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Developments in the management of exposures from radon in natural gas in the UK
1064
Developments in the management of exposures from radon in natural gas in the UK D.W. Dixon a , C.K. Wilson b a Group Leader, Radon Studies, National Radiological Protection Board, Chilton, Didcot, Oxon OX11 0RQ,
United Kingdom b Radioactive Substances Division, Department for Environment, Food and Rural Affairs, Ashdown House,
123 Victoria Street, London SW1E 6DE, United Kingdom
Some of the radon in homes where natural gas is used for cooking and heating comes from trace amounts of radon carried from the underground source of gas and which is released during its combustion. Although automatically included by normal UK measurement procedures, doses from natural gas are estimated in this paper to illustrate its contribution to radiation exposures in different circumstances and the factors that affect doses. Doses are estimated from the results of measurements on samples of UK gas from process plant and with assumptions about the transport of radon and daughters and typical distribution conditions. Levels of radon in blended gas received by most users are about 170 Bq m−3 which is comparable with the levels that are present naturally in room air in some buildings as a result of ingress from the ground; this level is greatly diluted during the combustion process. For typical rates of use of gas with an average radon level, the annual dose for domestic consumers from the use of natural gas is estimated at 2 µSv, less than 1% of the dose from radon exposure at the average level in UK homes. These estimates of dose provide support for the introduction of new legislation in the UK which allows producers and shippers of gas to operate without administrative control of the radioactivity in the gas up to a radon content of 5 Bq g−1 , which corresponds for UK gas to about 4000 Bq m−3 . In order to assist suppliers to monitor levels of radon in gas from different sources, a standard sampling and measurement protocol has been developed. 1. Introduction Comprehensive radon programmes in buildings have been developed in many countries to identify, measure and mitigate radon that enters buildings from the ground and progress with the UK programme is reported elsewhere in these proceedings [1]. Although the ground is generally the largest source of radon, there are other potentially significant sources of radon RADIOACTIVITY IN THE ENVIRONMENT VOLUME 7 ISSN 1569-4860/DOI 10.1016/S1569-4860(04)07131-1
© 2005 Elsevier Ltd. All rights reserved.
Developments in the management of exposures from radon in natural gas in the UK
1065
in buildings [2] including the structural materials, the water supply and natural gas, which is considered in this paper. Exposures from radon in the ground can often readily be reduced so this source of exposure receives considerable attention [3]. Nevertheless, the potential for exposure to other radon sources should be recognised, particularly where there is relatively little information or where exposure levels change significantly over the long term. Use of natural gas as a fuel, for example, has increased greatly in the last three decades and it is now supplied to very large numbers of domestic and occupational consumers. Occasional studies and measurements of radon in natural gas in many parts of the world show levels to fall within a wide range and that some sources contain substantially elevated radon levels [4]. With the increasing diversity of supply of natural gas and structure of the supply industry in many countries, it is prudent to consider the factors that affect potential exposures to radon from natural gas in the UK and the need for surveillance or management of exposures.
