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Imperial College of Science, Technology & Medicine, (University of London) Imperial College Reactor Centre, Silwood Park, Berkshire

 

SEA-TO-LAND TRANSFER OF RADIONUCLIDES: MATERIALS & METHODOLOGY OF ENVIRONMENTAL AIR SAMPLING

 

ELLIS I. EVANS (1991)

A report submitted in partial fulfillment of the requirements

For the Msc degree and/or DIC

ABSTRACT

A series of wind tunnel experiments were carried out to quantify the aerosol capture efficiency of muslin screens at 2 wind speeds and three mono-dispersed particle sizes. Three other fabrics were also tested under the same experimental conditions. The low wind speed experiments indicated that muslin cloth had the greatest capture efficiency at the 1 mm particle size with a value of 12 % which increased to 13 % and 23 % at the 5 mm and 10 um particle size range. Polyester and Lab-cloth had significantly increased screen efficiencies over muslin at the 5 um and 10 um particle size range. The high wind speed data indicated that Muslin and Polyester have similar screen capture efficiencies. Data from this wind tunnel study were used to select materials for field trials.

Air flow deviation over screens from wind tunnel experiments relative to screen porosity was also determined for all fabrics. Screen capture efficiencies at low wind speed indicate that an increased screen porosity is not associated with an increased screen capture efficiency. The high wind speed data indicates that the amount of screen capture area relative to screen porosity is a significant factor of screen capture efficiency.

Field trials on the West Cumbrian coast indicated that both Muslin and Polyester have different screen capture efficiencies under identical environmental conditions. Muslin shows an increased screen capture efficiency for insoluble radionuclides, whilst, Polyester catches substantially more soluble caesium-137. The long term accumulation of radioactivity due to sea-to-land transfer in the area was also assessed. The concentration of caesium-137 within soils decreased with distance from the coast. The 137/134Cs Chernobyl 'fingerprint' was identified within the top two centimetres of soil along the Nethertown transect. The greater part of activity within soils was confined to the first six to eight centimetres down the soil profile, indicating a surface input source for this nuclide.

BRIEF DISCUSSION ON SEA-TO-LAND TRANSFER MECHANISMS

Mackay et al 1990 has detailed some of the more important processes and transfer mechanisms of sea to land transfer. Foam flotation and bubble scavenging are thought to be the major mechanisms of radionuclide enhancement and bubble-burst, the major transfer mechanism back to land. Foam flotation occurs when breaking waves force air below the sea surface. As the bubbles rise upwards, particulate materials are scavenged by rising bubbles and transported to the sea surface to form an enriched radionuclide micro layer at the sea-air interface. Surfactants such as industrial detergents and organic matter enhance this attraction or affinity for particulate-associated radioactivity to rising air bubbles (Eakins et al 1982, Walker et al 1986). The resultant enriched microlayer is either washed ashore, to be deposited within the surf zone or a combination of wind and wave action injects the material into the atmosphere as an aerosol. On-shore winds transport the actinide-heavy metal enriched marine aerosol back to land.

Bubble-scavenging occurs both over the open sea and at the surf zone. Bubble path length through a water column in the open sea is much greater than for water-bodies closer to shore, as a consequence, the opportunity for particulate scavenging is much greater within the open oceans.

Other sources of bubble production include the biological activities of detritivores and photosynthesising plants within the photic zones of the open oceans. Bubbles are also generated within the sea by the action of winds on waves. The energy dissipated into the surface layer of the sea from winds may produce 'whitecaps' capable of producing 10E08 m3 of bubbles (McKay et al 1990). The bubbles produced burst at the sea-air interface either as a thin film or jet drop. Jet drops can achieve a vertical height of 20 cm above the sea surface.

The concentration of radionuclides within the marine aerosol (relative to their bulk water concentrations) is largely due to the oxidation state of the element in question. The stable elements Mg, Na and Cl are not enriched whereas low valence plutonium, americium and a suite of heavy metals are. Surprisingly, radiocaesium, although soluble in nature has enrichment factors of between 1-2 within marine aerosols compared to its bulk seawater concentrations. The concentration of stable Na is used in this study as a 'marker' for marine aerosols advecting back to land.

