Radionuclides environmental significance

Sources, Ubiquity and Diversity of Environmentally Significant Radionuclides... 

Fig. 7-1. A Radioecologist s Periodic Table , showing those elements with radionuclides of particular environmental significance.

This chapter first introduces the fundamentals of radioactivity and its environmental significance. The following sections focus on the geochemistry of uranium and uranium ore deposits as the basis of the nuclear fuel cycle. Later sections consider nuclear power and the geochemistry of important radionuclides in nuclear wastes, with emphasis on the actinide elements and some of their fission products which make nuclear wastes a potential problem for future generations because of their very long half-lives. 

BEHAVIOUR OF RADIONUCLIDES AND ENVIRONMENTALLY SIGNIFICANT ELEMENTS IN NUCLEAR FUEL ELEMENT DEBRIS (FED)... 

Abatement of radionuclides and environmentally significant elements using pH adjustment with hydroxide is understood, however in this study, the efficacy of removal of these key species from a FED matrix at pH 7.5, chosen as this provided maximum solubility for magnesium, was unknown. 

Key environmentally significant elements and radionuclides not fully abated by the pH adjustment process described in this study, namely Cs, Co, and Sr, are fiirther abated a by passage of the supernatant through activated carbon and highly selective ion-exchange media respectively, specifically Co-Treat ( Co ( Sr) specific) and Cs-Treat ( Cs specific), which ensures that the final process effluent is fully compliant with Bradwell Site 1 Discharge Authorisations. 

Behaviour of Radionuclides and Environmentally Significant Elements Table 2 Elements/Radiomclides indicating full (95 5%) abatement... 

The principal abiotic processes affecting americium in water is the precipitation and complex formation. In natural waters, americium solubility is limited by the formation of hydroxyl-carbonate (AmOHC03) precipitates. Solubility is unaffected by redox condition. Increased solubility at higher temperatures may be relevant in the environment of radionuclide repositories. In environmental waters, americium occurs in the +3 oxidation state oxidation-reduction reactions are not significant (Toran 1994). 

The type of approach described here is obviously more important for systems where the solution chemistry of the nuclide (charge, oxidation state and degree of complexation) is more complicated. Without supporting laboratory data, it is possible that significant retention values may be incorrectly interpreted as being due to radionuclide association with material in a particular size fraction. The components of the environmental sample might contribute to the separation process and retain species which on a size basis should readily pass through the filter membrane. 

Fission products of uranium and other actinides are released to the environment during weapons production and testing, and by nuclear accidents. Because of their relatively short half-lives, they commonly account for a large fraction of the activity in radioactive waste for the first several hundred years. Important fission products are shown in Table 3. Many of these have very short half-lives and do not represent a long-term hazard in the environment, but they do constitute a significant fraction of the total released in a nuclear accident. Only radionuclides with half-lives of several years or longer represent a persistent environmental or disposal problem. Of primary interest are °Sr, Tc, and... 

According to Styron et al., (1979) for a realistic assessment of the magnitude of release of radionuclides, special attention needs to be given to lead-210 and polonium-210 since they appear to have a large potential for significant environmental impact and have not received sufficient attention in trace-element studies for power plants. Another potentially important parameter in determining radiation exposure to man centres on disposal and utilization of coal ash and refuse. Lee et al., (1977) have suggested that emanation of radon-222 from ash disposal ponds will be the most serious radionuclide problem associated with increased use of coal. A potential hazard can be associated with the use of fly ash in cement and concrete blocks and in roadway construction. The radium-226 in these concrete blocks used for home construction may constitute an important source of radon-222 dose to the public. 

The approach used is that outlined by Dodd B. and Humphries L.L., (1988) although significant input was also obtained from IAEA Safety Series No. 57, Generic Models and Parameters for Assessing the Environmental Transfer of Radionuclides from Routine Releases Exposures of Critical Groups (1982). 

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). 

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. 

The presence of humic and fulvic acids in surface waters and groundwaters will have a significant influence on the transport and fate of metals, radionuclides, and organic contaminants in the environment. These natural organic acids can either transport or immobilize contaminants, depending on the environmental conditions. Humic and fulvic substances can also retard or enhance the photochemical decomposition of pesticides or toxic organics. Therefore, to be sucessfiil any remediation strategies must consider the effects of humic materials. If properly understood, this behavior can be used to manipulate pollutant solubilization and facilitate containment or cleanup of contaminated sites. 

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