Radionuclides environmental pathways

Because estimates of health risk are based on the levels of radionuclides in or near the vicinity properties, the quality of the potential health risk estimates depends upon the availability of appropriate measurement data. Hence, the first steps involved the determination of the appropriate environmental pathways of exposure and developing the source term for the exposure of persons potentially at risk. For our work, the radiological source-term data was based on measurements made principally by the Oak Ridge National Laboratory and the Mound Laboratory. 

Fig. 7-2. Summary of environmental pathways by which terrestrial plants may become contaminated with radionuclides. In the case of an input from atmosphere, or as a result of the process of resuspension , any external radionuclide burden may be reduced by field loss mechanisms conversely, an initially external radionuclide deposit (Rat) may become internalised (i int) following foliar absorption and translocation. Radioactive contaminants of soils may be derived either from atmospheric inputs or from seepage in ground waters. Partitioning of radionuclides in soil—soil water systems controls their availability for root absorption, which normally occurs exclusively from the liquid phase. The chemical speciation of the nuclide in this phase, however, provides a further control on bioavailability which is highly radionuclide specific.

Radionuclides classified as activation products are created in nuclear reactors and other nuclear devices by the reactions of neutrons with fuel and construction materials. Activation products include the isotopes of the transuranic elements and radioisotopes of hydrogen, carbon, caesium, cobalt, iron, manganese, zinc, and a host of other radionuclides, all of which should be recognised and considered in determining the environmental pathways to human exposure. 

The environmental monitoring program should be designed to allow the assessment of the most significant radionuclides and pathways resulting in doses to humans. In selection of samples, measurements and monitoring locations, one should consider an environmental transfer model (as shown in Fig. 9.3, after CAS 1990). 

When environmental monitoring provides results on the radiation levels and radionuclide content of air, water and foodstuffs, metabolic and dosimetric models should be used for the purposes of dose assessment, in conjunction with data on the time spent in different exposure conditions by individuals of the critical groups, the volume of air inhaled and their consumption rates of foodstuffs and beverages. When only the results of source monitoring are available or when environmental monitoring does not provide sufficient data on radiation levels and the contamination of air, water and food, the use of radionuchde transfer models for transfer through the environmental pathways of exposure and the food chains is also necessary. 

People living in areas contaminated with natural or human made long lived radionuclides are usually subjected to exposure via multiple pathways. The contributions of external doses and internal (i.e. by ingestion and inhalation) doses depend on the isotopic compositions and the physical and chemical forms of the radionuclides, environmental conditions and the habits of the population. 

In this discussion, we follow lAEA-Report 295 (1989) and describe samples and pathways relevant to the analysis of radionuclides in foods, and of environmental materials that are part of the immediate pathways leading to contamination of food. 

It is very important to consider the pathways of radionuclides in the environment for design of the environmental monitoring program. Radionuclides enter the receiving environment via direct emissions to atmosphere, direct discharges to water bodies or releases from land burials of radioactive wastes. 

Eollowing a release of radionuclides from a uranium-fuelled reactor to the environment the most important radionuclides to be assessed for internal exposure from the ingestion of food and water, and for the contamination of environmental materials which are parts of the immediate pathways leading to contamination of food, are Cs, Csf Ba), l, and other gamma emitters, the beta emitters Sr, Sr and tritium, and the alpha emitters Pu, ""Am and " Cm. 

The levels of radionuclides in the environment and food have been extensively compiled by UNSCEAR. In general, the radionuclides of major importance in the contamination of food and environmental samples (materials which are part of the pathways leading to the food) are ... 

There is a disadvantage with the use of environmental samples to assess the radionuclide concentrations in the environment. This arises since there is the potential for contamination of such samples from previously deposited material. This material may or may not originate from the source of interest and there may be significant contributions from Chernobyl and weapons fallout. Such contamination pathways include soil splash, the deposition of resuspended material and root uptake. 

Man has artificially produced radioelements (technetium, promethium, transuranium elements) and also many radioactive isotopes of the naturally occurring elements. Thus, the natural radioactivity on the earth is to some extent increased by man-made radioactive materials. Many analytical research works focus on the determination of manmade radionuclides, their migration, pathways, and accumulation in the environment. The aim of this analytical research work is either to know about fate, pathway, and metabolism of materials or to obtain information about possible environmental protection problems. 

Table 3 Relative importance of environmental media as a function of effluent pathway and radionuclide(s) Effluent pathway - atmosphere ...

Indicator organisms are biota that may not be significant in relation to pathways of human exposure and are therefore not used for dose assessment pnrposes, bnt that concentrate radionuclides effectively and so can be utilized as sensitive indicators for assessing trends in environmental radiation levels and activity concentrations of radionuclides in the environment. 

In addition to measurements on direct pathways to humans, consideration should be given to the measurement of activity concentrations in natural or artificially added indicator organisms or materials such as seaweeds, lichen or suspended particulate matter which are not always direct parts of food chains. Indicator materials are selected not because they represent a component of the human diet but because they concentrate radionuclides and provide a measure of trends in activity levels. Because of the concentration mechanism, radionuclides in indicator materials are usually more readily detectable than in foodstuffs, so the indicator organisms or materials provide a more sensitive indicator of environmental contamination [6]. 

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