Comparison of soil/solution distribution of a fresh radionuclides contamination (137Cs, 129I and 233U) added under organic or water pathways with their natural endogenous isotopes
Résumé
Due to nuclear activities, radionuclides (RNs) could be
disseminated in environment following accidental or chronic
releases. In models used to predict their behaviour, their
distribution in the soil/solution system is often estimated using a
partition coefficient (Kd), relating the total concentration of RN
in the soil to that present in solution.
The use of Kd model assumes that RNs are in equilibrium
between soil and solution and, if the same value is used for short
to long term modelling, assumes Kd value is not time dependant.
However, for some RNs, several studies showed a stabilization
of RNs in soils over time. This stabilization depends on the
nature of RN considered, on soil geochemical parameters and/or
on the nature of the contamination. For this last point, recent
studies shown that 137Cs water available fraction from soil differs
if the contamination is due to contaminated water or
contaminated leaves.
Obtaining Kd values in line with the required time scale for
long-term assessment of RNs transfer could be difficult because
the time scale of prediction is longer than the existing
contamination. To compensate this lack of data, natural
endogenous isotopes of the targeted RNs, could be used to mimic
long term behavior of freshly added radionuclides. However,
some biases could exist in this transposition. The major one is
that a part of the natural isotopes could be included in the
mineral matrix and not available for soil to water transfer. In
addition to the Kd determination, the objective of this work is to
determine the isotope soil water available fraction and its
associated Kdavailable, for freshly added RNs and their natural
endogenous isotope.
To reach this objective a cocktail of radionuclides having
different soil behaviors (137Cs, 129I, 233U) was added under liquid
or organic (from plants that have been previously contaminated)
pathways to four soils presenting contrasted geochemical
properties. Their water available fraction and Kdavailable were
determined by a successive desorption batch method, whereas
their distribution within the solid phase was studied with
chemical sequential extraction experiments. The obtained results
for added RNs were compared with their natural endogenous isotopes (133Cs, 127I, 238U).