Elis Holm

A comparison of doses from 137Cs and 210Po in marine food: A major international study

Radioactivity levels of natural 210Po and anthropogenic 137Cs in sea water and biota (fish and shellfish) have been estimated for the FAO fishing areas on the basis of measurements carried out in recent years. Collective doses resulting from seafood consumption are calculated for each FAO area using radioactivity data for water and biota. Good agreement is observed between the results calculated by these two methods, with the exception of the doses from 210Po via shellfish consumption. The collective effective dose commitment from 137Cs in marine food in 1990 has been estimated at 160 man Sv with an uncertainty of 50%. The corresponding dose from 210Po is 30000 man Sv with an estimated uncertainty of a factor of 5. The results confirm that the dominant contribution to doses derives from natural 210Po in fish and shellfish and that the contribution from anthropogenic 137Cs (mainly originating from nuclear weapons tests) is negligible.

Polonium-210 and lead-210 in the terrestrial environment: a historical review.

The radionuclides (210)Po and (210)Pb widely present in the terrestrial environment are the final long-lived radionuclides in the decay of (238)U in the earths crust. Their presence in the atmosphere is due to the decay of (222)Rn diffusing from the ground. The range of activity concentrations in ground level air for (210)Po is 0.03-0.3 Bq m(-3) and for (210)Pb 0.2-1.5 Bq m(-3). In drinking water from private wells the activity concentration of (210)Po is in the order of 7-48 mBq l(-1) and for (210)Pb around 11-40 mBq l(-1). From water works, however, the activity concentration for both (210)Po and (210)Pb is only in the order of 3 mBq l(-1). Mosses, lichens and peat have a high efficiency in capturing (210)Po and (210)Pb from atmospheric fallout and exhibit an inventory of both (210)Po and (210)Pb in the order of 0.5-5 kBq m(-2) in mosses and in lichens around 0.6 kBq m(-2). The activity concentrations in lichens lies around 250 Bq kg(-1), dry mass. Reindeer and caribou graze lichen which results in an activity concentration of (210)Po and (210)Pb of about 1-15 Bq kg(-1) in meat from these animals. The food chain lichen-reindeer or caribou, and Man constitutes a unique model for studying the uptake and retention of (210)Po and (210)Pb in humans. The effective annual dose due to (210)Po and (210)Pb in people with high consumption of reindeer/caribou meat is estimated to be around 260 and 132 μSv a(-1) respectively. In soils, (210)Po is adsorbed to clay and organic colloids and the activity concentration varies with soil type and also correlates with the amount of atmospheric precipitation. The average activity concentration levels of (210)Po in various soils are in the range of 20-240 Bq kg(-1). Plants become contaminated with radioactive nuclides both by absorption from the soil (supported Po) and by deposition of radioactive fallout on the plants directly (unsupported Po). In fresh leafy plants the level of (210)Po is particularly high as the result of the direct deposition of (222)Rn daughters from atmospheric deposition. Tobacco is a terrestrial product with high activity concentrations of (210)Po and (210)Pb. The overall average activity concentration of (210)Po is 13 ± 2 Bq kg(-1). It is rather constant over time and by geographical origin. The average median daily dietary intakes of (210)Po and (210)Pb for the adult world population was estimated to 160 mBq day(-1) and 110 mBq day(-1), corresponding to annual effective doses of 70 μSv a(-1) and 28 μSv a(-1), respectively. The dietary intakes of (210)Po and (210)Pb from vegetarian food was estimated to only 70 mBq day(-1) and 40 mBq day(-1) corresponding to annual effective doses of 30.6 μSv a(-1) and 10 μSv a(-1), respectively. Since the activity concentration of (210)Po and (210)Pb in seafood is significantly higher than in vegetarian food the effective dose to populations consuming a lot of seafood might be 5-15 fold higher.

