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TRANSFER OF ACCIDENTALLY RELEASED RADIONUCLIDES IN AGRICULTURAL SYSTEMS AND DEVELOPMENT OF REMEDIAL ACTIONS TO REDUCE THE CONTAMINATION IN FOODSTUFFS

Objective

The assessment of the radiological consequences of an accident in a nuclear plant rely on the quantification of several processes in the transfer of radionuclides along the food chain. This is particularly important in the case of a contamination of a large area where the doses will be mainly influenced by the deposition and transfer processes as well as the ingestion pathways.

The response to the Chernobyl accident has demonstrated that certain known processes and parameters, such as the change in time of radionuclide availability, the dynamic behaviour of deposited activity and the influence of food processing on the radionuclide content in food, need to be investigated more accurately. These facts confirm the need for new experimental evidences to quantify the relevant processes with a source term as close as possible to real accidental conditions.

The use of reliable parameters and accurate description of processes are essential for the preaccident assessment modelling, have serious implications on the establishment of the derived intervention levels and for the implementation of protective measures designed to mitigate the radiological consequences of an eventual accident.

The aim of this project is to contribute to the reliability of radiological assessment methods and to establish sound scientific bases to be used in the design of postaccident countermeasures.
The experience following the Chernobyl accident in 1986 emphasized the need to improve the quantification of several processes and parameters in the transfer of radionuclides along the food chain. The use of reliable parameters and accurate descriptions od processes are esssential for preaccident accessment modelling and for the implementation of protective measures.

The objective of the project dealt with the following aspects: characterization of a synthetic radioactive aerosol which simulated the accidental source term of a PWR reactor; study of the physical and chemical processes involved in its behaviour after depositionon some common European soil-crop systems; derivation of relavant transfer factors.

Two radioactive aerosols were used in the whole period of the TASSAS project, both of them contained three radioelements. caesium-134, strontium-85 and silver-110. The crops selected were lettuce, as a representative of a directly consumed crop, the pea as a vegetable with fruit and alfalfa as a typical perennial crop used for animal feeding. Two types of soil were used, a sandy and a sandy-loam soil.

A number of experiments were performed and several results were obtained on the following aspects:

aerosol characterization;
aerosol deposit and interception by different crops;
loss of radionuclides from lettuce and peas via leaching;
direct absorption of radionuclides deposited on different cultures;
evaluation of radionuclide root uptake for lettuce and pea plants as well as the dynamics of soil to plant radionuclide transfer using alfalfa;
retranslocation of radionulcide to pea plants to fruit;
study of radionuclide partioning in soils as well as their distribution in depth.

The project involves the generation of heat generated polymetallic aerosols that are representative of a real accidental release from a pressurized water reactor (PWR). The aerosol is a mixture of 16 elements representing fuel and cladding, structural materials, control rods and fission products and is heated to 2750 degrees in the graphite tube of an electrically heated furnace. Radioactive elements can be introduced and their measurement made by gamma spectrometry. The aerosols ARE physically and chemically characterized.

The aerosols thus produced are scattered into tight containment in which plants can be cultivated.

Studies in the radionuclide contamination of crops were conducted in 5 areas: characterization of the soils, interception by mature lettuce, radionuclide migration during irrigation, soil speciation and deposition on leaf surfaces.

2 standardized soils were selected to compare radionuclide activity, one of a sand texture and the other a sandy loam. The sandy loam soil has a higher adsorption capacity and cation content.

Mature lettuce plants grown on both soils were polluted with radioactive aerosols. Radionuclide canopy interception was very high as the crop covered most of the soil surface.

After pollution with radioactive aerosols plants were irrigated (washed out) with distilled water on alternate days for 2 weeks. Lettuce was sampled before each irrigation and the soil at the beginning and end of the experiment. Radionuclide distribution in sandy soil layers showed a significant increase of soil activity after irrigation. The major part of each radionuclide remained in the top layer of the soil.

2 extraction schemes were applied to sandy soil to study soil phases where radionuclides may be bound. The first results conclude that distributions obtained depended on the methodology and thus a standardized extraction method is required.

