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Environmental and occupational impacts of natural radioactivity from some non-nuclear industries in the Netherlands
J. Environ.
Radioadvity,
Vol. 32, Nos 1-2, pp. 97-104,
Copyright
0
1996
1996 Elsevier Science Limited
Printed in Ireland. All rights reserved 0265-931X/96 ELSEVIER
$15.00 + 0.00
0265-931X(95)00082-8
Environmental and Occupational Impacts of Natural Radioactivity from Some Non-Nuclear Industries in The Netherlands
C. W. M. Timmermans & J. van der Steen KEMA Nederland B.V., PO Box 9035: 6800 ET Arnhem, The Netherlands (Received 1 September 1995; accepted 10 November 1995)
ABSTRACT The existence of natural radioactivity in ores and other raw materials and in their products and wastes has long been recognized. National governments have imposed certain levels, above which the use of radioactive materials is restricted. In the Netherlands, the levels are laid down in the Implementing Orders of the Nuclear Energy Act. In general, the present levels are such that they are not exceeded when the primordial radioactive materials are present only in the form of contaminants. However, the concentration of radioactive materials can be such that somewhere in the raw material-product-waste chain or at releases, these levels might be exceeded. Under such circumstances, the industries have to apply for a licence under the Nuclear Energy Act. Part of the application for a licence is that the industry has to demonstrate that the dose to the workers and to the public is ALARA and below certain dose and risk constraints. In some industries compliance with the individual risk criterion for members of the public (lop6 year-’ as a maximum risk for dying as a consequence of the practice) can be problematic. Direct irradiation from stock piles and contamination of the environment by releases in the atmosphere or surface waters might give rise to risks around the constraint value. In this paper some examples are given. Copyright 0 1996 Elsevier Science Ltd.
INTRODUCTION In the Netherlands the system of licensing a practice dealing with radioactive materials is primarily based on the three basic principles of the TCRP: 97
98
C. W. M. Timmermans. J. van der Steen
(1) Justification: the use of radioactive materials or, as in the nonnuclear industry, raw materials which contain naturally radioactive nuclides must have a net benefit to the society. (2) ALARA: all exposures whether occupational or public must be kept as low as reasonably achievable; social and economic factors taken into account. (3) Dose limits: all exposures must be kept below defined dose limits. The present dose limit for radiological workers is 50 mSv year-‘. This will be lowered to 20 mSv year-’ in the near future. Workers exposed to a dose less than 2mSv year-’ will not be monitored or subject to medical control (non-radiological workers). For members of the public, the dose limitation is derived from the risk approach in the Dutch environmental policy (Lower Chamber, 1990). In this policy the cumulative maximal tolerable risk (MTR) for an individual of dying due to a certain activity is set at lop5 a-‘. This risk limit applies to each of the three distinguishable policy areas: chemicals, radiation and external safety. For radiation, a risk constraint per source is set at one-tenth of the overall risk limit in order to account for the possibility of individuals being exposed to several sources of radiation at one time. This means that the actual risk limit for the public from one single source is set to 10e6. Up to now the Dutch government has adopted a risk coefficient for dying of 2.5% Sv-‘. This, in combination with the risk constraint of 10e6, gives a dose limit per source of only 40 ,uSv a-‘. A practice is exempted from having a licence if either the total quantity of radionuclides on hand or the concentration in the raw material, products or waste is below a certain level. In the non-nuclear industry the quantities are that large that the limit for total activity is always exceeded. So, in fact, the activity concentration of the material determines whether one has to have a licence or not. At present, the exemption level for activity concentration of natural radionuclides is 500 Bq gg’ for the more or less undisturbed ores, which means that the radionuclides are in their original matrix (Nuclear Energy Act, 1986). For physically or chemically treated materials in which the natural matrix is changed, the existing exemption level is 100 Bqg-‘. These levels are high enough for most non-nuclear industries to be excluded from reporting or licensing. However the European Community has recently issued a draft directive (CEC, 1994) in which the exemption levels, especially for natural radionuclides, are much lower (Table 1). With these new levels many non-nuclear industries are forced to enter the radiation protection system for practices. Examples of raw materials, products and waste which can cause problems to the non-nuclear industry
