate on a joint study 1) to determine the radioactivity levels in Irish peat and peat ash, 2) to compare the results with similar studies in other countries and with national and international legislation and guidance, 3) to investigate the extent of any radiation exposure of workers arising from the handling, burning and storage of peat ash and 4) to look at the radiological implications of the potential use of Irish peat ash in construction materials. 2. Irish peat-fired power generation and associated radiological issues Until recently, up to nine peat-fired power stations were in operation in Ireland. By the end of 2004, the existing generation of plants built between 1950 and the early 80s will be decommissioned and 1

2 only two new large power plants will remain operative. They will process just over tonnes of peat per annum and produce a combined capacity of 250 MW. Both new plants will be situated on sites already occupied by two existing peat-fired power plants at Shannonbridge and Lanesborough. This study was undertaken at the Shannonbridge plant, the largest peat-fired power station in the country. It is located in County Offaly (Midlands region, FIG. 1) and has been operating since The current plant consumes approximately tonnes of peat per annum and produces 125 MW of electricity. On average it produces tonnes of peat ash every year (1/3 of the ash produced by all peat-fired plants). Five million tonnes of ash are currently land filled on site at the plant. The Irish Peat Board (Bord na Mona) supplies the milled peat to Shannonbridge from a local bog spread over 3 different counties (Westmeath, Offaly and Galway). It is mechanically harvested by scraping the top of the bog to a depth of up to 30 cm, milled (72 mesh), solar dried (up to 15% moisture content) and transported to the power station by light rail. Each convoy carries 75 tonnes of peat. On arrival at the plant a tippler unloads the 15 wagons of each convoy sequentially into a hopper. In the power plant, the peat is milled further into a fine dust and blown into the boilers (or furnaces) for combustion. The peat burns in suspension at about 1,000-1,100 C. Approximately 5-10% of the total ash produced falls below the furnace as 'bottom ash' while the remaining 90-95% passes into the flue gas stream as 'fly ash'. This gaseous-particulate mixture leaves the boiler and is drawn through a series of grit arrestors. These remove about 90% of the fly ash and any unburned carbon. At furnace temperatures, some elements originally contained in the peat are partly or completely evaporated. Between the furnace and the grit arrestors, the gas and fly ash stream passes over banks of tubes containing water or air to give a more efficient removal of the heat from the gas prior to its emission to atmosphere. As the flue gases cool down from 1,000 to 200 C, the volatilized elements condense onto the fly ash particles, giving rise to an enrichment of their concentrations in the fly ash trapped by the grit arrestors. Only a small fraction of the fuel gases containing small quantities of radionuclides in gaseous form passes through the grit arrestors and is then discharged through the stack to the atmosphere. Sampling of fly ash is possible only when the boilers are not in operation. The number of samples that could be obtained was therefore limited. In Shannonbridge, the bottom ash is disposed of in 'wet' or 'dry' conditions. Dry bottom ash is produced by two of the three boilers in operation. It is transported in a trailer attached to a tractor, to a dry ash pile situated a couple of hundred metres outside the building. Wet bottom ash from the remaining boiler is hydraulically piped out by flexible tubing to two nearby wet ash ponds together with the totality of the fly ash trapped in the grit arrestors. In the ponds, the ash resides in a 50% minimum aqueous environment to minimize the production of airborne particles. In the future, peat ash may potentially be recycled as a concrete additive [4]. If the activity concentrations of radionuclides present in the ash are significant there could be a potential for an increased radiation exposure to persons occupying buildings constructed with such material. There could also be a potential for increased radiation exposure to workers handling and working with the ash. Environmental exposure to elevated levels of radionuclides could potentially result from the gaseous emissions from the stack, but this aspect is not covered by the present study. 