2. Sources and use of natural gas The concentration of radon in natural gas flowing from a production well depends on many factors, principally the radioactivity of the oil or gas bearing strata, but also on operational factors [5]. In particular, the production stream is often at elevated temperature and may, therefore, contain significant amounts of the volatile elements polonium and lead. Most of this activity, however, is usually deposited on inner surfaces of plant or filters near the well head leaving only the radon gas to be carried through into the gas distribution network that supplies customers. Many companies are aware of the potential accumulation of solid deposits containing natural radionuclides and have routine protection programmes. Gas from multiple individual wells on each platform is mixed at offshore manifolds with that from other platforms and fields before it reaches the processing terminal onshore [6] and radon levels measured at points downstream from individual wells or fields tend to vary less than individual sources. The average value does, however, reflect the proportions of gas mixed from different sources, and these can vary over short periods in response to economic factors and more gradually over long periods as fields become exhausted and new ones are developed. Clearly, therefore, the radon level that is measured in a sample of gas will depend critically on the point in the distribution system or process plant from which it is collected and sampling programmes should be designed with this in mind. There are very few published data on radon levels in gas from individual wells but levels are likely to vary over a considerable range reflecting local rock condition and extraction conditions, which can affect the migration of radon through the rock and into the product stream. Occasional measurements at UK onshore processing plant show marked differences in radon levels in gas from fields in the northern basin of the North Sea and those in the southern basin, where levels are considerably higher. The histogram in Fig. 1 shows the distribution of radon levels found in 31 samples of gas from various wells and fields in the southern basin and illustrates considerable variability in samples from essentially the same stratum. There is some indication that gas from particular
1066
D.W. Dixon, C.K. Wilson
Fig. 1. Distribution of radon-222 concentrations in 31 samples of natural gas from the southern basin of the North Sea. Table 1 Illustrative levels of radon in natural gas Country
Europe
Canada USA
Germany Netherlands United Kingdom Alberta Ontario California Colorado Texas, Kansas
Radon concentration (Bq m−3 ) Average
Range
– – 170 2300 6300 – 940 –
40–360 40–1600 40–3400 370–7600 150–3000 40–4000 410–1670 190–54 000
fields is consistently higher than the average for the basin but there is insufficient detailed information at present to characterise fields by radon level. The wide range within which radon levels varies in gas sources in continental Europe and more remote sources is illustrated in Table 1 [2] and demonstrates the widespread potential for exposure to radon in gas. There are also large regions of gas production in regions east of Europe for which very little published information is available. The dose received by individual occupants of houses in which gas is burned depends on its radon content as well as the use and exposure patterns and it is particularly important, when evaluating doses from radon, to ensure that sampling and measurement programmes provide a reliable indication of the average level in gas as supplied to consumers. Radon will, of course,
Developments in the management of exposures from radon in natural gas in the UK
1067
continue to decay while in transit so the level in gas entering houses will be lower than that measured at process terminals, by an amount that depends on the distance travelled by the gas and its velocity of flow. The principal activity leading to exposure is assumed to be the use of gas in cooking appliances without a flue so that the combustion products of gas, with radon, are dispersed into room air and available for inhalation by the occupants. Radon is unaffected by the combustion process but after dispersal will decay to subsequent nuclides in the decay series. Estimates of the radiation dose received by occupants requires information about the radon level in gas, the quantity and duration of use and the effect of ventilation on the accumulation. The average level of radon in UK gas has been estimated from measurement programmes at 170 Bq m−3 and the assumption is made that about 100 m3 is used annually during cooking periods of about one hour each day. A large proportion of gas supplied to many domestic buildings is used for central heating for which combustion products are discharged outside the building and which therefore does not contribute to radiation dose. Doses have been calculated with a dose conversion convention of 1.56 × 10−5 J h m−3 and on the assumption that the occupants remain in the room after cooking. The rate of accumulation and steady state concentration that is reached inside a building depends on lifestyle factors such as size and configuration of living areas and their rate of ventilation so calculation of the likely dose arising from the release of radon with gas combustion products will vary over a wide range. Under these conditions, the estimated dose from radon received by domestic consumers in premises receiving gas with the average level of radon is 2 µSv [7]. Ventilation also affects the degree of equilibrium that the decay products are able to reach, and as a conservative estimate it is assumed that the daughters are at 50% equilibrium, which will overestimate doses somewhat [8,9]. The larger quantities of gas and longer exposure times in commercial premises such as kitchens and restaurants would, in principle, lead to higher doses than are estimated for domestic premises. In practice, however, removal of gas combustion products with effective fume extraction system, particularly for larger operations, should prevent essentially all of this additional exposure. Additional variability will be introduced by the location of users premises within the UK in relation to different sources of gas. Occupants of premises in southern England, for example, are likely to receive gas containing a higher than average proportion of gas from the southern basin of the North Sea and higher levels of radon. The proportions of gas from different sources varies with demand, however, and relatively few people will receive gas from a single source.