Natural radioactivity and non-nuclear industries
Exemption
Nuclide
232Th ore 238U ore 226Ra + 2’0Pb + 2’0Po
99
TABLE 1 Levels for Natural Radionuclides According to Existing Legislation (Nuclear Energy Act, 1986) and Draft EC Directive (CEC, 1994) Activity (kBq) Existing Drqft
Concentration (Bqg-‘) Existing Draft
1 1 10 10 10
50 5000 5 5 5
500 500 100 100 100
1 I 10 10 10
are zircon sand, that is used in the ceramic industry; phosphate rock, fertilizers, slag and phosphogypsum from the phosphate industry; flyashes from electricity production; and scales and sludges from oil and gas production. In these materials the specific activities are mostly above 1 Bq g-‘, ranging from 0.6 Bq g-’ for some of the more radioactive flyashes up to 1000 Bq g -’ in some oil/gas scales (Scholten & Zuur, 1994; Thompson, 1994). If both the exemption levels for total activity and specific activity are exceeded, or when certain exempted releases to the environment are exceeded, the industry has to apply for a licence. Part of the application is that one has to prove by means of a risk study that the practice causes no higher individual risk to members of the population than 10-6. When risks are above this level the licence can be refused or the government can force the licencee to reduce the risks to below the maximal tolerable risk level within a given time path. Between a risk of 10e6 and lo-’ the licencee has to comply with the ALARA principle, imposed and controlled by the government. Below the secondary level of 10e8, ALARA is no longer enforced by the government, but the principle still exists. KEMA has performed a number of risk studies for non-nuclear industries such as the ceramic industry processing zircon sands and the phosphate industry. The studies dealt with occupational exposures as well as environmental exposures. A few examples are given below.
CERAMIC
INDUSTRY
Ores may contain enhanced levels of nuclides from the uranium and thorium decay chains. By processing, specifically micronizing, the radioactivity in the ores can give rise to radiological problems in workplaces and in the environment. The doses due to inhaled dust by employees may be considerable. Direct radiation from stock piles causes dose rates higher
C. W. M. Timmermans.
100
J. van der Steen
than 1 @v hh’ . So the potential dose for employees might exceed 2 mSv in a year. Mineral sands used in the ceramic industry contain a relatively large amount of radionuclides of natural origin. Specific activities of, for instance, bauxite and rutile are about 0.5 Bq gg’ on average for nuclides of the uranium and thorium series (Table 2). In zircon, which is the common name for zirconium silicate, the specific activities range from 3 to about 20 Bq g-’ for nuclides from the uranium series which are in nearly secular equilibrium and from 0.6 to 40 Bq kg-’ for the thorium series. The specific activities strongly depend on the origin of the zircon. The highest concentrations are found in Malaysian zircon, which is a by-product from tin mining. The mineral is used in ceramics, foundries and paints. In the Netherlands the mineral arrives by boat, is grinded to a median size of a few micrometres and then shipped for further processing. Milling is a dry process and unavoidably the workshop is dusty. The conventional maximal allowable dust concentration in work shops is 5 mgme3. It was calculated that this tine dust can give rise to an effective dose due to inhalation of about 1OmSv year-‘. The external radiation in the factory close to the stockpiles reached values up to 14@vhgiving rise to an additional potential external exposure of the workers of about 5 mSv year-‘. Discharges to the environment by resuspension, during unloading of the ships, followed by atmospheric dispersion are low. Doses to nearby population centres are less than 1 $v year-‘. The direct external radiation at the fence is of the order of 0.1 @v hh’ and can thus give rise to a dose to the population of about 1 mSv aal. This exceeds the maximal permissible dose of 4O$Sv in the Netherlands. However, as the mill is situated in an industrial zone, the need for measures is not urgent.
