Fractionation of radionuclides

In the following pages, we will summarise the main processes controlling the fractionation of radionuclides during weathering and transfers into surface waters. Subsequently, we will present the main results obtained on surface weathering and transport in the river waters. Throughout this chapter, we will use parentheses to denote activity ratios. 

Each sorption experiment was conducted by adding 5.0 mL of the appropriate traced solution, prepared as described above, to a weighed (-1 g) portion of Hanford sediment. To simulate advancement of a radioelement plume from a failed tank through previously waste-wetted sediment, each sediment sample was preequilibrated twice with the relevant untraced solution prior to introduction of the traced solution. Each pre-equilibration lasted at least 2 hr. Following a 7-day equilibration with the traced solution, each sediment-solution mixture was centrifuged, the solution was filtered through an ultrafilter, and the radionuclide solution concentration was determined. Distribution coefficients and fractions of radionuclides sorbed were determined for each sorption experiment. The distribution coefficient, Kd, is the activity per gram of sediment divided by the activity per mL of solution at equilibrium. 

The particle size of a fission aerosol, and the distribution of fission products between particulate and vapour phases, depends on the mechanism of release to the atmosphere. In a weapons explosion, some physicochemical fractionation of radionuclides may occur, particularly if the explosion is near the ground. Everything in the vicinity is vapourised by the heat of the explosion, but within less than a minute the fireball cools to a temperature in the range 1000-2000°C, and refractory materials such as metal oxides and silicates condense to form particles (Glasstone Dolan, 1977). Refractory fission products, and plutonium, are incorporated in these particles. 

Leonard, K.S., Harvey, B.R., Woodhead, R.J., Brooks, T. and McCubbin, D. (1994) Assessment of an ultrafiltration technique for the fractionation of radionuclides associated with humic material./. Radioanal. Nucl. Chem. Articles, 181, 309-320. 

The main components of these sediments are similar to those in the sediments formed at earlier times sand and clay minerals. However, river and lake sediments contain also relatively large amounts of organic material and microorganisms. Appreciable fractions of radionuclides present in rivers and lakes are sorbed in sediments, but usually it is difficult to discriminate between the influences of the various processes taking place and to correlate the fixation in the sediments with certain components. As far as the inorganic components are concerned, clay minerals play the most important role. 

Following a nuclear accident, deposited radionuclides may be present in different physico-chemical forms, ranging from mobile low molecular mass (LMM) ionic species to inert high molecular mass (HMM) colloidal forms or particles. Even in areas far from the actual site, the relative fraction of radionuclides associated with HMM formed in rain-water may be substantial (Salbu, 1988). The size distribution patterns of radionuclides deposited, the composition of the fallout, level of activities and the activity ratios, will depend on the accident scenario, course of event, distance from the source, wind dispersion and climatic or microclimatic conditions. Spatial and temporal variations in the behaviour of deposited radionuclides with respect to mobility and bioavailability are to be expected and may in part be attributed to differences in the physico-chemical forms of radionuclides in the fallout, at least during the first years after deposition (Salbu et al., 1994). 

Radionuclides in sediment are indicators of facility releases to bodies of water and runoff from surfaces in the drainage region. Samples represent the fraction of radionuclides precipitated or sorbed on suspended matter that has settled to the bottom. If such radionuclides are in sediment, elevated levels usually can be found where suspended material tends to settle as the water flow rate decreases, e.g., at widening channels, on the inner side of bends, and behind dams. 

The first two tests measure the fraction of radionuclide loss to walls, while the others only show relative retention in solution. A deficiency in these tests for samples with low radionuclide content is that the obtained count rate often is too low for precise determination of loss. A radioactive tracer solution can provide higher count rates but may not represent the conditions in the actual sample. 

For francium, it is easy to show that after 10 half-lives there is only a tiny fraction of radionuclide left. The reduction in mass after 10 half-lives is actually 1/(2 x 2 X2X2X2X2X2X2X2X2)= 1/2 = 1/1024. 

Based on measurements of air filters from 1965 to 1967 and rainwater samples from 1967, the Tc/ Cs ratio seems to be a factor of 10 higher than expected from the fission yield. The anomalous ratios of fission products observed in the atmosphere may partly be explained by fractionation of radionuclides during the detonation process. The precursors of Cs are gaseous or volatile elements, i.e., xenon and iodine, while the precursors of T c are refractory elements, i.e., zirconium and niobium, which are usually incorporated in radioactive particles. Thus, the Tc/ Cs ratio in the atmosphere may decrease with time after detonation due to the deposition of large radioactive particles. For deposited material releases of Tc with time should be expected due to weathering of particles. Howevei we cannot, at this stage, exclude additional sources contributing to releases of Tc to the atmosphere. 

Without either spray droplets or flooded pathways, substantial fractions of radionuclides released from the degrading reactor fuel can be retained within the reactor coolant system. Results of some example calculation for radionuclide retention in the reactor coolant systems for various types of accidents are shown in Table III-l. The natural retention of radionuclide vapors oeeurs because the vapors either condense on surfaces or react with these surfaces. Depending on the surface temperature and the duration of its exposure to high temperature steam, the surface material is either ehromium oxide (Cr203) or iron oxide (Fe304 y). Both of these materials are expected to be reactive toward cesium-bearing vapours and strontium or barium vapors. Stainless steel lead screws above the core at Three Mile Island were found to have captured cesium by reaction with silica impurities in the steel. Metallic nickel inclusions in the oxide films on surfaces within the reactor coolant system are reactive toward tellurium whether it is in the metallic state or present as TeO or SnTe. 

The largest fraction of radionuclides by far enters the containment attached to aerosol particles which are carried by the steam flow. As was shown in Table 7.3., according to thermodynamic calculations oxides and metals are the predominant species of the refractory aerosols generated in the reactor pressure vessel, and one has to assume that at the lower temperatures prevaihng in the containment no significant changes in the chemical nature of these primary aerosols will occur. The primary aerosol particles will be coated by layers of more volatile substances such as CsOH and Csl which are deposited in regions of lower temperatures in the primary system other fission product species may also be attached to the aerosol surfaces by condensation, chemical reactions and, probably to a lesser extent, by physi- and chemisorption. 

A on the vertical axis of the graph corresponds to 0.90, the fraction of radionuclide remaining. Moving horizontally to the curve and projecting down to the horizontal axis—again A —ogives 0.16 half-life. 

FCiN is the fraction of radionuclide i from the radioactive contents released into the containment system under normal conditions during transport (conservatively 10%) ... 

FE A is the fraction of radionuclide i which is available for release from the containment system into the environment under accidental conditions during transport ... 

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