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Secular Equilibruim

Determination of radionuclude daughter product abundances can be obtained under the assumption of secular equilibrium. Secular equilibrium is a situation in which the quantity of a radioactive isotope remains constant because its production rate (due, e.g., to decay of a parent isotope) is equal to its decay rate.

Secular equilibrium in radioactive decay

Secular equilibrium can only occur in a radioactive decay chain if the half-life of the daughter radionuclide B is much shorter than the half-life of the parent radionuclide A. In such a situation, the decay rate of A, and hence the production rate of B, is approximately constant, because the half-life of A is very long compared to the timescales being considered. The quantity of radionuclide B builds up until the number of B atoms decaying per unit time becomes equal to the number being produced per unit time; the quantity of radionuclide B then reaches a constant, equilibrium value. The quantity of radionuclide B when secular equilibrium is reached is determined by the quantity of its parent A and the half-lives of the two radionuclide. This can be seen from the time rate of change of the number of atoms of radionuclide B:

where λA and λB are the decay constants of radionuclide A and B, related to their half-lives t1/2 by λ = ln(2) / t1 / 2, and NA and NB are the number of atoms of A and B at a given time. How does this change.

Secular equilibrium occurs when dNB / dt = 0, or Over long enough times, comparable to the half-life of radionuclide A, the secular equilibrium is only approximate; NA decays away according to

Oxidation of Uranium Minerals

As with base-metal tailings, sub-aerial deposition of uranium tailings can result in the oxidation of the gangue and residual ore minerals. For example, if exposed to atmospheric conditions, uraninite [UO2] can undergo the following oxidation/dissolution reaction (DeWindt et al., 2003):



As oxygenated pore water infiltrates down through the tailings, the concentrations of oxidized species (i.e., Fe3+) increases, which can in turn further oxidize or dissolve uraninite in the absence of dissolved oxygen:


Sulphide minerals associated with the ore minerals can provide competition for the oxidized species and will preferentially oxidize before uraninite. Because sulphide minerals will preferentially oxidize before uranium minerals, sulphide minerals can reduce oxidized uranium species (e.g., U(VI)). The presence of pyrite, for example, can reduce U(VI) through the following reaction (Bain et al., 2001):


If there is an abundance of sulphide minerals, the oxidation of uraninite in tailings will be limited and the concentrations of uranium will be controlled by the solubility of reduced uranium species (i.e., uraninite). Conversely, sulphide-poor tailings containing uraninite are susceptible to the release and transport of uranium ([1]).

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  1. Bain et al., 2001

--Kdevos 11:35, 3 January 2008 (EST)