M.S. Baxter

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.

Radiometric investigations of Kara Sea sediments and preliminary radiological assessment related to dumping of radioactive wastes in the Arctic Seas

There has been world-wide concern over the possible health and environmental impacts of the dumping of radioactive wastes in shallow waters of the Kara and Barents Seas. During 1992, IAEA-MEL participated in a joint Russian-Norwegian expedition to the region and collected a suite of sediment cores for characterization of downcore radioactivity distribution patterns and inventories with the aim of assessing past and present inputs of radionuclides. The 137Cs, 238Pu, 239,240Pu, and 241Am contents of the 0–1 cm sections range from 17 to 32, 0·01 to 0·06, 0·4 to 1·3 and 0·2 to 0·5 Bq kg−1 (dry wt), respectively. The 238Pu/239,240Pu and 241Am/239,240Pu activity ratios are consistent with those reported for global fallout. Detailed information on downcore radionuclide distribution patterns and inventories, as well as an account of sedimentation parameters, are presented. A preliminary assessment of the long-term radiological impact of radioactive waste disosal in the Arctic Seas, on global and regional scales, has been made. The modelling results suggest that the global radiological impact of the disposals will be comparable to or less than those resulting from other anthropogenic and natural sources of radioactivity. However, radiological effects on the regional scale may be of importance. IAEA is carrying out a 4 year International Arctic Seas Assessment Project (IASAP) that will comprehensively cover all aspects of radioactive dumping in the Arctic Seas and it will be reported to the London Convention in 1996. The IAEA-MEL study described here represents a preliminary contribution to IASAP.

Underwater gamma-spectrometry with HPGe and NaI(Tl) detectors

Abstract An in situ γ-spectrometer designed for underwater operations consisting of HPGe and NaI(Tl) detectors with electronics, data acquistion and processing electronics, and a supporting system consisting of a hydraulic winch with 1200 m conducting cable is described. The characteristics of the system and results obtained during operational tests and deployment in the Irish and Kara Seas are presented. The spectra measured with the HPGe detector represent the first set of high resolution sea-bed γ-spectra ever recorded in situ. Further, a possible utilisation of underwater γ-spectrometry for in situ monitoring of leakages of radionuclides from dumped or sunken nuclear objects/wastes or discharges from nuclear plants is discussed. Remote stationary γ-spectrometers operating on the sea-bed, in the open sea or in any aquatic environment with satellite data transmission would be a very efficient means of long-term monitoring. Such systems could also be equipped with other sensors, like current, temperature and salinity meters, and thus provide comprehensive information for the region.

Observations on plutonium in the oceans

Abstract This review describes various studies of marine plutonium distributions, behaviour and transfer carried out at the Marine Environment Laboratory of IAEA. Plutonium in sea water is primarily present in solution as Pu(V), probably as PuO + 2 , and on particles in reduced form as Pu(IV), most likely as species such as adsorbed Pu(OH) 4 . In nearshore sediments and coastal soils, plutonium is complexed with high molecular weight humic and low molecular weight fulvic compounds. Results of oxidation state studies in the Black Sea and Norwegian fjords show that major reduction of dissolved Pu(V) occurs [to Pu(III)] in anoxic waters in association with complexation of reduced plutonium with these dissolved and colloidal humics and fulvics. The IAEA-MEL has also been active in developing and applying laboratory radiotracer methods to study rates and pathways of plutonium flux through marine organisms, particularly using the γ-emitting tracer, 237 Pu. Plutonium concentration factors spanning five orders of magnitude have been observed, with no significant dependence on oxidation state. Almost complete excretion of ingested plutonium by zooplankton makes their defecation an extremely important mechanism in the scavenging of plutonium from surface waters. This phenomenon has been well documentated in many field studies using time-series sediment traps to quantify and characterize elemental flux. Plutonium fluxes in the northwestern Mediterranean have recently been shown to correlate well with mass flux and suggest residence times of 24 and 3.5 years, respectively, for 239,240 Pu and 241 Am in the upper mixed layers, values which are rather longer than those measured previously. The IAEA-MEL has recently studied plutonium distributions in the vicinity of former waste dumpsites in the NE Atlantic Ocean and in the Kara Sea. The results of these surveys provide evidence of radiologically negligible but measurable leakage at the former site and of no leakage whatsoever at the latter. Computer predictions of potential future dispersion of plutonium-containing wastes from the Kara Sea suggest that the maximum possible global collective dose (30 manSv) would be extremely low. Finally, the review notes the increasing analytical contribution to environmental plutonium studies which can be made by mass-spectrometric techniques. At IAEA-MEL, the ETV-ICPMS technique along with low-level radiometric method is being refined in order to maximize the possibility of using aquatic isotopic signals to characterize and identify nuclear source-terms.