3. Materials and methods Gamma spectrometry analysis of peat, peat ash and effluent samples collected at the plant, airborne peat dust analysis, aerial radon gas measurements and ambient gamma dose rate measurements were carried out. Samples for gamma spectrometry analysis were counted in Marinelli geometry (0.5 l beakers) and analyzed using a low background n-type HPGe GMX gamma ray detector with a relative efficiency of 34% and a resolution of 2 kev (FWHM) at 1.33 MeV. Each sample was counted for a 24-hour period. Activity concentrations of 226 Ra, 232 Th and 40 K were determined, as well as 137 Cs, 238 U, 234 Th, 214 Pb and 210 Pb. Ra-226 activities were ascertained using the two gamma-ray lines at 93 kev and 186 kev and corrected for the interference of 235 U at 186 kev. Th-232 was determined from the gamma-ray emissions at 911, 969, 338, 965, 795, and 463 kev from 228 Ac. K-40 and 137 Cs activities were determined from their respective lines at 1461 kev and 662 kev. 2

3 FIG. 1. Schematic sketch of the Shannonbridge peat-fired power plant with locations of the measurements undertaken and samples analyzed during this study (the scale of the objects are not respected) GDR = gamma dose rate measurement, Rn = radon measurement. Inserted is a map of Ireland showing the location of Shannonbridge River Shannon Wet ash pond: 1 GDR 2. Effluent from ash pond: 2 samples 3. Bunker: 2 peat samples, 2 Rn and 2 GDR 4. Boilers: 2 GDR and 2 Rn 5. Offices and workshop: 2 Rn 6. Dry ash pile: 2 GDR and 4 bottom ash samples 7. Fly ash: 2 samples 8. Tippler: 1 peat samples, 1 Rn and 1 GDR 9. Incoming peat from bog: 2 samples 10. Control site (Shannonbridge church) outside the plant perimeter: 1 GDR 11. Chimney Peat and peat ash fluxes through the process 6 9 3

4 Calculations were undertaken to determine the radiological health significance of the potential use of peat ash in building materials. The EC guidance document [5] advises on the determination of an Activity Concentration Index (I) to convert the specific activity of a building material (in Bq kg -1 ) into a measure of radiation dose (in msv) that may be received by an individual occupying a 'model room constructed from a building material with a certain specific radioactivity. The formula used for I calculation is: CRa CTh C Ι = + + K (1) where C Ra, C Th, and C K are the 226 Ra, 232 Th and 40 K activity concentrations (Bq kg -1 ) in the building material, respectively. I was calculated for all the peat ash samples. Airborne peat dust concentration was measured to assess the potential radiation dose through inhalation of airborne particles using a filtration sampling method (AEA Technology filter holder, Casella London Ltd.) where a known volume of air is drawn through a pre-weighed glass fibre filter paper (25 mm diameter, pore size 80 microns) by means of an air pump. A set of filter papers was used as controls or spares in case of accidental damage or contamination. On site, the filter holder was placed at a height of approximately 1.60 m (approximate breathing zone height) in a static position. The flow rate of the pump was set at 2 litres per minute. The pump was allowed to run for a standard work shift, from 9.30 am until 5.15 pm (465 minutes). Passive long-term radon measurements were carried out on the premises to determine if the concentrations exceeded the national Reference Level for workplaces, 400 Bq m -3 averaged over a minimum period of 3-months. Passive alpha track detectors consisting of a two-part polypropylene holder and a CR-39 (poly allyl diglycol carbonate) detection plastic were used. Upon completion of the measurements the tracks recorded on the plastics were analysed and counted using a Leitz Ergolux AMC microscope coupled to a Leica Quantimet Q520 image analysis system. A track density was determined for each plastic and converted into radon concentration C (Bq m -3 ) after subtraction of a fixed background value and taking into account a pre-determined calibration factor as well as the exposure duration. A seasonal correction was applied to C because the detectors were exposed for less than twelve months [6]. Gamma dose rate measurements were carried out using a NE Technology portable gamma dose rate meter (type PDR1) and a Mini Instruments integrating Geiger Müller-Background Monitor-Type 6-80 (GM6-80). Instantaneous gamma dose rate readings were taken with the PDR1 meter and an average value was calculated from the lowest and highest readings. The GM6-80 meter was fixed to a tripod at each location for 1000 seconds. The readings were then converted to an ambient gamma dose rate (µsv hr -1 ) using a calibration conversion table relevant to the instrument. 