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A unique characteristic of the Chernobyl accident was the release to the environment of large amounts of uranium oxide fuel particles. Within the 30 km exclusion zone, the major fraction of deposited radionuclides was associated with fuel particles, and fallout in this region contained enhanced levels of refractory radionuclides including transuranics and 90Sr. The hypothesis was that 90Sr would become more mobile after release from the particle matrix, and that weathering rates would be influenced by particle composition and environmental conditions. Little was known on the fate of transuranics. The main objectives of the project, therefore, were:
to study the physical and chemical characteristics of fuel particles released to the environment; to investigate the effect of particle composition and environmental conditions on the weathering rates of particles; to investigate the change in speciation of radionuclides released from fuel particles in the soil-water system. The results of the project have highlighted the significance of uranium oxide fuel particles in determining the fate of radionuclides in the Chernobyl area. Electron microscopy studies on individual fuel particles have confirmed the presence of different types of fuel particles, suggesting differences in structures and possibly the degree of uranium oxidation. The influence of particle composition and soil pH on the solubility and weathering rate of particles in the natural environment has been demonstrated for the first time. Particles deposited to the West of the reactor (the initial releases) had slower dissolution rates than those deposited to the North and South ("oxidised" particles released during the reactor fire), and rates increase with decreasing soil pH. Dissolution rate constants ranged from0.006 to 0.33 yr-1.
Laboratory dissolution studies on isolated particles have shown that the solubility of particles decreases with time: possibly indicating surface oxidation, and have confirmed the influence of pH, as well as redox conditions and microbial activity/temperature, on particle dissolution. Laboratory derived dissolution rate constants showed good agreement with field derived constants. Thus, it follows that well-controlled laboratory experiments could provide important modelling data on the behaviour of uranium oxide particles under environmental conditions other than those observed in the Chernobyl area. In addition, kinetic tracer studies have provided information on Cs, Sr and Pufixation rates in Chernobyl soils: for Pu- and Cs- isotopes, fixation rates exceed particle dissolution rates; for Sr- isotopes the rate of fixation is much slower than particle dissolution, Radiocaesium sorption rates to soils were not influenced by moisture content, supporting a diffusional model for interactions with soil components.
Sequential extraction studies illustrate that radionuclide speciation incontaminated soils varies as a function of time and not always in ways that could be predicted from other environmental sources, e.g. weapons' fallout. Inthe ten years following deposition, 90Sr has become more mobile, 137Cs has become more strongly-bound to soils, whilst Pu-isotopes show little variation in distribution between extraction fractions. It is clear that the major rate determining parameter for changes in soil-to-plant transfer of 90Sr,and the subsequent increase in dose to humans, is the fuel particle weathering rate.
The results of this project make a significant contribution to our knowledge on the behaviour and significance of fuel particles ill the environment. In particular, information on the variables influencing particle dissolution rate scan explain some of the large spatial and temporal variations in 90Sr mobility observed in the Chernobyl area. Incorporation of particle weathering constants into models will help to improve prognoses and reduce uncertainties in predictions of' the future migration of90Sr and other radionuclides within the food chain. In addition to being of specific relevance to studies on the consequences of the Chernobyl accident, such information is also essential for increasing preparedness in the event of another nuclear accident.
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