Radionuclide fractionation processes

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. 

The purpose of this test was to ascertain the hazard that would result from a rapid reactivity insertion into a Kiwi reactor. The test provided an occasion to study radionuclide fractionation in debris from a reactor excursion. Because fractionation processes distribute hazardous radionuclides among debris particles in different manners, their effects require documentation and study. Chapter 17 by Crocker and Freiling in this volume will provide background for the reader who is unfamiliar with fractionation phenomena. 

The block PSM simulates the pollution processes over territory Q as a result of atmospheric transport, river and surface coastal outflow, navigation, and other human activity (Mohler and Arnold, 1992 Muller and Peter, 1992). The variety of pollutants is described by three components oil hydrocarbons O, heavy metals (e solid particles, -0 dissolved fraction), and radionuclides e. It is supposed that pollutants only enter living organisms through the foodchain. 

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. 

The majority of the longer-lived transuranic nuclides produced by neutron capture reactions decay primarily by a-emission. Most environmental samples contain radionuclides from the natural uranium and thorium series in concentrations often many times greater than transuranic concentrations. As a result, the chemical problems encountered in these measurements are derived from the requirement that separated trans-uranics should be free of a-emitting natural-series nuclides which would constitute a-spectrometric interferences. Table I lists those transuranic nuclides detected to date in marine environmental samples, together with some relevant nuclear properties. Their relative concentrations (on an activity basis) are indicated although the ratios may be altered by environmental fractionation processes which enrich and deplete the relative concentrations of the various transuranic elements. Alpha spectrometric measurements do not distinguish between 239p Pu, so these are... 

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. 

For any given radionuclide, the rate of decay is a first-order process that is constant, regardless of the radioactive atoms present and is characteristic for each radionuclide. The process of decay is a series of random events temperature, pressure, or chemical combinations do not effect the rate of decay. While it may not be possible to predict exactly which atom is going to undergo transformation at any given time, it is possible to predict, on average, the fraction of the radioactive atoms that will transform during any interval of time. 

The TRUclean soil washing system is a patented, ex situ modular process that uses soil washing, size fractionation, and gravimetric separation techniques to remediate soils contaminated with radionuclides and heavy metals. The technology developer, Lockheed Martin Corporation, claims... 

Although comets are not expected to have experienced the thermal processing that asteroids have, some of the larger KBOs and Oort cloud objects may have been heated by decay of radionuclides. The relative proportions of rock and ice may determine the amount of heating, as radionuclides occur in the rock fraction. Comets and asteroids may have had similar impact histories, and many of these bodies may now be collisional fragments. 

Touring the formation of radioactive fallout particles, one of the most important processes is the uptake, in the cooling nuclear fireball, of the vaporized radioactive fission products by particles of molten soil or other environmental materials. Owing to the differences in the chemical nature of the various radioactive elements, their rates of uptake vary, depending upon temperature, pressure, and substrate and vapor-phase composition. These varying rates of uptake, combined with different residence times of the substrate particles in the fireball, result in radiochemical fractionation of the fallout. This fractionation has a considerable effect on the final partition of radioactivity, exposure rate, and radionuclides between the ground surface and the atmosphere. 

Aerosol particles of the size likely to contain the nuclear weapon debris radionuclides and to come from stratospheric air masses are also more likely to serve efficiently as precipitation nuclei (13). Either of these two precipitation removal processes could result in the actual fractionation of potentially fractionated nuclear weapon debris. 

The analysis was made on the assumptions that the differences in gaseous-precursor fission products resulted in a potential fractionation of the three alkaline-earth radionuclides and the radioaerosols participating in the precipitation processes in the Pacific cyclones over the... 

Part of the difficulty in applying such techniques is that the extraction processes operationally assign a given fraction (or species) of the radionuclide to a particular component phase but it has been shown that extractants are not as selective as sometimes claimed. 

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. 

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