In terms of the environment around the Sellafield area it is noteworthy to mention that Albright & Wilson discharge surfactants (mostly phosphates) into the Irish Sea and that many of the coastal resorts along the NW coast including the bigger towns discharge raw and partially treated sewage into the receiving environment of the Irish Sea (see figure 1 below). Thus, sewage and surfactants probably increase the rate at which foam-flotation processes occur with consequent increase in particle-associated radioactivity and heavy metal concentrations within coastal zones and areas.

 

Figure 1, EC bathing water standards for the NW

Of England (Observer newspaper 25.7.1990)

ENVIRONMENTAL CHARACTERISTICS AND SOURCES OF 226Ra &241Am

Rocks and sediments contain primordial remnants of 232Th and 238U. The greatest concentrations of these naturally-occurring radionuclides are found in sandstones, particularly the black shales laid down in the Jurassic period and these deposits predominantly underlay the North Sea oil fields (Miller 1983). Over geologic timescales, the shales are in contact with seawater and some radioactivity is desorbed into solution to form what is known as 'formation waters'. Formation waters are found in structures known as anticlines which also contain oil and gas reserves which are exploited by oil companies. Formation waters contain most of the Group II cations i.e. Ba, Sr, Ca and Ra. Sulphates are also present in formation waters and co-precipitation between elements commonly occurs to produce Ba(Sr)SO4, Ba(Ra)SO4 .

As oil reserves within anticlines become depleted, pressure drops and oil companies usually inject 'treated seawater' into the structure. Treated seawater is usually more saline than the formation waters and decay products such as 234U, 234Pa(m), and 234Th become more soluble. On the other hand, elements such as radium, polonium, bismuth and Pb become even more insoluble. The insoluble nature of these elements pose a radiological risk to both oil workers who handle/dismantle oil pipes and to the local marine environment where some of these pipes are dumped.

Over a period of time, oil pipes build up an insoluble layer of decay products, most notably, BaSO4 and precipitates of radium. These deposits are difficult to remove and workers either dispose of the pipes to the marine environment or haul the pipes to shore and use high pressure steam as de-scaling tools. Either disposal route releases radium and barium precipitates into the marine and terrestrial environment.

Radium-226 is both an g and a -emitter and is strongly bound by most soil types. However, sandy soils which are low in organic matter have a much lower cation exchange capacity and as a consequence radium is much more mobile in these environmental media. The decay products of 226Ra are both gaseous and particulate and decay via a ,b and g -emissions. Pathways back to Man include inhalation, ingestion, and external irradiation. Importantly, radium is a nutrient analogue of Ca and follows the same metabolic pathways within the human body i.e. the target tissue is bone (Green NRPB 1988).

SOURCES OF 241Am:

This nuclide does not occur in Nature and is formed anthropogenically from successive neutron captures of uranium. The reaction of most importance and quantity is the irradiation of 239Pu which yields 241Pu which has a half-life of 14.4 years. Plutonium-241 decays to produce 241Am which has a half life of 433 years. Americium-241 forms part of the liquid waste stream which is discharged from Sellafield into the Irish Sea. Most of the 241Am present in these waste streams are tri-valent and consequently, largely insoluble. Americium-241 is particle-reactive and will therefore adsorb onto silts and sediments. Its pattern of environmental transport will be dependent on the dynamics of water movement into and out of the Irish Sea.

Peak discharges of 241Pu occurred between 1973-1974 and Playford (1990) detailed the increase of 241Am in 1987 due to 241Am ingrowth. During 1985-1989 the deposition of 241Pu increased which suggests that activities of 241Am may peak again sometime in 2002-2003 because of the half life of 241Pu being 14.4 years.

DESCRIPTION OF CUMBRIAN STUDY SITE

A transect extending some 5 km inland at Nethertown was selected for soil and air sampling. Nethertown is approximately 5 km due north of the Sellafield nuclear reprocessing plant in Cumbria UK. A sample of Irish Sea seawater was also collected for actinide and radiocaesium analyses. This part of Cumbria is largely agricultural and the average yearly rainfall is approximately 900 mm (Fulker pers. comm. 1991). The soils are classified as stagnoley, i.e. non-calcareous and loamy which are underlain by clays. These soils are also associated with shales and till (Soil Survey of England & Wales 1976).