3. Discussion The dose received by domestic users who cook with natural gas with average radon levels for the UK is a few microsievert which is less than 1% of the average exposure that occurs naturally in homes and unlikely to be of concern to most people. Although large numbers of people may be exposed in domestic circumstances, the dose is very small compared with that caused by radon from the ground and other sources such as water. It should be noted, of course, that the contribution from radon in natural gas is automatically included by normal measurement
1068
D.W. Dixon, C.K. Wilson
programmes for radon in houses so these estimates serve principally as a reference against which future doses can be compared. A further perspective on the significance of radon in gas as a source is provided by considering the input rate of radon from gas and from the ground. Ingress from the ground into a typical UK building commonly amounts to a cubic metre or so per hour [10] which implies a daily input of perhaps 50 kBq of radon. By contrast, usage rates of natural gas with the average radon for UK gas amounts to a daily input of only about 0.20 kBq so the scope for exposure from radon in natural gas seems always likely to remain small in relation to other natural sources. Annual doses of the order of a few tens of microsievert are generally considered to be within the range that might reasonably be regarded as trivial and below which regulatory control would therefore generally be considered unnecessary. Furthermore, any reasonable method of reducing exposures either by removal of radon from gas, or be increasing transit time would be prohibitively expensive and potentially disruptive to the national supply. Nevertheless, the measurement data shows the considerable variability of radon and with the introduction of new or different sources of supply, the possibility that exposures might increase to undesirably or unnecessarily high levels should be recognised. The estimates of dose in this paper allow projection of the doses that would be received by domestic users of gas with higher radon levels and suggest that doses would still reasonably be considered as trivial, under the assumed conditions of use, with radon levels considerably higher than has been measured in current gas supplies. These judgements have been used in the UK in support of recent legislation [11] which will exempt producers and shippers of gas from administrative controls, which would otherwise apply to the use of gas and disposal of waste products, if the radon level and the levels of its daughter products, in gas are below 5 Bq g−1 . This corresponds for UK gas to a concentration by volume of about 4000 Bq m−3 . This legislation has been laid before the Parliament of the United Kingdom and will come into force on the 17th May 2002. The new legislation is drafted specifically to exclude any solid deposits that collect on filters or inner surfaces of plant. Existing legislation [12] in the United Kingdom provides a system of control for radioactive waste by instituting a “prior permission” regime whereby holders of radioactive substances are required to register their premises and to seek authorisation for the accumulation or disposal of radioactive waste. The scope of the legislation includes naturally occurring radioelements and for radon from any source applies to concentrations above 0.037 Bq g−1 . The threshold set by the new legislation, specifically for natural gas and natural gas products is at a level that minimises the need for control measures across virtually the whole gas industry so as to benefit a wide range of producer and distributor companies as well as customers. There may be some locations, principally offshore, where the level of radioactivity in gas exceeds the threshold and in such cases the operator will need to register the premises and seek the regulatory body’s approval to accumulate or dispose of what is, legally, radioactive waste. Generally, gas at offshore locations is rigorously contained within vessels and pipework providing little opportunity for exposure of personnel, however a broad range of activities and processes are conducted on such sites and significant amounts of radioactivity may accumulate in some circumstances, requiring practical radiological protection measures.
Developments in the management of exposures from radon in natural gas in the UK
1069
Although control measures would not be required formally even at considerably higher radon levels than currently occur in UK gas, the large number of sources and distribution procedures will ensure that, in all practical circumstances, radon levels will be close to the average value noted above and quite stable over the medium term. It is recognised, nevertheless, that average radon levels could change significantly over the long term if the sources and structure of supply change radically. Some surveillance of radon levels is desirable, and indeed necessary in the few cases where the threshold might be approached, and enables the trends and significance of radon levels in different sources to be evaluated. A standard protocol has been developed [13] to assist companies to collect and measure samples of gas in a reproducible and consistent manner. This protocol has been developed in association with the professional body that represents many of the largest oil and gas producers and should assist harmonisation of radon measurement and management procedures across national boundaries. The Protocol includes detailed guidance on the practicalities of sample collection, measurement and interpretation of results and should maximise the benefit of measurement programmes. Discussions are underway on the extent to which routine surveillance under the Protocol should be required to support the legislative control system.