The natural radioactivity of the rock phosphates, phosphatic products and their environmental implications

The radioactivity of the Minjingu phosphorite deposit in Arusha-Tanzania was determined by radiochemical and radiometric methods of analysis. The main radioisotopes analysed in the rock phosphate, phosphate concentrate (ground rock phosphate), phosphate fertilizers (superphosphate, and triple superphosphate) and phosphogypsum were 238U, 235U, 232Th and their progenies. Uranium-238 and its progenies had high activity concentrations of ≈4000 Bq kg−1 in all materials analysed, whereas phosphogypsum had high 226Ra, in contrast to triple superphosphate fertilizer. Extensive use of ground rock phosphate, and phosphate fertilizers was assessed in relation to their radiological implications to the farmers. The external radiation arising from the use of phosphate fertilizers in agricultural fields was found to be less than 2% of normal background radiation (50 nGy h−1). A model for pathways of the natural radionuclides from the ground rock phosphate, phosphate fertilizers and by-products is proposed in order to assess their long-term impacts on the secondary environment.

Determination of 99Tc in environmental samples

Abstract A method for the analysis of 99Tc in environmental samples has been developed. The pre-treatment of samples to avoid losses of technetium during ashing has been carefully studied. Hydrochloric acid is in this case superior to ammonia. Technetium-99m ( t 1 2 = 6.0 h ) is used as a radiochemical yield determinant. For large water samples technetium is reduced to +IV oxidation state and co-precipitated with iron hydroxide. Technetium is extracted from sulphuric acid solution with tributylphosphate and back-extracted with sodium hydroxide solution. Back-extraction with ammonia has also been shown to be successful. Technetium is electroplated from 2M sodium hydroxide solution (or from ammonium sulphate solution at pH 4–9) onto stainless steel discs. The radiochemical yield is determined either by counting conversion electrons from 99mTc using a surface barrier detector or by gamma-spectrometry using the 140 keV transition from 99mTc. After decay of 99mTc is measured by an anti-coincidence shielded GM-gas flow counter. The radiochemical yield of 10–100 g biological samples is 50–80%.

Removal of radionuclides at a waterworks.

A waterworks with an average production rate of 1.3 m3 s(-1), providing several large cities in the province of Scania with drinking water has been studied regarding its capacity to remove several natural and anthropogenic radionuclides. The raw water is surface water from lake Bolmen which is transported through an 80 km long tunnel in the bedrock before it enters the waterworks. The method used for purification is a combination of coagulation-flocculation and filtration in sand filters. Two different purification lines are currently in use, one using Al2(SO4)3 as a coagulant and one using FeCl3. After coagulation and flocculation the precipitate is removed and the water is passed through two different sand filters (rapid filtration and slow filtration). Water samples were collected at the lake, the inlet to the waterworks, after each of the flocculation basins (Al2(SO4)3 and FeCl3), after rapid filtration and from the municipal distribution network. The samples were analysed with respect to their content of uranium, thorium, polonium, radium, plutonium and caesium. The results show a high removal capacity for uranium (about 85%), thorium (>90%), plutonium (>95%) and polonium (>90% in the coagulation-flocculation process) while caesium, strontium and radium pass through the purification process with almost unchanged activity concentrations. During transportation of the water in the tunnel it was also observed that infiltration of groundwater leads to a change in isotopic ratios and/or activity concentrations for the naturally occurring radionuclides and plutonium.

Further Studies of Plutonium and Americium at Thule, Greenland

Eleven years after the accidental loss of nuclear weapons in 1968, the fourth scientific expedition to Thule occurred. The estimated inventory of 1 TBq 239,240Pu in the marine sediments was unchanged when compared with the estimate based on the 1974 data. Plutonium from the accident had moved further away from the impact point and at some locations the vertical distribution indicated a downward displacement of Pu in the sediment column since 1974. Seawater and seaplants showed no evidence of the presence of Pu from sources other than fallout; but Pu in benthos varied nearly proportionally with the levels in sediments. From the measurements of 239,240Pu in the Macoma community since 1970, the transfer factor to this community was estimated at 0.01 Bq a per Bq released to the sediments. The inventory of 241Am was 0.1 TBq. The vertical distribution of Am in sediments did not differ from that of Pu, but in benthos 241Am/239,240Pu were two times higher than in sediments. Seaplants showed the same value of Am/Pu as seawater. There was no indication of any biomagnification of Pu or Am through the marine food chains at Thule.