Radioactivity
in Some
TABLE 2 ores and Mineral Sands (Hu & Kandaiya, Johnston, 1991; Scholten et al., 1993) Activity 238 u
Bauxite Rutile Ilmenite Zircon Australia Africa Malaysia
226
(Bqg-‘) Ra
1985; RIVM,
232 T/,
0.5 0.7 1.5
0.3 0.5 2.3
0.4 0.2 1.2
3.9 6.0 18
3.5 7.0 16
0.6 1.0 40
1990;
Natural radioactivity and non-nuclear industries
PHOSPHATE
101
INDUSTRY
The specific activity of phosphate rock is between 0.1 and 5 Bqg-’ for radionuclides of the uranium series (UNSCEAR, 1982). The thorium concentrations are always much lower, having more or less the same magnitude as found in most soils. For the commonly used ores in the Netherlands the specific activity is between 1 and 1.5 Bq g-i. In the Netherlands there are three major plants which process phosphate ore. Two plants use the wet-acid process for the production of phosphoric acid. Wet-acid process
The wet acid process using sulphuric acid runs according to the reaction: raw phosphate
ore + H$04 -+ HsP04 + CaS04.xH20.
Of the radionuclides in the ore, which go into solution, radium, lead and polonium concentrate in the phosphogypsum, while the remaining radionuclides (uranium and thorium) stay in the phosphoric acid. The atmospheric discharge of radionuclides is negligible. In the Netherlands the phosphogypsum is not stockpiled as in other countries but discharged to surface water. The two plants, which are situated near to each other in the Rotterdam harbour area, together process about 1.2 million tons of raw phosphate ore per year with an average activity concentration of about I.1 Bqg-’ of uranium in secular equilibrium with its daughters. The production of phosphogypsum amounts to about 2 million tons per year which is discharged directly to the surface water from which it is for the greater part transported to open sea. The phosphogypsum contains enhanced levels of the radionuclides 226Ra. 2*0Pb and 210Po. Each year about 1300GBq of each of these radionuclides is discharged. Of the radionuclides in the gypsum 90-95% dissolves and is eventually transported to sea. Five to ten per cent is believed to remain attached to small insoluble particles which settle in the harbour. Measurements of harbour sediment indicate that locally radium levels are enhanced with 10-70 Bq kg-’ which is ascribed to the discharges of the two phosphate plants. From model calculations it was found that the annual phosphogypsum discharge of the smaller plant (one-third of the total capacity) can enhance the radium concentration in the harbour sediment up to 4-25 Bq kg-‘, which is consistent with the measurements. The average annual effective dose from the consumption of aquatic organisms caught in the Dutch coastal waters is only 1-24~ due to these
102
C. W. 44. Timmermans,
J. van der Steen
discharges. However, consumption of fish caught in the Rotterdam harbour close to the points of discharge might lead to doses as high as the maximal tolerable dose from one source. In the past, harbour sediment was used for landfill of polders close to the Rotterdam harbour area and it is the policy of the Dutch government that in the future the harbour sediment is clean enough to be used for this purpose again. When houses are built on this sediment, as has been done in the past in a few locations, the radon which emanates from the soil and enters the houses can cause risks above the maximal tolerable risk level. The average radium concentration in the Dutch soils is about 25Bqkg-‘. The average radon concentration in Dutch dwellings is 29 Bq m-3 and it is assumed that two-thirds of this is due to emanation of radon from the soil, i.e. 20 Bq rne3. This causes an average individual risk to the population in the Netherlands of 1.3 x lo-’ a-‘. Since the radium concentration in harbour sediment is on average raised with about 4 8 Bq kgg’ (from one source), the additional risk to an individual will be (2-4) x 10e6 in the absence of remedial actions. However, when remedial actions are taken, such as covering the soil with a clean layer of clay or sand, it is expected that the enhanced individual risks will be reduced to below the MTR. Thermal process
The third phosphate plant in the Netherlands uses the thermal process to produce phosphor. This process runs at high temperatures, which cause large atmospheric discharges of 210Pb and 210Po. The chemical reaction of this process is given by the equation: 2 Ca3(P04)2 + 6 Si02 + 10 C -+ 6 Casio3 + 10 CO + Pq. Annually about 600,000 tons of raw phosphate ore, 360,000 tons of silicon dioxide and 120,000 tons of cokes are processed. The phosphate ore contains about 1 Bqg-’ of uranium in secular equilibrium with its radioactive daughters up to 210Po. At the very high temperatures at which the process is run, most of the lead and all of the polonium, which have relatively high vapour pressures, go into the gas phase and will be released. The remaining radionuclides go into the calcium silicate slag. An amount of about 1000 tons of solid waste per year remains with a high specific activity of the radionuclides 2’oPb (750 Bq g-‘) and 2’oPo (60 Bq g-l). This waste is disposed of as low-level radioactive waste. The annual atmospheric discharges of the plant are about 45 GBq 2’oPb and 540 GBq 210Po. The liquid discharges to the sea are 25 and 100 GBq, respectively. The risks to the public-separated into two critical groups,
Natural radioactivity
and non-nuclear industries
103
namely workers of an adjacent plant and the population of a nearby village-are just below the maximal tolerable dose of 40 &Sv a-‘. The inhalation path causes the largest contribution to the dose, the contribution of polonium being about two-thirds of the total dose. Consumption of foodstuffs with enhanced lead and polonium activities gives only a minor contribution to the individual dose ( < 2 @Sv year-‘).