A successive extraction procedure was designed to study radionuclide deposition on leaves. Caesium-strontium, caesium-silverand silver-strontium ratios were compared before and after irrigation and before and after successive extraction. An increase of the caesium-strontium and caesium-silver ratios after successive extractions before irrigation indicate that caesium is selectively absorbed. After irrigation isotopic ratios following successive extractions do not change significantly.

The project objective was to quantify the transfer of radionuclides during food processing.

Measurements were carried out either on different raw and processed vegetables sampled in factories and analysed for potassium, caesium, cobalt and strontium, or on vegetables that were artificially contaminated and processed in the laboratory.

Measurements of stable caesium and strontium in cereals before and after industrial processing show an important activity decrease; retention factors vary from 0.1 to 0.2 for wheat milling and 0.1 to 0.4 for rice processing; wine production and distillation leads to a noticeable radioactivity decrease.
Canning of vegetables contaminated by caesium, strontium, cobalt, ruthenium and plutonium, after paring and bleaching, leads to residual radioactivity varying from 30% to 50% for green beans and from 5% to 20% for carrots.
The influence of different processing technologies and additives on the caesium, strontium, cobalt, ruthenium content of vegetables after canning was also studied. Parameters tested were the influence of the currently used additives (or process additives) added to the brine of canned vegetables (green beans, carrots, tomatoes), and the effect of pH variation of the brine, of the time of storage, and different blanching and peeling techniques. The results show no significant change in the retention of radionuclides when modifying these parameters.
Information on the fate of factory by-products and wastes from the processing of oilseed, wheat and sugar beet show that levels of caesium, strontium and potassium are significantly elevated in wastes used as animal feed.
These results which were compiled in a data file, will help to improve the estimations of the ingested activity when evaluating radionuclide impact on the environment through the foodchain pathway.

Measurements were carried out on canned pears, rice processings and wine production and distillation. In studies on canned pears the caesium analysis seemed unreliable; the strontium value was aberrant; there was a concentration increase in lead, chromium and nickel probably from the can metal and an appreciable concentration decrease in cobalt and potassium. Rice processing involves 2 physical steps and each step showed significant decreases in the levels of potassium, cobalt, strontium and caesium. 3 wine processes were studied: rose, red from the grape only and red made from grape and stalk. Rose is the process leading to the lowest concentrations of the measured elements. No significant amounts of potassium, cobalt, strontium or caesium were found in the resulting ALCOOLS following distillation of the wines.

The project involved cultivation or collection of food products from a region having relatively high concentrations of radionuclides in the soil, preparation of raw and processed foods for analysis of radionuclides and stable elements and to analyse the samples for potassium-40, cobalt-60, strontium-90, ruthenium-106 and caesium-137 together with their stable isotopes.

The Sellafield region of England was chosen for the cultivation of peas, potatoes, brussels sprouts, carrots, wheat and collection of field mushrooms. Milk was also obtained from this area to provide dairy products. Sellafield was chosen because it has a relatively high soil concentration of caesium-137 owing to fallout from the Chernobyl accident and historical releases of radionuclides from the Sellafield Works.

Radionuclide analysis. Results are available for caesium-137, potassium-40 and stable elements in 26 samples of raw and processed foods. Strontium-90 was analyzed in 13 samples. Food processing retention factors for radionuclides were derived. A wide range of retention factors was recorded for caesium-137, potassium-40 and stable cobalt, from 0.1 to 3 according to the processing treatment; thus an increased concentration of radionuclides in fresh foods was sometimes evident. Stable element analysis. Analysis was made for cobalt, caesium, potassium, ruthenium and strontium. It was noticed that raw and processed mushrooms contained large amounts of silver and lead. Canning increased the concentrations of tin in peas and potatoes by an order of magnitude.

Soil to plant transfer factors have been derived from soil concentrations of stronium-90 and caesium-137 at locations where crops were cultivated. Thus the counter effect of food processing on the removal of radionuclides taken up by crops following an accidental release may also be assessed.

Project objectives were to establish the main characteristics of the Spanish mediterranean diet, analyze any differences with other European Community (EC) areas and to obtain information on soil to plant transfer factors for strontium-90 and caesium-137.