STUDY SITES: SOILS

Soil sampling was based on the following criteria (after Cawse 1983). (a) soils which were relatively undisturbed i.e. rough grazing pasture, (b) from the same lithologic parent series, (c) level ground not subject to run-off, (d) open aspect to the shoreline without any topographic complications which may affect rainfall, (e) as far as possible from major or minor roads which may resuspend previously deposited material (f) where possible, sampling took place away from any overhanging hedges.

Site 1 was a coastal site situated 2 m above the high water mark. The soil was an acidic brown earth underlain by till and sand. It was relatively free-draining with evidence of 'cutback' at its base from wave action with some slump features extending onto the beach. Site 2 was a field of rough grazing pasture as was site 3. Site 4, however, was a cultivated field and soil samples were taken from an undisturbed spot near one edge of a fence. Sites 5 and 6 were rough grazing pasture and both samples were taken close to one field edge.

EXPERIMENTAL METHODS: SOILS

Soils were oven dried at a temperature of 350C for 24 hours, lightly disaggregated and sieved through a 2 mm sieve. The dried soils were coned and quartered for gamma spectroscopic analyses. Gamma-emitting nuclides were identified and quantified using a lead-shielded Ge-Li drifted spectrometer. The output from the detector was fed to a 4096 multi-channel analyser (Nuclear Data 6600). Americium-241 was resolved using its 59.6 keV spectral line. Confirmation of this nuclide is usually accepted if its low order peaks at 26.3 keV and 20.133 are also identified.

A 5 g aliquot of soil was filtered through a Whatman no. 42 filter paper with 50 ml of double-distilled water and the solution analysed for sodium using atomic emission spectroscopy (AES) using a Thermo Jarret Ash S12. Because of its anticipated high content, a low order wavelength of 330.3 nm was used in its identification. Soil pH was measured through Gentry's (1988) method. 5 g of soil was added to a conical flask containing 12.5 ml of double-distilled water. The suspension was thoroughly mixed and left to equilibrate for a period of 3 hours. Soil pH was determined using a Gallenkamp stick pH meter.

SEAWATER ANALYSES:

A 10 l sample of seawater from the Irish Sea was collected by dipping a polythene container just below the surface at a distance of about 1 m from the water's edge. Four litres of seawater were pre-filtered through a weighed Whatman no.1 filter which provided a rough size fraction of <63 m m suspended sediment size. The filters were dried in an oven at a temperature of 350C for 24 hours, re-weighed and analysed for g -emitters. Following g -spectroscopy, the filters were ashed at a temperature of 5000C for 24 hours to determine carbon/organic content. The residue was filtered through Whatman no. 42 filters with 10 ml of 2M HNO3, made up to 25 ml with water and analysed for sodium content. The remaining pre-filtered seawater was filtered through 2 m m PTFE membrane filters (using a peristaltic pump) which defines very roughly the soluble/insoluble fractions of Irish Sea waters.

SEAWATER RESULTS:

The activities of americium-241 in bulk seawater were below the limits of detection. The activities of caesium-137 and radium-226 were 5.2E-02 Bq l-1 and 7-02 Bq l-1 respectively. The concentration of Na loading within seawater was high at 106 mg l-1. Activities normalised to Na suggest that 226Ra is more strongly associated with Na than radiocaesium. This is consistent with the fact that 137Cs acts conservatively within water and is more soluble than particle-reactive 226Ra.

In contrast, the activities of 241Am within suspended sediments were detected at 5.75 mBq l-1 indicating its largely insoluble nature within seawater. The activities of 137Cs within suspended sediments were lower than the bulk seawater activities which suggested its largely conservative behaviour and / or low particle loading within this sample location. Kd coefficients for 226Ra (concentration in aqueous/particulate phase) were found to be just under an order of magnitude which underline its largely insoluble nature.

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