Radiocaesium from Sellafield effluents in Greenland waters

Since the middle of the 1970s, discharges of radiocaesium to the Irish Sea from the British Nuclear Fuels Ltd Sellafield (formerly Windscale) installation in Cumbria have increased the concentrations of 137Cs and 134Cs in the northern North Atlantic. In coastal British waters1,2 enhanced concentrations have also been seen in the North Sea3–5, the Danish Straits and western part of the Baltic Sea4,5, the Norwegian coastal current6,7, and the Barent and Greenland Seas7. We report here that radiocaesium from Sellafield is now detectable in the East Greenland polar current. The transit time from Sellafield is estimated at 6–8 yr. The concentrations in the polar current are approximately one-thousandth of those found in the North Channel by the outlet from the Irish Sea.

Radioactivity levels in the Red Sea coastal environment of Sudan

Measurements of natural and fallout radionuclides in marine surface sediments, seagrass and algae collected from the Sudanese coastal waters of the Red Sea have been made using high resolution γ-spectrometry, radiochemical separation and α-spectrometry. Activity levels of uranium isotopes, thorium isotopes, 226Ra, 210Po, 40K and 137Cs were determined in the samples. Comparison of the data on natural radionuclides from coastal marine sediments with those collected from 30 km offshore (Sanganeb atoll) reveals that both anthropogenic and terrestrial influx from the hinterland is negligible. However, values for 226Ra and 210Po are higher in the sediments of Port Sudan harbour relative to those from the adjacent fringing reefs. Uranium content is higher in shallow-water sediments and the authigenic fraction constitutes 12% on the average. The 228Th:232Th disequilibrium in sediments indicates rapid rate of sedimentation at the sampling sites. The activity levels detected for 137Cs in sediments collected from the Port Sudan harbour area are fairly high as compared with values from other sampling locations. On the basis of individual data, the variations are insignificant with regard to the uptake of natural radionuclides by marine species considered in this study. However, 137Cs activity in algae ranged from 0.33 to 1.32 Bq kg−1 with Sargassum (brown algae) showing the highest level.

The natural radioactivity in phosphate deposits from Sudan

Abstract The natural radionuclide content of phosphate deposits at Uro and Kurun in Eastern Nuba mountains in the state of Kordofan (Western Sudan) and soils was determined radiochemically and gamma spectrometrically. The analyses were performed for natural uranium and thorium, 210 Po, 226 Ra and 40 K. The data indicate that 238 U and its decay products contribute primarily to the high natural radioactivity of phosphate ores. At maximum, we found an activity concentration of 2600 Bq/kg natural 238 U. From the values obtained, the dose rates over agricultural areas, resulting from the use of ground rock phosphate as fertilizers estimated under extremely conservative assumptions, result in an additional external radiation exposure for members of the population of 5.42 × 10 −9 nGy/a and 1.13 × 10 −9 nGy/a for Uro and Kurun rock phosphates, respectively. These results show that the natural radionuclides contained in Uro and Kurun ground rock phosphate contribute very little to the average terrestrial radiation exposure to the population.

Mercury and major essential elements in seals, penguins, and other representative fauna of the Antarctic.

Concentrations of total Hg and major essential elements, Ca, Mg, Na, and K, were measured in the muscle, liver, and kidney of three species of seals, crabeater seal (Lobodon carcinophagus), leopard seal (Hydrurga leptonyx), and Weddell seal (Leptonychotes weddelli) collected in the Antarctic. The muscle and liver of three species of penguins, i.e., gentoo penguin (Pygoscelis papua), Adelie penguin (P. adeliae), chinstrap penguin (P. antarctica), and other representative fauna were also analyzed for the elements. Distinct inter-tissue differences in the metal concentration were observed; liver had the greatest concentrations of Hg, kidney showed maximum concentration of Ca and Na, while muscle was characterized by the greatest content of Mg and K. Inter-specimen differentiation of concentrations among the same species was distinctly visualized for Hg but not for the major essential elements. The Hg concentration in the seals analyzed are in keeping with those reported previously by other authors. Such comparison for Ca, Mg, Na, and K was impossible because of the lack of available literature data for their concentrations in marine seals. Numerous significant correlations were observed between concentrations of the several metals analyzed. There was no correlation between Hg, which is a non-essential element and both Zn and Cd which are essential and non-essential elements, respectively in the liver or kidney of seals. However, there were significant correlations between concentration of Zn and the sum of molar concentrations of Hg + Cd in kidney (r=0.82) and liver (r=0.76). The results suggest that several control mechanisms operate to maintain physiologically required levels that decreases any effect of heavy metal toxicants such as Hg and Cd.