DISCUSSION
AND CONCLUSIONS
Many materials of natural origin, such as ores and minerals, often contain high levels of radioactivity caused by radionuclides of the uranium and thorium series. A number of the radionuclides of these series decay through alpha-particle emission which makes them potentially hazardous for both workers in the non-nuclear industries (usually unaware of the dangers) and for the general public which can get exposed to these radionuclides through emissions to the environment. In the Netherlands, a system of licensing is set up for those practices which have raw materials, products or waste on hand with activity concentrations above the current exemption levels or which discharge radioactivity to the environment above the general limits set by the government. By means of a risk study it has to be proven that these practices cause no higher mortality risk to the public than 1O-6 year-’ and that occupational doses are below the dose limit for non-radiological workers, i.e. 2 mSv year-‘. A number of risk studies has been performed by these industries from which in general the following conclusions can be drawn: (1) Occupational exposures in the non-nuclear industry may exceed 2mSv in a year, especially in very dusty workshops and close to large stockpiles. (2) Environmental risks from most non-nuclear industries are within 10-7-10-6 pe r source per year. (3) Use of harbour sediment contaminated with (low levels of) radium activity for landfill, followed by residence on this landfill, can give rise to risks above 1O-6 due to the emanation of radon from the soil. However, the additional risk can be reduced considerably by applying relatively simple remedial actions such as covering the sediment with a layer of non-contaminated soil. (4) The new exemption levels as proposed by the EC may cause considerable problems to the non-nuclear industry in general, in that it will influence its economic position on the world market.
104
C. W. M. Timmermans, J. van der Steen
REFERENCES (1994). Amended proposal for a Council Directive (EURATOM) laying down the basic safety standards for the protection of the health of workers and the general public against the dangers arising from ionizing radiation. Hu, S. J. & Kandaiya, S. (1985). Ra-226 and 232Th concentration in Amang. CEC
Health Phys., 49, 1003-7.
Johnston, G. (1991). An evaluation of radiation and dust hazards at a mineral sand processing plant. Health Phys., 60, 781-7. Lower Chamber (1990). Omgaan met risico’s van straling (Managing radiation risks). Dutch Lower Chamber, meeting year 198991990, 21483 (I), State Publishers, The Hague. Nuclear Energy Act (1986). Besluit stralenbescherming Kernenergiewet (Decree on radiation protection under the Nuclear Energy Act). Staatsblad 465. RIVM (1990). Measurements on the natural radioactivity of mineral sands. RIVM Report No. 240801001, Bilthoven, the Netherlands (in Dutch). Scholten, L. C., Roelofs, L. M. M. & Steen, J. van der (1993). A survey of potential problems for non-nuclear industries posed by implementation of new EC standards for natural radioactivity. KEMA, Report 40059-NUC 935203, Arnhem, the Netherlands. Scholten, L. C. & Zuur, C. (1994). Problems arising at the non-nuclear industry in the Netherlands by introduction of new EC rules. Proc. 17th IRPA Regional Congress, Portsmouth, 6-10 June 1994, pp. 13-16. Thompson, H. M. (1994). Environmental radioactivity: a perspective on industrial contributions. 17th IRPA, Regional Congress, Portsmouth, 6-10 June 1994, pp. 253-6. UNSCEAR (1982). Ionizing radiation: sources and biological effects. United Nations Scientific Committee on the Effects of Atomic Radiation, Report to the General Assembly, with annexes.