4. Results 4.1. Peat, peat ash and effluent from the wet ash pond Radionuclide analysis of peat, bottom ash and fly ash from Shannonbridge indicate a great variability of activity concentrations (Table I). In general, fly ash presents significantly higher concentrations than the bottom ash in the U-series, while the bottom ash contains more 40 K than the fly ash. Table II shows that there is a wide range of activities between the fly ash produced at different peat-fired power stations in Ireland [11]. Compared with other types of NORM or with the average Irish soils, it is clear that the peat and the peat ash produced in Shannonbridge contain lower levels of naturally occurring radionuclides. With regard to the radioactivity enhancement in the fly ash and the bottom ash arising from the combustion process, some radionuclide concentrations could be enhanced by a factor 20 to 25 compared with concentrations in the original peat as indicated in [19]. Pb-210 shows the largest enrichment onto small fly ash particles (< 1.3 µm) according to Mustonen and Jantunen [7], indicating a volatile behaviour at the furnace temperature. 4

5 Table I. Specific activities of U-series radionuclides, 232 Th, 40 K and 137 Cs (in Bq kg -1, dry weight) measured in peat, peat ash and effluent from the ash pond. I is the Activity Concentration Index (see text for explanations). Errors quoted are the counting uncertainties at one standard deviation from the mean count. BDL = Below Detection Limit of 0.19Bq kg -1 Sample Type 238 U 234 Th 226 Ra 214 Pb 210 Pb 232 Th 40 K 137 Cs I PEAT entering plant 2.8± ± ± ± ±1.5 BDL 6.1± ±0.1 entering plant 4.0± ± ± ± ± ±0.0 BDL 2.2±0.1 in tippler 10.9± ± ± ± ±1.6 BDL 6.5± ±0.3 in bunker 7.4± ± ± ± ±2.6 BDL BDL 11.5±0.3 dust in bunker BDL 3.8± ± ± ±1.6 BDL BDL BDL MAX VAL FLY ASH 301.2± ± ± ± ± ± ± ± ± ± ± ± ±14.9 BDL BDL BDL 0.11 MAX VAL BOTTOM ASH 77.1± ± ± ± ±0.8 BDL 7.6± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±0.1 BDL BDL 0.02 MAX VAL EFFLUENT 0.31± ±0.5 BDL 0.5± ±0.3 BDL BDL 0.6±0.0 BDL 1.1± ±0.1 BDL 0.27±0.2 BDL BDL BDL 5

6 Table II. Comparison of results from this study and other references in the literature (activity concentrations in Bq kg -1 ) Sample Type 238 U 234 Th 226 Ra 210 Pb 232 Th 40 K 137 Cs References RAW MATERIAL Irish peat This study Finnish peat [7] Coal Moneypoint 19 (5-45) 30 (6-67) 14 (4-27) 8 (2-13) 61 (20-100) [8] UK coal [9] Coal world average [10] FLY ASH This study This study Shannonbridge [11] Ferbane [11] Lanesborough [11] Rhode [11] Bellacorick [11] Finland [12] Moneypoint [8] UK [9] BOTTOM ASH This study This study Moneypoint [8] World average [9] OTHER NORM Bauxite Boké [13] Bauxite [14] Red mud [15] Red mud [16] Phosphogypsum 1000 [17] Phosphate ore [14] Zircon sands [14] Average Irish soils [18] 6

7 Enrichment factors (EF) for different radionuclides can be calculated using the formula: EF [ c ] [ c ] = r ash r peat [ Ra] ash [ Ra] (2) peat where [c r ] and [ 226 Ra] are the activity concentrations of a potentially enriched (or depleted) radionuclide r and of 226 Ra, respectively. Ra-226 is used as a reference nuclide because of its nonvolatile nature at furnace temperature [7]. To simplify the calculations, a single activity concentration for each radionuclide in the peat, in the fly ash and in the bottom ash was assumed by rounding up to the maximum concentration measured (conservative end of the range of concentrations measured). The values are displayed in italics in Table I where they are quoted as MAX VAL. EF values in the fly ash were calculated to be 1.4, 4, 2, 1 and 0.5 for 210 Pb, 238 U, 232 Th, 40 K and 137 Cs, respectively. For the same radionuclides, EF values in the bottom ash are 2.5, 3.3, 2.5, 10 and Airborne dust concentration The dustiest location of the plant is the bunker area, an indoor type of warehouse where a 4-hour supply of milled peat is stored at any time before it is fed into the mills. In this area, employees are carrying out dry sweeping duties of spilled peat dust, regularly generating large amounts of fine airborne