Technologically enhanced radioactivity: An overview

Abstract Man first increased his radiation exposure above the natural baseline when he began cave-dwelling and mining and working metals and minerals. Since then the understanding of technologically enhanced radioactivity and of the associated exposure pathways has increased to the present state which is described by UNSCEAR (1993, Sources and effects of ionizing radiation, UN Scientific Committee on the Effects of Atomic Radiation, Report to the General Assembly, UN, New York) as ‘sketchy’ and ‘not sufficient to enable reliable estimates’. This review of present-day knowledge suggests that many non-nuclear industries are indeed capable of generating significant critical group or collective exposures or both. Doses can be delivered to workers and to populations living in the vicinity of industrial sites. Notable amongst these types of industry are the phosphate-processing industries (fertilizers, detergents, chemicals), fossil-fuel burning, oil and gas extraction, mining of almost all kinds, ore and heavy mineral processing and the forest industries, such as pulp making and biofuel use. There is a growing awareness of the problem but a two-tier system of radiation protection and discharge control is liable to continue, particularly in developing countries, with an imbalance in the understanding and limitation of radioactivity emissions from non-nuclear versus nuclear industries and also in radiological protection of workers. This review, covering historical and current aspects, suggests that there is a need for further study of technological enhancement of radioactivity and that the first international conference dedicated to the subject is extremely worthwhile and should become the first landmark meeting in a long and important series.

Technological enhancement of natural radionuclides in the marine environment

This review summarizes aspects of technologically enhanced radioactivity in the UK marine environment, considers briefly related investigations in western Europe and then discusses some models for the kinetics of series decay and ingrowth which can be applied to technological inputs of series members to the marine environment and to their differential elemental biogeochemistries.

Distribution of radionuclides in mussels, winkles and prawns. Part 1. Study of organisms under environmental conditions using conventional radio-analytical techniques

Abstract Mussels ( Mytilus edulis ) and winkles ( Littorina littorea ), collected from Ravenglass, Cumbria, England in the vicinity of the British Nuclear Fuels plc nuclear reprocessing plant at Sellafield, and prawns ( Palaemon serratus ), landed nearby at Whitehaven, have been investigated to determine the distributions of α-emitting ( 210 Po, 238 Pu, 239 + 240 Pu, 241 Am) and γ-emitting ( 95 Nb, 95 Zr, 103 Ru, 106 Ru, 137 Cs, 241 Am) radionuclides in their tissues and organs. Ravenglass mussels exhibited 239 + 240 Pu concentrations ranging from 43 Bq kg −1 dry in muscle tissue to 1658 Bq kg −1 dry in byssal threads, the corresponding 137 Cs range being 131–1340 Bq kg −1 . Although 210 Po concentrations were not determined in byssal threads, muscle tissue still displayed the lowest polonium concentration (124 Bq kg −1 ), whilst the viscera (containing digestive gland, stomach and kidneys) contained the highest (596 Bq kg −1 ). Subsequent concentration factor estimates for 137 Cs, 210 Po and 239 + 240 Pu in the total soft parts of Ravenglass mussels were, respectively, 9, 25 800 and 1400. In Cumbrian winkles, nuclide concentrations ranged for 239 + 240 Pu, from 18·5 Bq kg −1 dry (muscle tissue) to 457 Bq kg −1 dry (pallial complex); for 137 Cs, from 103 (foot tissue) to 1495 Bq kg −1 (pallial complex) and for 210 Po, from 12·2 (muscle tissue) to 145 Bq kg −1 (digestive gland). Total soft part concentration factors (CFs) were calculated to be 16 for 137 Cs, 5500 for 210 Po and 5700 for 239 + 240 Pu. The magnitude of these CFs, as for those in the mussel, is consistent with the respective CF values recommended by the International Atomic Energy Agency for molluscs. Radionuclide concentrations in Whitehaven-landed prawns were much lower than those observed in mussels or winkles; no artificial γ-emitter activities were present above detection limits and the highest 239 + 240 Pu concentration was 5·96 Bq kg −1 dry in the carapace. 210 Po activities, however, were more readily detectable throughout the prawns tissues, concentrations ranging from 2·7 Bq kg −1 (abdomen muscle) to 144 Bq kg −1 (cardiac foregut), producing CFs of the order of 2 × 10 4 in tissues associated with feeding and digestion. Previous studies have attempted to determine the principal nuclide source to marine organisms by comparing nuclide activity ratios in their tissues, sea water and particulate material. From the environmental samples studied here, no single transport medium appears to dominate uptake. The primary radiological implication of the observed radionuclide concentrations in Ravenglass mussels and winkles is that, from seafood ingestion, the critical group receives only a small percentage (ca. 10%) of the ICRP-recommended subsidiary dose limit. Dose contributions from 210 Po are higher than those from 239 + 240 Pu in mussels but are less than those from 239 + 240 Pu in winkles.

