Abstract
National Radiological Protection Board, Chilton, Didcot, Oxon OX11 0RQ, UK
In this issue (page 101), Hunt reports studies in which volunteers consumed cockles from the Irish Sea and measurements were made of the absorption of radionuclides, including the alpha-emitting actinide nuclides, 239Pu and 241Am. Previously, Hunt and colleagues (1986, 1990, 1993) have reported similar studies of the absorption of 239Pu and 241Am from Cumbrian winkles and mussels and of the naturally occurring alpha-emitter, 210Pu, from crabmeat. Their principal motivation has been to improve estimates of radiation doses to specific critical groups of seafood consumers in the coastal communities; in so doing they have provided valuable information for more general assessments of dose and risk. Studies of the absorption of 239Pu and 241Am have involved the measurement of declining levels as a result of continued reductions in marine discharges from Sellafield and somewhat delayed reductions in concentrations in shellfish. The greatest critical group doses from 239Pu and 241Am are now substantially lower than doses from natural 210Po.
Doses to local population groups due to marine discharges of radionuclides from Sellafield and other sites have been kept under continuing surveillance from the 1950s by the Ministry of Agriculture, Fisheries and Food (MAFF 1995). Large variations in doses have been recorded over the years, with changes in the relative importance of different critical groups (Hunt 1997). Discharges of the actinides, principally 239Pu and 241Am, increased in the 1960s and peaked in the 1970s, corresponding to an increase in reprocessing activities. Subsequently, there has been a dramatic reduction in discharges due to a number of improvements in procedures at Sellafield, most recently the use of the Enhanced Actinide Removal Plant from 1994. Monitoring data showed that shellfish concentrate radionuclides with an affinity for adsorption to sediments, including 239Pu and 241Am. A critical group of fish and shellfish consumers were receiving the highest doses attributable to Sellafield discharges by the 1970s. Doses estimated for this group, due largely to 137Cs in fish and 239Pu and 241Am in shellfish, peaked in the late 1970s and early 1980s and have since reduced progressively.
Table 1. Human data on gut transfer of plutonium and americium.
Those responsible for the estimation of critical group doses have had to contend with a number of changes in values of dose per unit intake (dose coefficients: Sv Bq-1 intake). Doses from the ingestion of 239Pu and 241Am are largely due to their long-term retention in the skeleton and liver, despite their very low levels of absorption from the gut. Uncertainties in estimates of their gut transfer dominate uncertainties in dose coefficients (Kendall et al 1990). The values of gut transfer used by the International Commission on Radiological Protection (ICRP) to calculate doses from 239Pu and 241Am have changed over the years as more information has become available. Thus, the ICRP value of fractional absorption (241Am value) for 239Pu was 3 × 10-5 from 1959, 1 × 10-1 from 1979, 1 × 10-3 from 1986 and 5 × 10-4 from 1993. Values for 241Am were < 1 × 10-4 from 1959, 5 × 10-4 from 1979, 1 × 10-3 from 1986 and 5 × 10-4 from 1993. In addition to theses changes in gut transfer values, in 1993 ICRP introduced new biokinetic models for the behaviour of Pu and Am in the body, taking account of their movement between body tissues and within the skeleton. At the same time, new values were used for the relative contribution of irradiation of individual tissues to overall detriment including risk of cancer (new tissue weighting factors), based on revised estimates of risks (ICRP 1991).
The gut transfer factors used by ICRP were based, as for all biokinetic parameters, on the available human and animal data. Table 1 summarises the human data on the absorption of Pu, Am and related actinide elements which provided the basis for the current ICRP value of 5 × 10-4 for environmental forms of each of these elements. Also included in table 1 are the results reported by Hunt in this issue for the absorption of 239Pu and 241Am from cockles. The values obtained are within the range of previously reported results, providing support for the use of a general value of 5 × 10-4. The approach taken by ICRP in recommending 239Pu values and other biokinetic parameters is that more appropriate values should be substituted in situations where their use can be justified. The case of the absorption of 239Pu and 241Am from Cumbrian winkles is a good example for which the data of Hunt et al (1986, 1990) justified the use of a lower 241Am value of 2 × 10-4 (Hunt et al 1990, NRPB 1990). The results presented in this issue suggest that it would be reasonable to consider the effect on doses of the use of the same value of 2 × 10-4 for 239Pu and 241Am in Cumbrian cockles.
The naturally occurring alpha-emitter 210Po is among the radionuclides that concentrate in shellfish. Interest in doses from this nuclide was raised in the late 1980s when monitoring data showed increased accumulation in Cumbrian shellfish due to discharges from a phosphate processing plant in Whitehaven (Rollo et al 1993, MAFF 1995). Doses are delivered largely from absorbed 210Po reaching blood and depositing in tissues including liver, spleen and bone marrow. An 210Pu value of 0.1 was used by ICRP in 1979, based on animal data and a single measurement of absorption in a volunteer (Silberstein 1950). In 1993, Hunt and Allington reported measurements of the absorption of 210Po by volunteers consuming crabmeat. Estimated absorption was around 0.8 (0.6-1). In the same year, ICRP introduced a number of changes in biokinetic assumptions for Po, including a general 239Pu value of 0.5 which took account of the results obtained by Hunt and Allington (1993) and other available data. New tissue weighting factors were also used at this time.
In the most recent NRPB analysis of the radiation exposure of the UK population (Hughes and O'Riordan 1993), average annual dietary intakes were estimated to result in an alpha dose from 210Po of 50-60 µSv, calculated using the most recent dose coefficients (ICRP 1991, 1993). The greatest critical group dose reported by MAFF in 1994 for fish and shellfish consumers in the vicinity of Sellafield was 80 µSv, with 55 µSv due to 241Am and isotopes of Pu. Greater doses, largely attributable to 210Po, were estimated by MAFF (1995) for a critical group of fish and shellfish consumers in the Whitehaven area of Cumbria. These estimates, which apply only to the dose attributable to discharges from the Whitehaven phosphate processing plant, were 220 µSv based on the ICRP (1993) 241Am value of 0.5 and 340 µSv based on a value of 0.8 (Hunt and Allington 1993). In addition, the assumed levels of seafood consumption would result in doses from naturally occurring background levels of 210Po of 100-200 µSv using an 239Pu value of 0.5 or 150-300 µSv using 0.8.
An important issue in determining biokinetic parameters, dose coefficients and dose estimates is the extent of uncertainties associated with the values given. For the example of 241Am values for Pu and Am (table 1), results show considerable variation between individuals as well as uncertainties in individual estimates. Although the approach of ICRP and others has been to specify parameters, doses and risks without explicit consideration of uncertainties, thought is being given to this subject. Noteworthy is a study in progress, organised jointly by the European Commission and the US Nuclear Regulatory Commission, to provide uncertainty analyses of their accident consequence codes (Harrison 1997). These programs predict the consequences of radionuclide releases from nuclear installations in terms of early and late health effects. The study has addressed separately each of the areas of uncertainty in the programs, including internal dosimetry. It should be completed later this year.
Human data of the quality provided by Hunt and his colleagues greatly improve the confidence with which we can estimate doses and risks from exposure to radionuclides. Doses to volunteers in such studies are invariably low. The ingestion of about 5 Bq 239Pu and 15 Bq 241Am in Cumbrian cockles (p 101) corresponds to a dose of 2-4 µSv, or less than 10 µSv allowing for uncertainties. Nevertheless, there is a tendency among the general population to question the wisdom of scientists who involve themselves in volunteer studies, particularly with alpha-emitting radionuclides. Those concerned with the development of biokinetic and dosimetric models understand the particular value of the limited human data available.
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