4. Conclusion The results of measurements of radon in natural gas show considerable variability, which is likely to be particularly marked at upstream locations where individual source wells or fields can have a disproportionate influence. The average level in the UK gas supply is about 170 Bq m−3 , and perhaps a factor of two or so higher for gas from the southern basin of the North Sea. Annual doses estimated for typical domestic users are about 2 microsievert. Some variation in levels might be expected over the long term as new gas fields are exploited, but the general levels of dose are very low compared to the exposures that most people receive from the average level of radon that enters buildings from the ground. Exposures to radon in gas are well below the level at which legislative control is generally considered appropriate and the results have been used to identify a threshold level of radon below which companies are exempt from legislative controls. This threshold level of 5 Bq g−1 , or about 4000 Bq m−3 for UK gas, has been incorporated in new UK legislation which is expected to take effect on May 17th, 2002. Although radon levels in virtually all sources of UK gas are currently well below the threshold, the value of surveillance to identify long term trends or sources with particularly high levels is recognised. A standard sampling and measurement Protocol has been developed in association with members of the oil and gas industry, which may promote harmonisation of the approach to radon in natural gas across national boundaries.
References [1] B.M.R. Green, L. Davey, The new radon programme in England, in: J.P. McLaughlin, S.E. Simopoulos, F. Steinhäusler (Eds.), The Natural Radiation Environment VII, Proc. VIIth International Symposium on the NRE, Rhodes, Greece, 20–24 May 2002, in: Radioactivity in the Environment, vol. 7, Elsevier, Amsterdam, 2004, this volume.
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[2] UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiation), Sources and Effects of Ionising Radiation, Report to the General Assembly, with Annexes, United Nations, New York, 1993. [3] M. Kendall, H. Miles, D. Cliff, R. Green, R. Muirhead, W. Dixon, R. Lomas, M. Goodridge, Exposure to Radon in UK Dwellings, NRPB-R272, HMSO, London, UK, 1994. [4] B.T. Wilkins, The assessment of radon and its daughters in North Sea gas used in the United Kingdom, in: Proceedings of the International Congress of the International Radiation Protection Association, Jerusalem, Israel, 9–14 March 1980, IRPA, Washington, DC, 1980. [5] American Petroleum Institute, Bulletin on management of naturally occurring radioactive materials in oil and gas production, API Bull. E2 (April 1) (1992). [6] Department of Trade and Industry, The Energy report, vol. 2, Oil and gas reserves of the United Kingdom, SO, London, 1996. [7] D.W. Dixon, Radon exposures from the use of natural gas in buildings, Radiat. Prot. Dosim. 97 (2001) 259–264. [8] A.C. James, J.C. Strong, K.D. Cliff, E. Stranden, The significance of equilibrium and attachment in radon daughter dosimetry, Radiat. Prot. Dosim. 24 (1988) 451–455. [9] ICRP Publication 65: Protection against radon-222 at home and at work, Ann. ICRP 23 (2) (1993). [10] K.D. Cliff, J.C.H. Miles, L. Brown, The incidence and origin of radon and its decay products in buildings, NRPB-R159, Chilton, SO, London, 1984. [11] Department for Environment, Food & Rural Affairs, The radioactive substances (natural gas) exemption order 2001, A consultation paper, September 2001. [12] Parliament, Radioactive Substances Act, SO, London, 1993. [13] D.W. Dixon, Protocol for the collection and analysis of natural gas samples for radon-222 concentrations, NRPB-M1211, 2000.