Natural and Artificial Radioactivity in Coastal Regions of UK

The transport and bioaccumulation of natural and artificial radionuclides in the coastal regions of UK have been investigated. The magnitude of radionuclide concentration variation throughout the UK coastline has been quantified and where appropriate source terms have been identified. Sites bordering the Irish Sea show the highest levels of artificial radioactivity which are derived from past Sellafield discharges. Enhanced concentrations of natural radioactivity were found in all Whitehaven samples (due to discharges from a phosphate ore processing plant at Whitehaven), in sediment at Blackhall Colliery near Hartlepool (from disposal of coal mining spoil directly into the North Sea) and in mussels at Aberdeen (due to discharges from the de-scaling of pipes and valves by the oil industry). Highest radionuclide concentrations were present in Whitehaven mussels -3124Bqkg-1 dry weight of 210Po. Excluding sites bordering the Irish Sea, levels of natural radionuclides (210Pb, 210Po, 232Th, 238U) were greatly in excess of the artificial radionuclide concentrations in all sample types. The biological preference for accumulation of Po relative to its grandparent 210Pb is evident. 210Po/ 210Pb activity ratios range between 2 and 40 in mussels, winkles and seaweed but only between 0.24 and 1.3 in sediments and sea water. Radiologically, the highest radiation exposure to the public evident from the results would be from the ingestion of Whitehaven mussels (3.2mSv y-1), the greatest single contribution being from technologically enhanced 210Po.

Distribution coefficients (Kd's) for use in risk assessment models of the Kara Sea

As a prerequisite for most evaluations of radionuclide transport pathways in marine systems, it is necessary to obtain basic information on the sorption potential of contaminants onto particulate matter. Kd values for use in modeling radionuclide dispersion in the Kara Sea have been determined as part of several international programs addressing the problem of radioactive debris residing in Arctic Seas. Field and laboratory Kd experiments were conducted for the following radionuclides associated with nuclear waste: americium, europium, plutonium, cobalt, cesium and strontium. Emphasis has been placed on two regions in the Kara Sea: (i) the Novaya Zemlya Trough (NZT) and (ii) the mixing zones of the Ob and Yenisey Rivers (RMZ). Short-term batch Kd experiments were performed at-sea on ambient water column samples and on samples prepared both at-sea and in the laboratory by mixing filtered bottom water with small amounts of surficial bottom sediments (particle concentrations in samples = 1-30 mg/l). Within both regions, Kd values for individual radionuclides vary over two to three orders of magnitude. The relative particle affinities for radionuclides in the two regions are americium approximately equal to europium > plutonium > cobalt > cesium > strontium. The values determined in this study agree with minimum values given in the IAEA Technical Report [IAEA, 1985. Sediment Kds and Concentration Factors for Radionuclides in the Marine Environment. Technical Report No. 247. International Atomic Energy Agency, Vienna.]. Given the importance of Kds in assessments of critical transport pathways for radionuclide contaminants, we recommend that Kd ranges of values for specific elements rather than single mean values be incorporated into model simulations of radionuclide dispersion.

Modelling the potential radiological consequences of radioactive waste dumping in the Kara Sea

Abstract There has recently been growing concern over the dumping of high- and medium-level solid radioactive wastes in the Kara Sea by the former Soviet Union. The largest amounts of radioactive wastes were dumped primarily as nuclear reactors containing spent nuclear fuel. The present radionuclide inventory in dumped nuclear reactors is estimated at 4·7 PBq. Compartmental and hydrodynamic models have been developed and applied to describe the possible dispersal of radioactive contaminants and to predict the long-term radiological impact on global, regional and local scales. The collective committed effective dose to the world population based on the marine food ingestion pathway has been calculated as 2·2 man Sv. Modelling results suggest that only radiological effects on a local scale may be of importance. The global radiological impact of the disposals in the Kara Sea will be smaller than from other anthropogenic sources of radioactivity.