Radioactive nuclides diffusion

If one component is at a trace level but with variable concentrations (e.g., from 1 to 10 ppb) and concentrations of other components are uniform, the diffusion is called tracer diffusion. An example of tracer diffusion is diffusion into a melt of uniform composition (Watson, 1991b) when the concentration of is below ppb level. Usually only for a radioactive nuclide such as or " Ca, can such low concentrations be measured accurately to obtain concentration profiles. If a radioactive nuclide diffuses into a melt that contains the element (such as Ca diffusion into a Ca-bearing melt), it is still called tracer diffusion although it may be through isotopic exchange. 

Electrokinetic screens are applied for preventing radioactive nuclide diffusion from the pollution foci into the environment (groundwater, soil, and other subsurface materials), and they play the role of protective screens against possible migration of radioactive nuclides. Figures 5.1 and 5.2 show schemes of such electrokinetic... 

Because the above equation is identical to the diffusion equation of a stable component, it can be solved the same way. After solving for w, then C can be found as we . For diffusion of two isotopes, one stable and one radioactive, because they have the same diffusivity, the concentration profile for the radioactive nuclide is simply the concentration profile of the stable isotope multiplied by either (i) Toe , where Fq is the initial isotopic ratio, or (ii) F, where F is the isotopic ratio at the time of measurement of the profiles. 

The second effect is on the diffusive loss of daughters of radioactive nuclides. For example, decays into many daughters and finally becomes AH the... 

It is also straightforward to extend the equations to allow for only partial equihbration during transport. Iwamori (1993a) presents a one-dimensional steady-state single-porosity model for stable elements that includes diffusive re-equili-bration between melt and solid. He does not extend it to radioactive nuclides in this paper but includes this effect in his two porosity model (Iwamori, 1994) (see Section 3.14.4.3.4). The expected effects of chemical disequilibrium should be similar to those in the Qin (1992) dynamic melting model, namely he effective bulk partition coefficients of all elements will be driven towards unity. 

In many experiments described and referenced in Chapter 1 an important role was played by diffusional deposition of nonvolatile molecular species (and of particulate matter) on the surface of gas ducts. First of all, in chromatographic columns, the nonvolatile molecules deposit within a short distance from the inlet and contribute to decontamination of the elements under study from interfering radioactive nuclides. Meanwhile, especially in chemical experiments with the aid of aggressive gases, unwanted aerosols occasionally form. The latter diffuse much more slowly, and when they catch and then carry a fraction of the nonvolatile molecular species, they make the depositional decontamination less efficient. 

The release of radioactivity is calculated specifically for those individual demanding accidents which lead to the most release of radioactive nuclides fi om the fuel elements. The release of radio-nuclides fi om the fuel kernel through the surrounding coatings and the matrix graphite to the helium coolant is calculated on a diffusion-sorption basis, where the selection of calculation parameters is based mostly on the German experimental results and literature. Where necessary, conservative factors are put into the analysis. 

Following an accident at a nuclear power station, great amounts of radioactive aerosols are emitted into the troposphere, because airborne fission product radionuclides interact with the environment and a carrier is responsible for their long-range transport and atmospheric diffusion. Radioactive nuclides, such as Ru, Te and Cs, characterise these aerosols. 

Hanson and Eberhardt (1971) found the models of the cation binding and diffusive translocation developed by Tuominen to be applicable to their observations in natural lichen communities, when they tried to simulate behavior of Cs in the lichen-caribou-Eskimo food chains. They postulated a very interesting seasonal cycle of Cs concentration in lichens as described in more detail in the subsection on the radioactive nuclides. The radionuclide cycling within natural arctic lichen communities seems to be a more dynamic process than previously noted. 

Impurities in the water and water activation products also contribute to the radioactivity of the coolant water. Tritium is produced as a low yield ( 0.01%) fission product that can diffuse out of the fuel, by activation of boron or fiLi impurities in PWRs. 24Na and 38C1 are produced by neutron activation of water impurities. In BWRs, the primary source of radiation fields in the coolant and steam systems during normal operations is 7.1s 16N. This nuclide is produced by 160(n, p)16N reactions from fast neutrons interacting with the coolant water. This 16N activity can exist as N07, NO in the coolant and NHj in the steam. 

This bulk state of secular equilibrium applies to the total amount of the U-series nuclides, but does not necessarily say where the different elements reside within the system. If the bulk system has a single phase (such as a melt or a monomineralic rock) then that phase will be in secular equilibrium. If the material has multiple phases with different partitioning properties, however, the individual phases can maintain radioactive dis-equilibria even when the total system is in secular equilibrium. There are two basic sets of models that exploit this fact, the first assumes complete chemical equilibrium between all phases and the second assumes transient diffusion controlled sohd exchange. 

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