Developments in the management of exposures from radon in natural gas in the UK D.W. Dixon a , C.K. Wilson b a Group Leader, Radon Studies, National Radiological Protection Board, Chilton, Didcot, Oxon OX11 0RQ,
United Kingdom b Radioactive Substances Division, Department for Environment, Food and Rural Affairs, Ashdown House,
123 Victoria Street, London SW1E 6DE, United Kingdom
Some of the radon in homes where natural gas is used for cooking and heating comes from trace amounts of radon carried from the underground source of gas and which is released during its combustion. Although automatically included by normal UK measurement procedures, doses from natural gas are estimated in this paper to illustrate its contribution to radiation exposures in different circumstances and the factors that affect doses. Doses are estimated from the results of measurements on samples of UK gas from process plant and with assumptions about the transport of radon and daughters and typical distribution conditions. Levels of radon in blended gas received by most users are about 170 Bq m−3 which is comparable with the levels that are present naturally in room air in some buildings as a result of ingress from the ground; this level is greatly diluted during the combustion process. For typical rates of use of gas with an average radon level, the annual dose for domestic consumers from the use of natural gas is estimated at 2 µSv, less than 1% of the dose from radon exposure at the average level in UK homes. These estimates of dose provide support for the introduction of new legislation in the UK which allows producers and shippers of gas to operate without administrative control of the radioactivity in the gas up to a radon content of 5 Bq g−1 , which corresponds for UK gas to about 4000 Bq m−3 . In order to assist suppliers to monitor levels of radon in gas from different sources, a standard sampling and measurement protocol has been developed. 1. Introduction Comprehensive radon programmes in buildings have been developed in many countries to identify, measure and mitigate radon that enters buildings from the ground and progress with the UK programme is reported elsewhere in these proceedings [1]. Although the ground is generally the largest source of radon, there are other potentially significant sources of radon RADIOACTIVITY IN THE ENVIRONMENT VOLUME 7 ISSN 1569-4860/DOI 10.1016/S1569-4860(04)07131-1
© 2005 Elsevier Ltd. All rights reserved.
Developments in the management of exposures from radon in natural gas in the UK
1065
in buildings [2] including the structural materials, the water supply and natural gas, which is considered in this paper. Exposures from radon in the ground can often readily be reduced so this source of exposure receives considerable attention [3]. Nevertheless, the potential for exposure to other radon sources should be recognised, particularly where there is relatively little information or where exposure levels change significantly over the long term. Use of natural gas as a fuel, for example, has increased greatly in the last three decades and it is now supplied to very large numbers of domestic and occupational consumers. Occasional studies and measurements of radon in natural gas in many parts of the world show levels to fall within a wide range and that some sources contain substantially elevated radon levels [4]. With the increasing diversity of supply of natural gas and structure of the supply industry in many countries, it is prudent to consider the factors that affect potential exposures to radon from natural gas in the UK and the need for surveillance or management of exposures.
