Nuclide migration

The migration rate of a groundwater constituent, relative to the groundwater flow rate, is controlled by the retardation factor, where Ri = 1 + Ki. Where Ki 1 (e.g., for Th and Ra), Ri Ki, and Iads + Iw = IwRi- Note that ki and k-i are element-specific but not isotope-specific. All isotopes that decay slower than desorption, so that k-i have a value of Ki that is equal to that of a stable isotope (Eqn. 3). The value of Ki may be lower for the shortest-lived nuclides (see Fig. 2b), and so a series of equations derived from Equation (3) applied to different isotopes of the same element may be used to obtain absolute values for the separate rate constants. 

Yanase N, Payne TE, Sekine K (1995) Groundwater geochemistry in the Koongarra ore deposit, Australia 2. Activity ratios and migration mechanisms of uranium series nuclides. Geochem J 29 31-54... 

There has been considerable interest recently in the migration of long-lived nuclides involving technetium. The behavior of technetium in groundwater, sorption and permeation under subterranean conditions needs to be studied for the purpose of assessing environmental safety in connection with the disposal of spent nuclear fuel. Chemical and physicochemical data on technetium under such conditions are necessary. 

Tsvetnova O.B., Shcheglov A.L, 1996. Accumulation of Cs by higher macromycetes and their role in biogeochemical migration of nuclide in forest ecosystems. Review of Moscow State University, Ser. 17, Soil Science, 4 59-69. 

Deep geological disposal is the most favored solution for the permanent disposal of nuclear wastes with long half-lives. Although the locations of the burial places are selected with outmost care to avoid migration of the wastes in nature over a very long period of time, no barrier can be safe forever, so, numerous studies are in progress to determine the main factors that could cause leaks of radioactive nuclides. Soluble compounds in ground water are likely to play a major role in the release of actinides. 

Retention in the Backfill Material. The radionuclide holdup time in the backfill barrier for the KBS-concepts is in the order of thousands of years or less (15). Thus, only a few of the long-lived radionuclides in HLW and SUF ( °Sr, Cs and lAm) will be able to decay within the clay barrier and the more long-lived nuclides will just be delayed in their migration out into the bedrock. In the long-term time span the backfill material is of minor importance, as far as the retaining effect is concerned. 

Grundfelt, B., "Transport of Radioactive Effluents with Ground-water from a Repository", KBS TR 43, 1977 Grundfelt, B., "Nuclide Migration from a Repository for Spent Fuel", KBS TR 77, 1978... 

Landstrom, 0., Klockars, C.-E., Holmberg, K.-E. and Westerberg, S., "In Situ Experiments on Nuclide Migration in Fractured Crystalline Rocks", KBS TR 110, 1978... 

In previous work (l.> > ) it was found that the kinetics of sorption was an important parameter affecting the migration of nuclides in geologic media. For example, in experiments designed to measure the kinetics of reaction for radionuclides in solution with tablets of rock, it was found that periods from several minutes to several hours were required for the radionuclides to reach steady state concentrations on the rock tablets and in the solutions. Figure 1 shows the reaction curves found for the sorption of plutonium and americium from solution by a tablet of granite. The reaction rates for the sorption of plutonium and americium from solution are not the same, and both require a number of hours to reach steady state concentrations. 

This paper describes an experimental study of the applicability of the ARDISC model to laboratory studies of nuclide migration in geologic media. 

If the rate of adsorption, the rate of desorption, and the equilibrium partitioning of a nuclide between a solid medium and solution are known, then the rate of migration of a nuclide through the medium can be predicted with the ARDISC model. 

The rate of adsorption and the rate of desorption are assumed to be dependent on the geometric relationship of the rock and solution (i.e., dependent upon the volume of solution and on the shape and area of the solid medium in contact with the solution). Because the dependence of sorption kinetics on the geometric relationship is not known, the rates for sorption are determined by experiment for the particular geometry (surface area of rock to volume of solution) for which the prediction of nuclide migration is desired. 

---