Agricultural production and food consumption data was gathered from many sources for the Spanish mediterranean area and the rest of the EC. The data was analyzed and production and consumption ratios (Spanish mediterranean to total Spanish, Spanish mediterranean to EC) developed. In spite of some differences in the consumption of oil, vegetables, fruits and wine, the Spanish mediterranean diet could be considered representative of the global EC mediterranean diet. A general conclusion is that mediterranean foods have a progressive influence in the EC countries due to expanding commercial exchanges. This indicates the importance of determining radionuclide dose estimations of typical mediterranean diet could be considered representative of the global EC mediterranean diet. A general conclusion is that mediterranean foods have a progressive influence in the EC countries due to expanding commercial exchanges. This indicates the importance of determining radionuclide dose estimations of typical mediterranean foods.

A bibliographic search on soil to plant transfer factors indicated such information is available mainly for generic foods. The lack of such information for typical mediterranean products and foods is evident. Experimental work is commencing to obtain this data.
Three main aspects will be considered in the project:
a simulated accidental source term will be used and the behaviour of aerosol deposits containing Sr, Cs and Ag isotopes will be followed in some European soil-crop systems;
the extent of radionuclide transfer rates through the food chain as modified by well established food processing techniques will be studied for Sr, Cs, Co and Ru;
a study of specific Mediterranean data and environmental factors aimed at an identification of the relevant differential characteristics and lack of appropriate data for the dose evaluation in southern Europe.

These studies are intended to be complementary with other CEC funded projects such as the RESSAC and COSYMA programmes and consequently special attention will be paid to the coordination with other related projects.

WORK PROGRAM
The behaviour of radioactive aerosol deposits will be studied using a source term as similar as possible to accidental conditions. To achieve this objective CEN-CEA Cadarache has developed a particle generating system. Some of the relevant radionuclides in nuclear accidents, and 12 other elements present in the reactor core, will be volatilized by heating at high temperature (3000 degrees celsius).

These particles will be characterized and the physicochemical processes involved in their deposition on typical crops will be studied.

The research will start with lettuce (Lactuca sativa), one of the most universal plants in vegetable gardens and wheat (Triticum aestivum), one of the most important cereals in Europe.

Two types of soil will be used, a French one and a Spanish one. Both types of soils were chosen taking into account the fact that they will also be used in the CEC RESSAC Programme.

The fractions intercepted by plants, adhered to leaves, directly absorbed, as well as the dynamics of leaf washout, root uptake, migration in soil and speciation will be studied in different growth stages of the lettuce plants. The soil to plant dynamics, the activity transfer to mature grain will be described for wheat sown in soil contaminated by aerosol deposits.

The factors that modify the radionuclide content in food processing and the potential contribution of those factors for dose control in the event of an accident will also be studied. Radionuclides and/or stable elements to be analyzed include Cs, Sr, Co and Ru.

The processes to be studied include freezing, drying, canning, milling, juice extraction, dairy processing and normal culinary operations. In Britain, vegetables to be studied are potatoes, carrots, peas, brussels sprouts and mushrooms. Wheat (milling) will also be included. In 1991, oil production from rapeseed and fruit juice extraction (apple and blackcurrant) will be studied. In France, studies will be made on the processing of haricot beans, tomato and grape juice, salad vegetables, sugar beet, rice and olive oil. In both Britain and France some dairy products will be examined including milk, fats, cheese, and powders of milk and whey.

Finally, the project will include a study on the specific Mediterranean data that will be compared with currently used generic parameters, and the relevant data that are lacking will be identified. Additionally some relevant soil to plant transfer factors for Sr and Cs isotopes will be obtained.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

CENTRO DE INVESTIGACIONES ENERGETICAS, MEDIAMBIENTALES Y TECNOLOGICAS
Address
22,Avenida Complutense 22
28040 Madrid
Spain

Participants (4)

Commissariat à l'Energie Atomique (CEA)
France
Address
Centre D'études De Cadarache Sere-ders
13113 Saint-paul-lez-durance
UNIVERSITAT DE BARCELONA
Spain
Address
Gran Via De Les Corts Catalanes 585
Barcelona
UNIVERSITY OF READING
United Kingdom
Address
Whiteknights, Whiteknights House, Palmer Building
RG6 6AH Reading / Silchester
United Kingdom Atomic Energy Authority
United Kingdom
Address
353,Harwell
OX11 0RA Didcot - Oxfordshire