Radioadvity,
Vol. 32, Nos 1-2, pp. 97-104,
Copyright
0
1996
1996 Elsevier Science Limited
Printed in Ireland. All rights reserved 0265-931X/96 ELSEVIER
$15.00 + 0.00
0265-931X(95)00082-8
Environmental and Occupational Impacts of Natural Radioactivity from Some Non-Nuclear Industries in The Netherlands
C. W. M. Timmermans & J. van der Steen KEMA Nederland B.V., PO Box 9035: 6800 ET Arnhem, The Netherlands (Received 1 September 1995; accepted 10 November 1995)
ABSTRACT The existence of natural radioactivity in ores and other raw materials and in their products and wastes has long been recognized. National governments have imposed certain levels, above which the use of radioactive materials is restricted. In the Netherlands, the levels are laid down in the Implementing Orders of the Nuclear Energy Act. In general, the present levels are such that they are not exceeded when the primordial radioactive materials are present only in the form of contaminants. However, the concentration of radioactive materials can be such that somewhere in the raw material-product-waste chain or at releases, these levels might be exceeded. Under such circumstances, the industries have to apply for a licence under the Nuclear Energy Act. Part of the application for a licence is that the industry has to demonstrate that the dose to the workers and to the public is ALARA and below certain dose and risk constraints. In some industries compliance with the individual risk criterion for members of the public (lop6 year-’ as a maximum risk for dying as a consequence of the practice) can be problematic. Direct irradiation from stock piles and contamination of the environment by releases in the atmosphere or surface waters might give rise to risks around the constraint value. In this paper some examples are given. Copyright 0 1996 Elsevier Science Ltd.
INTRODUCTION In the Netherlands the system of licensing a practice dealing with radioactive materials is primarily based on the three basic principles of the TCRP: 97
98
C. W. M. Timmermans. J. van der Steen
(1) Justification: the use of radioactive materials or, as in the nonnuclear industry, raw materials which contain naturally radioactive nuclides must have a net benefit to the society. (2) ALARA: all exposures whether occupational or public must be kept as low as reasonably achievable; social and economic factors taken into account. (3) Dose limits: all exposures must be kept below defined dose limits. The present dose limit for radiological workers is 50 mSv year-‘. This will be lowered to 20 mSv year-’ in the near future. Workers exposed to a dose less than 2mSv year-’ will not be monitored or subject to medical control (non-radiological workers). For members of the public, the dose limitation is derived from the risk approach in the Dutch environmental policy (Lower Chamber, 1990). In this policy the cumulative maximal tolerable risk (MTR) for an individual of dying due to a certain activity is set at lop5 a-‘. This risk limit applies to each of the three distinguishable policy areas: chemicals, radiation and external safety. For radiation, a risk constraint per source is set at one-tenth of the overall risk limit in order to account for the possibility of individuals being exposed to several sources of radiation at one time. This means that the actual risk limit for the public from one single source is set to 10e6. Up to now the Dutch government has adopted a risk coefficient for dying of 2.5% Sv-‘. This, in combination with the risk constraint of 10e6, gives a dose limit per source of only 40 ,uSv a-‘. A practice is exempted from having a licence if either the total quantity of radionuclides on hand or the concentration in the raw material, products or waste is below a certain level. In the non-nuclear industry the quantities are that large that the limit for total activity is always exceeded. So, in fact, the activity concentration of the material determines whether one has to have a licence or not. At present, the exemption level for activity concentration of natural radionuclides is 500 Bq gg’ for the more or less undisturbed ores, which means that the radionuclides are in their original matrix (Nuclear Energy Act, 1986). For physically or chemically treated materials in which the natural matrix is changed, the existing exemption level is 100 Bqg-‘. These levels are high enough for most non-nuclear industries to be excluded from reporting or licensing. However the European Community has recently issued a draft directive (CEC, 1994) in which the exemption levels, especially for natural radionuclides, are much lower (Table 1). With these new levels many non-nuclear industries are forced to enter the radiation protection system for practices. Examples of raw materials, products and waste which can cause problems to the non-nuclear industry