2. Sources and use of natural gas The concentration of radon in natural gas flowing from a production well depends on many factors, principally the radioactivity of the oil or gas bearing strata, but also on operational factors [5]. In particular, the production stream is often at elevated temperature and may, therefore, contain significant amounts of the volatile elements polonium and lead. Most of this activity, however, is usually deposited on inner surfaces of plant or filters near the well head leaving only the radon gas to be carried through into the gas distribution network that supplies customers. Many companies are aware of the potential accumulation of solid deposits containing natural radionuclides and have routine protection programmes. Gas from multiple individual wells on each platform is mixed at offshore manifolds with that from other platforms and fields before it reaches the processing terminal onshore [6] and radon levels measured at points downstream from individual wells or fields tend to vary less than individual sources. The average value does, however, reflect the proportions of gas mixed from different sources, and these can vary over short periods in response to economic factors and more gradually over long periods as fields become exhausted and new ones are developed. Clearly, therefore, the radon level that is measured in a sample of gas will depend critically on the point in the distribution system or process plant from which it is collected and sampling programmes should be designed with this in mind. There are very few published data on radon levels in gas from individual wells but levels are likely to vary over a considerable range reflecting local rock condition and extraction conditions, which can affect the migration of radon through the rock and into the product stream. Occasional measurements at UK onshore processing plant show marked differences in radon levels in gas from fields in the northern basin of the North Sea and those in the southern basin, where levels are considerably higher. The histogram in Fig. 1 shows the distribution of radon levels found in 31 samples of gas from various wells and fields in the southern basin and illustrates considerable variability in samples from essentially the same stratum. There is some indication that gas from particular
1066
D.W. Dixon, C.K. Wilson
Fig. 1. Distribution of radon-222 concentrations in 31 samples of natural gas from the southern basin of the North Sea. Table 1 Illustrative levels of radon in natural gas Country
Europe
Canada USA
Germany Netherlands United Kingdom Alberta Ontario California Colorado Texas, Kansas
Radon concentration (Bq m−3 ) Average
Range
– – 170 2300 6300 – 940 –
40–360 40–1600 40–3400 370–7600 150–3000 40–4000 410–1670 190–54 000
fields is consistently higher than the average for the basin but there is insufficient detailed information at present to characterise fields by radon level. The wide range within which radon levels varies in gas sources in continental Europe and more remote sources is illustrated in Table 1 [2] and demonstrates the widespread potential for exposure to radon in gas. There are also large regions of gas production in regions east of Europe for which very little published information is available. The dose received by individual occupants of houses in which gas is burned depends on its radon content as well as the use and exposure patterns and it is particularly important, when evaluating doses from radon, to ensure that sampling and measurement programmes provide a reliable indication of the average level in gas as supplied to consumers. Radon will, of course,
Developments in the management of exposures from radon in natural gas in the UK
1067
continue to decay while in transit so the level in gas entering houses will be lower than that measured at process terminals, by an amount that depends on the distance travelled by the gas and its velocity of flow. The principal activity leading to exposure is assumed to be the use of gas in cooking appliances without a flue so that the combustion products of gas, with radon, are dispersed into room air and available for inhalation by the occupants. Radon is unaffected by the combustion process but after dispersal will decay to subsequent nuclides in the decay series. Estimates of the radiation dose received by occupants requires information about the radon level in gas, the quantity and duration of use and the effect of ventilation on the accumulation. The average level of radon in UK gas has been estimated from measurement programmes at 170 Bq m−3 and the assumption is made that about 100 m3 is used annually during cooking periods of about one hour each day. A large proportion of gas supplied to many domestic buildings is used for central heating for which combustion products are discharged outside the building and which therefore does not contribute to radiation dose. Doses have been calculated with a dose conversion convention of 1.56 × 10−5 J h m−3 and on the assumption that the occupants remain in the room after cooking. The rate of accumulation and steady state concentration that is reached inside a building depends on lifestyle factors such as size and configuration of living areas and their rate of ventilation so calculation of the likely dose arising from the release of radon with gas combustion products will vary over a wide range. Under these conditions, the estimated dose from radon received by domestic consumers in premises receiving gas with the average level of radon is 2 µSv [7]. Ventilation also affects the degree of equilibrium that the decay products are able to reach, and as a conservative estimate it is assumed that the daughters are at 50% equilibrium, which will overestimate doses somewhat [8,9]. The larger quantities of gas and longer exposure times in commercial premises such as kitchens and restaurants would, in principle, lead to higher doses than are estimated for domestic premises. In practice, however, removal of gas combustion products with effective fume extraction system, particularly for larger operations, should prevent essentially all of this additional exposure. Additional variability will be introduced by the location of users premises within the UK in relation to different sources of gas. Occupants of premises in southern England, for example, are likely to receive gas containing a higher than average proportion of gas from the southern basin of the North Sea and higher levels of radon. The proportions of gas from different sources varies with demand, however, and relatively few people will receive gas from a single source.