dust. A single air sampling experiment over an 8-hour working shift was carried out during which the dust concentration was measured at 25.6 mg m -3. This is very significant in terms of occupational dust exposure (the Irish Occupational Exposure Limit (OEL) for nuisance dust is set at 10 mg m -3 [20]) but employees working in this area are required to wear personal protection equipment (PPE) including protective clothing and a face dust mask, and only work in this location for very short periods of time Radon gas In industries dealing with NORM an important radiation exposure pathway can be radon and radon daughters inhalation from storage of large volume of materials. This is because these materials are often crushed or powdered before they are processed (allowing for radon to escape more easily from the matrix) and may be stored in poorly ventilated spaces (allowing radon concentrations to build up). The associated radiation dose may substantially vary as it is very strongly dependent on a wide range of parameters such as the emanating fraction, the dose equilibrium factor, the dose conversion factor, the ventilation rates, the room size, the surface to volume ratio and the diffusion coefficients [16]. Two radon surveys were carried out in Shannonbridge over the last 8 years and the results are displayed in Table III. Not only are all the measurements below 400 Bq m -3, the national Reference Level for workplaces but they are similar to outdoor radon concentrations commonly measured in Ireland. As such, they are of no radiological significance from the point of view of radon occupational exposure Ambient gamma dose rate measurements The locations of the measurements carried out are displayed on FIG. 1 and the results are shown in Table IV. The values given by the two dose rate meters are in good agreement and range from 0.06 to 0.18 µsv h -1. They are not significantly different from the ambient gamma dose rate recorded outside the perimeter of the plant and used as a control measurement of the natural background ( µsv h -1 ). More than likely, the readings given by the GM6-80 (0.07 µsv h -1 on average) give a better idea of the real situation as these are inegrated counts over 20 minutes instead of instantaneous values given by the PDR1 (0.10 µsv h -1 on average). In Ireland, the average absorbed dose rate in air is 33 ngy h -1, with a range of 2 to 110 ngy h -1 [18]. Using a conversion factor of 1 Sv Gy -1 [10], it corresponds to an average effective dose for adults of 0.03 µsv h -1 (range of to 0.11 µsv h -1 ). Therefore, the dose rates measured in Shannonbridge are within the range of natural variations, although clearly in the upper part of this range. 7

8 Table III. Results of passive radon measurements carried out in the Shannonbridge peat-fired power plant and associated effective dose (µsv y -1 ) Location Measurement period Radon concentration (Bq m -3 ) Assumed exposure duration (h y -1 ) (1) Effective dose from radon inhalation (µsv y -1 ) (3) Maintenance room 02/1995 to 05/ (2) 110 Conference room 02/1995 to 05/ Tippler area 12/2002 to 03/ Bunker 12/2002 to 03/ Control room in bunker 12/2002 to 03/ (unoccupied) 0 Boiler 1 12/2002 to 03/ Boiler 2 12/2002 to 03/ (1) Based on the characteristics of each work practice on site (2) Employees in the maintenance room spend the whole working year at this location (3) ICRP 65 [21] dose coefficients and F factor of 0.4 used for the calculations (4) Calculated assuming a maximum radon concentration of 15 Bq m -3 in the boiler room (4) Table IV. Ambient gamma dose rate measurements at the Shannonbridge plant and associated effective dose (µsv y -1 ) arising from external exposure to gamma radiation Locations Dose rate recorded (µsv h -1 ) PDR1 GM6-80 Assumed exposure duration (h y -1 ) (2) Effective dose (µsv y -1 ) Tippler area Bunker area Boiler 1 Bottom ash area Boiler 2 Bottom ash area Bottom ash pile (inactive disposal area) Bottom ash pile (active disposal area) Wet ash pond Control measurement (outside plant perimeter) Irish average (1) 0.03 (absorbed dose rate in air 33 ngy h -1 ) (1) [18] (2) Based on the characteristics of each work practice on site