Natural radioactivity and non-nuclear industries
Exemption
Nuclide
232Th ore 238U ore 226Ra + 2’0Pb + 2’0Po
99
TABLE 1 Levels for Natural Radionuclides According to Existing Legislation (Nuclear Energy Act, 1986) and Draft EC Directive (CEC, 1994) Activity (kBq) Existing Drqft
Concentration (Bqg-‘) Existing Draft
1 1 10 10 10
50 5000 5 5 5
500 500 100 100 100
1 I 10 10 10
are zircon sand, that is used in the ceramic industry; phosphate rock, fertilizers, slag and phosphogypsum from the phosphate industry; flyashes from electricity production; and scales and sludges from oil and gas production. In these materials the specific activities are mostly above 1 Bq g-‘, ranging from 0.6 Bq g-’ for some of the more radioactive flyashes up to 1000 Bq g -’ in some oil/gas scales (Scholten & Zuur, 1994; Thompson, 1994). If both the exemption levels for total activity and specific activity are exceeded, or when certain exempted releases to the environment are exceeded, the industry has to apply for a licence. Part of the application is that one has to prove by means of a risk study that the practice causes no higher individual risk to members of the population than 10-6. When risks are above this level the licence can be refused or the government can force the licencee to reduce the risks to below the maximal tolerable risk level within a given time path. Between a risk of 10e6 and lo-’ the licencee has to comply with the ALARA principle, imposed and controlled by the government. Below the secondary level of 10e8, ALARA is no longer enforced by the government, but the principle still exists. KEMA has performed a number of risk studies for non-nuclear industries such as the ceramic industry processing zircon sands and the phosphate industry. The studies dealt with occupational exposures as well as environmental exposures. A few examples are given below.
CERAMIC
INDUSTRY
Ores may contain enhanced levels of nuclides from the uranium and thorium decay chains. By processing, specifically micronizing, the radioactivity in the ores can give rise to radiological problems in workplaces and in the environment. The doses due to inhaled dust by employees may be considerable. Direct radiation from stock piles causes dose rates higher
C. W. M. Timmermans.
100
J. van der Steen
than 1 @v hh’ . So the potential dose for employees might exceed 2 mSv in a year. Mineral sands used in the ceramic industry contain a relatively large amount of radionuclides of natural origin. Specific activities of, for instance, bauxite and rutile are about 0.5 Bq gg’ on average for nuclides of the uranium and thorium series (Table 2). In zircon, which is the common name for zirconium silicate, the specific activities range from 3 to about 20 Bq g-’ for nuclides from the uranium series which are in nearly secular equilibrium and from 0.6 to 40 Bq kg-’ for the thorium series. The specific activities strongly depend on the origin of the zircon. The highest concentrations are found in Malaysian zircon, which is a by-product from tin mining. The mineral is used in ceramics, foundries and paints. In the Netherlands the mineral arrives by boat, is grinded to a median size of a few micrometres and then shipped for further processing. Milling is a dry process and unavoidably the workshop is dusty. The conventional maximal allowable dust concentration in work shops is 5 mgme3. It was calculated that this tine dust can give rise to an effective dose due to inhalation of about 1OmSv year-‘. The external radiation in the factory close to the stockpiles reached values up to 14@vhgiving rise to an additional potential external exposure of the workers of about 5 mSv year-‘. Discharges to the environment by resuspension, during unloading of the ships, followed by atmospheric dispersion are low. Doses to nearby population centres are less than 1 $v year-‘. The direct external radiation at the fence is of the order of 0.1 @v hh’ and can thus give rise to a dose to the population of about 1 mSv aal. This exceeds the maximal permissible dose of 4O$Sv in the Netherlands. However, as the mill is situated in an industrial zone, the need for measures is not urgent.