3. Discussion The dose received by domestic users who cook with natural gas with average radon levels for the UK is a few microsievert which is less than 1% of the average exposure that occurs naturally in homes and unlikely to be of concern to most people. Although large numbers of people may be exposed in domestic circumstances, the dose is very small compared with that caused by radon from the ground and other sources such as water. It should be noted, of course, that the contribution from radon in natural gas is automatically included by normal measurement
1068
D.W. Dixon, C.K. Wilson
programmes for radon in houses so these estimates serve principally as a reference against which future doses can be compared. A further perspective on the significance of radon in gas as a source is provided by considering the input rate of radon from gas and from the ground. Ingress from the ground into a typical UK building commonly amounts to a cubic metre or so per hour [10] which implies a daily input of perhaps 50 kBq of radon. By contrast, usage rates of natural gas with the average radon for UK gas amounts to a daily input of only about 0.20 kBq so the scope for exposure from radon in natural gas seems always likely to remain small in relation to other natural sources. Annual doses of the order of a few tens of microsievert are generally considered to be within the range that might reasonably be regarded as trivial and below which regulatory control would therefore generally be considered unnecessary. Furthermore, any reasonable method of reducing exposures either by removal of radon from gas, or be increasing transit time would be prohibitively expensive and potentially disruptive to the national supply. Nevertheless, the measurement data shows the considerable variability of radon and with the introduction of new or different sources of supply, the possibility that exposures might increase to undesirably or unnecessarily high levels should be recognised. The estimates of dose in this paper allow projection of the doses that would be received by domestic users of gas with higher radon levels and suggest that doses would still reasonably be considered as trivial, under the assumed conditions of use, with radon levels considerably higher than has been measured in current gas supplies. These judgements have been used in the UK in support of recent legislation [11] which will exempt producers and shippers of gas from administrative controls, which would otherwise apply to the use of gas and disposal of waste products, if the radon level and the levels of its daughter products, in gas are below 5 Bq g−1 . This corresponds for UK gas to a concentration by volume of about 4000 Bq m−3 . This legislation has been laid before the Parliament of the United Kingdom and will come into force on the 17th May 2002. The new legislation is drafted specifically to exclude any solid deposits that collect on filters or inner surfaces of plant. Existing legislation [12] in the United Kingdom provides a system of control for radioactive waste by instituting a “prior permission” regime whereby holders of radioactive substances are required to register their premises and to seek authorisation for the accumulation or disposal of radioactive waste. The scope of the legislation includes naturally occurring radioelements and for radon from any source applies to concentrations above 0.037 Bq g−1 . The threshold set by the new legislation, specifically for natural gas and natural gas products is at a level that minimises the need for control measures across virtually the whole gas industry so as to benefit a wide range of producer and distributor companies as well as customers. There may be some locations, principally offshore, where the level of radioactivity in gas exceeds the threshold and in such cases the operator will need to register the premises and seek the regulatory body’s approval to accumulate or dispose of what is, legally, radioactive waste. Generally, gas at offshore locations is rigorously contained within vessels and pipework providing little opportunity for exposure of personnel, however a broad range of activities and processes are conducted on such sites and significant amounts of radioactivity may accumulate in some circumstances, requiring practical radiological protection measures.
Developments in the management of exposures from radon in natural gas in the UK
1069
Although control measures would not be required formally even at considerably higher radon levels than currently occur in UK gas, the large number of sources and distribution procedures will ensure that, in all practical circumstances, radon levels will be close to the average value noted above and quite stable over the medium term. It is recognised, nevertheless, that average radon levels could change significantly over the long term if the sources and structure of supply change radically. Some surveillance of radon levels is desirable, and indeed necessary in the few cases where the threshold might be approached, and enables the trends and significance of radon levels in different sources to be evaluated. A standard protocol has been developed [13] to assist companies to collect and measure samples of gas in a reproducible and consistent manner. This protocol has been developed in association with the professional body that represents many of the largest oil and gas producers and should assist harmonisation of radon measurement and management procedures across national boundaries. The Protocol includes detailed guidance on the practicalities of sample collection, measurement and interpretation of results and should maximise the benefit of measurement programmes. Discussions are underway on the extent to which routine surveillance under the Protocol should be required to support the legislative control system.