9 5. Discussion 5.1. Peat harvesting Radiation dose arising from exposure to external gamma radiation of terrestrial origin for workers involved in the harvesting of the peat all year round should be lower than the natural background value. It should also be lower than the dose arising from a normal outdoor work activity. This is because activity concentrations measured in the raw peat are lower than in average Irish soils. Harvesting is carried out in open-air by machineries and workers are wearing facial masks and protective clothing to protect them from any windborne peat dust. Radiation dose arising from inhalation of peat dust is therefore minimized Enrichment factors Enrichment factors calculated in this study are not significant compared to other published values [19]. It is recognised that the levels of enhancement of radionuclide concentrations in ashes are very variable. This is mostly due to differences in the raw peat, the type of furnace, the combustion temperature and the operational characteristics of the plant [9]. For example, the temperature in the furnace at Shannonbridge is C, which is lower than the combustion temperature of C quoted in Mustonen and Jantunen [7] Inhalation of airborne peat dust in the bunker Radiological assessments usually refer to the inhalation of contaminated dust as a major pathway by which workers dealing with NORM are likely to be receiving the largest radiation dose. Calculations were undertaken to determine the committed effective dose arising from inhalation of peat dust likely to be received by an employee in the bunker over the working year. A sample of airborne peat dust that had settled on shelving adjacent to the personal sampling pump was collected and analyzed by gamma spectrometry. This enabled the amount and types of radionuclides likely to be in the airborne peat dust to be determined (Table I, dust in bunker). The committed effective dose from inhalation of peat dust was calculated using the formula: ( g c ) D inh = texp V inh, r r (3) where t exp is the exposure duration (assumed to be 100 hours over the year), V is the breathing rate (1.18 m 3 h -1 for light work, [9], g inh,r is the inhalation dose factor for the nuclide r (in Sv Bq -1, [22]) and c r is the ambient air activity concentration for the radionuclide r (Bq m -3 ). Results of the calculations are shown in Table V. The committed effective dose resulting from inhalation of peat dust in the bunker over the working year is less than 1 µsv (0.89 µsv y - 1) and therefore insignificant. It should be noted that this dose is the maximum likely to be received by any worker as it was calculated assuming no PPE Radon and radon daughters inhalation Another significant exposure pathway in workplaces where NORM materials are processed is radon (and thoron) inhalation from storage of important quantities of materials in a warehouse [23]. In our case, it could be possible that the peat (bunker area) and peat ash (bottom ash in the boiler area) stored onsite may contribute significantly to the total occupational exposure due to the quantities involved. Another exposure situation which would arise from large quantities of fly ash stored in an enclosed space would be the cleaning of the grit arrestors or the freeing of blockages in the hoppers. The radiological assessment of these work activities was not carried out as they did not occur at the time of our site visits. This maintenance work would arise 3 times in a year approximately, would take up to 5 days to be completed and would be undertaken under very strict conditions (obligation to wear respiratory equipment, over clothing, gloves, etc) using water sprays for dust suppression. The annual effective doses from inhalation of radon and radon daughters were calculated for the levels measured 9

10 across the plant taking into account the exposure duration at each location (Table III). The highest dose calculated would be received in the maintenance room and is 0.11 msv y -1, which is only 10% of the annual limit under S.I. 125 of Table V. Committed effective dose from inhalation of airborne peat dust in the bunker area Radionuclide r 226 Ra 210 Pb 210 Po 228 Ra 228 Th Units Assumed activity concentrations in peat dust (1) Bq kg -1 Dust concentration (2) 25.6 mg m -3 c r (3) Bq m -3 g inh,r (4) Sv Bq -1 g inh,r c r Sv m -3 g inh,r c r Sv m -3 Exposure duration t exp 100 h y -1-1 Breathing rate V 1.18 m 3 h t exp V g inh,r c r 0.89 µsv y -1 (1) See Table I. Pb-210 and Po-210 are assumed to be in equilibrium (2) Dust concentration is equal to A / v where A is the amount of peat dust breathed in during an 8-hour shift (23.78 mg) and v is the flow rate