Radioactivity
in Some
TABLE 2 ores and Mineral Sands (Hu & Kandaiya, Johnston, 1991; Scholten et al., 1993) Activity 238 u
Bauxite Rutile Ilmenite Zircon Australia Africa Malaysia
226
(Bqg-‘) Ra
1985; RIVM,
232 T/,
0.5 0.7 1.5
0.3 0.5 2.3
0.4 0.2 1.2
3.9 6.0 18
3.5 7.0 16
0.6 1.0 40
1990;
Natural radioactivity and non-nuclear industries
PHOSPHATE
101
INDUSTRY
The specific activity of phosphate rock is between 0.1 and 5 Bqg-’ for radionuclides of the uranium series (UNSCEAR, 1982). The thorium concentrations are always much lower, having more or less the same magnitude as found in most soils. For the commonly used ores in the Netherlands the specific activity is between 1 and 1.5 Bq g-i. In the Netherlands there are three major plants which process phosphate ore. Two plants use the wet-acid process for the production of phosphoric acid. Wet-acid process
The wet acid process using sulphuric acid runs according to the reaction: raw phosphate
ore + H$04 -+ HsP04 + CaS04.xH20.
Of the radionuclides in the ore, which go into solution, radium, lead and polonium concentrate in the phosphogypsum, while the remaining radionuclides (uranium and thorium) stay in the phosphoric acid. The atmospheric discharge of radionuclides is negligible. In the Netherlands the phosphogypsum is not stockpiled as in other countries but discharged to surface water. The two plants, which are situated near to each other in the Rotterdam harbour area, together process about 1.2 million tons of raw phosphate ore per year with an average activity concentration of about I.1 Bqg-’ of uranium in secular equilibrium with its daughters. The production of phosphogypsum amounts to about 2 million tons per year which is discharged directly to the surface water from which it is for the greater part transported to open sea. The phosphogypsum contains enhanced levels of the radionuclides 226Ra. 2*0Pb and 210Po. Each year about 1300GBq of each of these radionuclides is discharged. Of the radionuclides in the gypsum 90-95% dissolves and is eventually transported to sea. Five to ten per cent is believed to remain attached to small insoluble particles which settle in the harbour. Measurements of harbour sediment indicate that locally radium levels are enhanced with 10-70 Bq kg-’ which is ascribed to the discharges of the two phosphate plants. From model calculations it was found that the annual phosphogypsum discharge of the smaller plant (one-third of the total capacity) can enhance the radium concentration in the harbour sediment up to 4-25 Bq kg-‘, which is consistent with the measurements. The average annual effective dose from the consumption of aquatic organisms caught in the Dutch coastal waters is only 1-24~ due to these
102
C. W. 44. Timmermans,
J. van der Steen
discharges. However, consumption of fish caught in the Rotterdam harbour close to the points of discharge might lead to doses as high as the maximal tolerable dose from one source. In the past, harbour sediment was used for landfill of polders close to the Rotterdam harbour area and it is the policy of the Dutch government that in the future the harbour sediment is clean enough to be used for this purpose again. When houses are built on this sediment, as has been done in the past in a few locations, the radon which emanates from the soil and enters the houses can cause risks above the maximal tolerable risk level. The average radium concentration in the Dutch soils is about 25Bqkg-‘. The average radon concentration in Dutch dwellings is 29 Bq m-3 and it is assumed that two-thirds of this is due to emanation of radon from the soil, i.e. 20 Bq rne3. This causes an average individual risk to the population in the Netherlands of 1.3 x lo-’ a-‘. Since the radium concentration in harbour sediment is on average raised with about 4 8 Bq kgg’ (from one source), the additional risk to an individual will be (2-4) x 10e6 in the absence of remedial actions. However, when remedial actions are taken, such as covering the soil with a clean layer of clay or sand, it is expected that the enhanced individual risks will be reduced to below the MTR. Thermal process
The third phosphate plant in the Netherlands uses the thermal process to produce phosphor. This process runs at high temperatures, which cause large atmospheric discharges of 210Pb and 210Po. The chemical reaction of this process is given by the equation: 2 Ca3(P04)2 + 6 Si02 + 10 C -+ 6 Casio3 + 10 CO + Pq. Annually about 600,000 tons of raw phosphate ore, 360,000 tons of silicon dioxide and 120,000 tons of cokes are processed. The phosphate ore contains about 1 Bqg-’ of uranium in secular equilibrium with its radioactive daughters up to 210Po. At the very high temperatures at which the process is run, most of the lead and all of the polonium, which have relatively high vapour pressures, go into the gas phase and will be released. The remaining radionuclides go into the calcium silicate slag. An amount of about 1000 tons of solid waste per year remains with a high specific activity of the radionuclides 2’oPb (750 Bq g-‘) and 2’oPo (60 Bq g-l). This waste is disposed of as low-level radioactive waste. The annual atmospheric discharges of the plant are about 45 GBq 2’oPb and 540 GBq 210Po. The liquid discharges to the sea are 25 and 100 GBq, respectively. The risks to the public-separated into two critical groups,
Natural radioactivity
and non-nuclear industries
103
namely workers of an adjacent plant and the population of a nearby village-are just below the maximal tolerable dose of 40 &Sv a-‘. The inhalation path causes the largest contribution to the dose, the contribution of polonium being about two-thirds of the total dose. Consumption of foodstuffs with enhanced lead and polonium activities gives only a minor contribution to the individual dose ( < 2 @Sv year-‘).