4. Conclusion The results of measurements of radon in natural gas show considerable variability, which is likely to be particularly marked at upstream locations where individual source wells or fields can have a disproportionate influence. The average level in the UK gas supply is about 170 Bq m−3 , and perhaps a factor of two or so higher for gas from the southern basin of the North Sea. Annual doses estimated for typical domestic users are about 2 microsievert. Some variation in levels might be expected over the long term as new gas fields are exploited, but the general levels of dose are very low compared to the exposures that most people receive from the average level of radon that enters buildings from the ground. Exposures to radon in gas are well below the level at which legislative control is generally considered appropriate and the results have been used to identify a threshold level of radon below which companies are exempt from legislative controls. This threshold level of 5 Bq g−1 , or about 4000 Bq m−3 for UK gas, has been incorporated in new UK legislation which is expected to take effect on May 17th, 2002. Although radon levels in virtually all sources of UK gas are currently well below the threshold, the value of surveillance to identify long term trends or sources with particularly high levels is recognised. A standard sampling and measurement Protocol has been developed in association with members of the oil and gas industry, which may promote harmonisation of the approach to radon in natural gas across national boundaries.
References [1] B.M.R. Green, L. Davey, The new radon programme in England, in: J.P. McLaughlin, S.E. Simopoulos, F. Steinhäusler (Eds.), The Natural Radiation Environment VII, Proc. VIIth International Symposium on the NRE, Rhodes, Greece, 20–24 May 2002, in: Radioactivity in the Environment, vol. 7, Elsevier, Amsterdam, 2004, this volume.
1070
D.W. Dixon, C.K. Wilson
[2] UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiation), Sources and Effects of Ionising Radiation, Report to the General Assembly, with Annexes, United Nations, New York, 1993. [3] M. Kendall, H. Miles, D. Cliff, R. Green, R. Muirhead, W. Dixon, R. Lomas, M. Goodridge, Exposure to Radon in UK Dwellings, NRPB-R272, HMSO, London, UK, 1994. [4] B.T. Wilkins, The assessment of radon and its daughters in North Sea gas used in the United Kingdom, in: Proceedings of the International Congress of the International Radiation Protection Association, Jerusalem, Israel, 9–14 March 1980, IRPA, Washington, DC, 1980. [5] American Petroleum Institute, Bulletin on management of naturally occurring radioactive materials in oil and gas production, API Bull. E2 (April 1) (1992). [6] Department of Trade and Industry, The Energy report, vol. 2, Oil and gas reserves of the United Kingdom, SO, London, 1996. [7] D.W. Dixon, Radon exposures from the use of natural gas in buildings, Radiat. Prot. Dosim. 97 (2001) 259–264. [8] A.C. James, J.C. Strong, K.D. Cliff, E. Stranden, The significance of equilibrium and attachment in radon daughter dosimetry, Radiat. Prot. Dosim. 24 (1988) 451–455. [9] ICRP Publication 65: Protection against radon-222 at home and at work, Ann. ICRP 23 (2) (1993). [10] K.D. Cliff, J.C.H. Miles, L. Brown, The incidence and origin of radon and its decay products in buildings, NRPB-R159, Chilton, SO, London, 1984. [11] Department for Environment, Food & Rural Affairs, The radioactive substances (natural gas) exemption order 2001, A consultation paper, September 2001. [12] Parliament, Radioactive Substances Act, SO, London, 1993. [13] D.W. Dixon, Protocol for the collection and analysis of natural gas samples for radon-222 concentrations, NRPB-M1211, 2000.