of the pump (2 l min) multiplied by the duration of the experiment (465 min) and divided by 1000 (3) Ambient air activity concentration for the radionuclide r (Bq m -3 ) is the product of the assumed activity concentration by the dust concentration (4) Inhalation dose factor for the nuclide r (AMAD 5 µm, [22]) 5.5. Exposure to external gamma radiation in the plant and on the landfill sites Annual effective doses arising from exposure to external gamma radiation were calculated on the basis of the maximum dose rates measured at each location in the plant multiplied by the exposure duration at each location (Table IV). All the doses received on the premises of the plant are below the dose received at the location used as control (or background) Use of fly ash in building materials As described previously, the radiation dose that an individual may receive from occupying a building is indicated by the activity concentration index I rather than the activities of the materials themselves. For materials used in bulk amounts I should be less than 1 to ensure that an external radiation dose of greater than 1 msv per annum is not received, and less than 0.5 to ensure a dose of less than 0.3 msv per annum. For superficial materials the I values corresponding to 1 msv and 0.3 msv per annum are 6 and 2, respectively. Regulatory control should be considered for materials that give rise to external doses of between 0.3 msv and 1 msv per annum. Materials giving external doses below 0.3 msv should be exempt from all restrictions while those above 1 msv must be regulated. Calculated values of I for the peat ash are displayed in Table I. Results indicate that the I values for all the fly ash samples are below 1. When the lower index (I = 0.5) is considered, none of the fly ash samples are 10

11 found to exceed it. This implies that if fly ash were to be re-used in bulk in building works, occupiers of such buildings would be unlikely to receive an external radiation dose in excess of 0.3 msv per annum. Taking the above results into account, it can be concluded that peat ash from Shannonbridge can be exempt from restrictions or regulatory control. With an external exposure to gamma radiation limited to levels below 1 msv per annum following the EU recommendations, the 226 Ra concentrations in the materials are also unlikely to cause indoor radon concentration to exceed the European Commission guidance level of 200 Bq m Conclusions Table VI summarises all the doses arising from different pathways calculated in the framework of this study. The total annual effective dose likely to be received by a worker involved in the processing of the peat and handling of the peat ash in Shannonbridge is approximately 0.3 msv (312 µsv). The exposure pathways taken into account are the peat dust inhalation in the bunker area, the inhalation of radon and radon progeny and the external gamma irradiation at different locations in the plant. Therefore, most of the exposure situations where workers are involved on a regular basis are covered, with the exception of maintenance duties like the cleaning of the hoppers and the freeing of blockages in the grit arrestors. These duties are the only ones where workers are directly in contact with the peat fly ash. One would not expect the annual effective dose associated with these duties to be significant as this type of work is always carried out with PPE, is undertaken in wet conditions, occurs nonroutinely (3 times in a year) and is usually completed within a week. Another exposure situation not covered in this study is the inhalation of peat ash dust on the landfill sites arising from the generation of windborne ash on the ash pond. However the top layer of the pond, when dried out, usually forms a crust underneath which the ash is trapped. It is therefore unlikely to be wind blown. However, it would be advantageous to investigate these pathways further. Table VI. Occupational radiation doses calculated for workers at Shannonbridge Location / exposure duration Dust inhalation (µsv) Inhalation of radon and progeny (µsv) External gamma irradiation (µsv) TOTAL (µsv) Tippler / 100 h y Bunker area / 100 h y Boiler area / 680 h y Bottom ash pile (inactive) / 50 h y -1 3 (1) 4 7 Bottom ash pile (active) / 500 h y (1) Wet ash pond / 400 h y (1) Maintenance duties / 170 h y -1 undetermined undetermined undetermined undetermined TOTAL / 2000 h y (1) calculated assuming outdoor radon concentration of 10 Bq m -3 [10] and a F factor (for outdoors) of 0.8 (instead of 0.4 indoors) References 1. Stationery Office, Radiological Protection Act, 1991 (Ionising Radiation) Order. Statutory Instrument 125 of 2000, Department of Public Enterprise, Government Publications Office, Dublin (2000). 11