DISCUSSION
AND CONCLUSIONS
Many materials of natural origin, such as ores and minerals, often contain high levels of radioactivity caused by radionuclides of the uranium and thorium series. A number of the radionuclides of these series decay through alpha-particle emission which makes them potentially hazardous for both workers in the non-nuclear industries (usually unaware of the dangers) and for the general public which can get exposed to these radionuclides through emissions to the environment. In the Netherlands, a system of licensing is set up for those practices which have raw materials, products or waste on hand with activity concentrations above the current exemption levels or which discharge radioactivity to the environment above the general limits set by the government. By means of a risk study it has to be proven that these practices cause no higher mortality risk to the public than 1O-6 year-’ and that occupational doses are below the dose limit for non-radiological workers, i.e. 2 mSv year-‘. A number of risk studies has been performed by these industries from which in general the following conclusions can be drawn: (1) Occupational exposures in the non-nuclear industry may exceed 2mSv in a year, especially in very dusty workshops and close to large stockpiles. (2) Environmental risks from most non-nuclear industries are within 10-7-10-6 pe r source per year. (3) Use of harbour sediment contaminated with (low levels of) radium activity for landfill, followed by residence on this landfill, can give rise to risks above 1O-6 due to the emanation of radon from the soil. However, the additional risk can be reduced considerably by applying relatively simple remedial actions such as covering the sediment with a layer of non-contaminated soil. (4) The new exemption levels as proposed by the EC may cause considerable problems to the non-nuclear industry in general, in that it will influence its economic position on the world market.
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C. W. M. Timmermans, J. van der Steen
REFERENCES (1994). Amended proposal for a Council Directive (EURATOM) laying down the basic safety standards for the protection of the health of workers and the general public against the dangers arising from ionizing radiation. Hu, S. J. & Kandaiya, S. (1985). Ra-226 and 232Th concentration in Amang. CEC
Health Phys., 49, 1003-7.
Johnston, G. (1991). An evaluation of radiation and dust hazards at a mineral sand processing plant. Health Phys., 60, 781-7. Lower Chamber (1990). Omgaan met risico’s van straling (Managing radiation risks). Dutch Lower Chamber, meeting year 198991990, 21483 (I), State Publishers, The Hague. Nuclear Energy Act (1986). Besluit stralenbescherming Kernenergiewet (Decree on radiation protection under the Nuclear Energy Act). Staatsblad 465. RIVM (1990). Measurements on the natural radioactivity of mineral sands. RIVM Report No. 240801001, Bilthoven, the Netherlands (in Dutch). Scholten, L. C., Roelofs, L. M. M. & Steen, J. van der (1993). A survey of potential problems for non-nuclear industries posed by implementation of new EC standards for natural radioactivity. KEMA, Report 40059-NUC 935203, Arnhem, the Netherlands. Scholten, L. C. & Zuur, C. (1994). Problems arising at the non-nuclear industry in the Netherlands by introduction of new EC rules. Proc. 17th IRPA Regional Congress, Portsmouth, 6-10 June 1994, pp. 13-16. Thompson, H. M. (1994). Environmental radioactivity: a perspective on industrial contributions. 17th IRPA, Regional Congress, Portsmouth, 6-10 June 1994, pp. 253-6. UNSCEAR (1982). Ionizing radiation: sources and biological effects. United Nations Scientific Committee on the Effects of Atomic Radiation, Report to the General Assembly, with annexes.