12 2. Council of the European Union, Basic Safety Standards for the Health Protection of the General Public and Workers Against the Dangers of Ionising Radiation, Council Directive 96/29/EURATOM, Luxembourg (1996). 3. Lee, E.M., Menezes, G., Finch, E.C. Natural radioactivity in building materials in the Republic of Ireland. Health Physics, in press. 4. Lyons, J., Electricity Supply Board, personal communication, European Commission, Radiological protection principles concerning the natural radioactivity of building materials. Radiation Protection 112. EC Directorate-General Environment, Luxembourg (1999). 6. Madden, J.S., Radon in dwellings in selected areas of Ireland, Report RPII-94/3, Radiological Protection Institute of Ireland, Dublin (1994). 7. Mustonen, R., Jantunen, M., Radioactivity of size fractionated fly-ash emissions from a peatand oil-fired power plant. Health Physics, 49: , (1985). 8. McAulay, I.R., Department of Physics, Trinity College Dublin. Unpublished data ( ) 9. Smith, K.R., Crockett, G.M., Oatway, W.B., Harvey, M.P., Penfold, J.S.S., Mobbs, S.F., Radiological impact on the UK populations of industries which use or produce materials containing enhanced levels of naturally occurring radionuclides: Part I: Coal-fired Electricity Generation, Report NRPB-R327, National Radiological Protection Board, Chilton, Didcot (2001). 10. United Nations Scientific Committee on the Effects of Atomic Radiation, Sources and Effects of Ionizing Radiation. Report to the General Assembly, with Scientific Annexes. United Nations, New York (2000). 11. Finch, E.C., A radiological analysis of peat ash samples supplied by ESB International, Unpublished report to the Electricity Supply Board, Trinity College, Ireland (1998). 12. Mustonen, R., Building Materials as sources of indoor exposure to ionising radiation. Report STUK-A105, Strålsäkerhetscentralen, Helsinki (1992). 13. Von Philipsborn, H., Kuhnast, E., Gamma spectrometric characterisation of industrially used African and Australian bauxites and their red mud tailings. Rad. Prot. Dos., 45: , (1992). 14. International Atomic Energy Agency, Radioactivity in material not requiring regulation for purposes of radiation protection, Draft Safety Guide DS-161, Safety Standards Series, IAEA, Vienna (2003). 15. European Commission, Practical use of the concepts of clearance and exemption Part II Application of the concepts of exemption and clearance to natural radiation sources. Radiation Protection 122. EC Directorate-General Environment, Luxembourg (2001). 16. Hofmann, J., Leicht, R., Wingender, H.J., Worner, J., Natural radionuclide concentrations in materials processed in the chemical industry and the related radiological impact, Report EUR 19264, Nuclear Safety and the Environment, European Commission, Luxembourg (2000). 17. O Grady, J., Radioactivity and fertilisers. Technology Ireland, 24:41-45, (1992). 18. Marsh, D., Radiation mapping and soil radioactivity in the Republic of Ireland, MSc. Thesis, Trinity College, National University of Ireland, Dublin (1991). 19. European Commission, Enhanced radioactivity of building materials. Radiation Protection 96. EC Directorate-General Environment, Luxembourg (1999). 20. National Authority for Occupational Safety and Health, Code of Practice for the Safety Health and Welfare at Work (Chemical Agents) Regulations 2001, Government Publications Office, Dublin (2002). 21. International Commission on Radiological Protection, Protection against Radon-222 at home and at work. Publication 65. Annals of the ICRP, 23, No. 2, Pergamon Press, Oxford (1994). 22. International Commission on Radiological Protection, Dose coefficients for intakes of radionuclides by workers. Publication 68. Annals of the ICRP, 24, No. 4, Pergamon Press, Oxford (1994). 23. Penfold, J.S.S., Mobbs, S.F., Degrange, J.P., Schneider, T., Establishment of reference levels for regulatory control of workplaces where materials are processed which contain enhanced levels of naturally-occurring radionuclides. Radiation Protection 107. EC Directorate-General Environment, Luxembourg (1999). 12