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Influence of transient flows on the mobility of strontium in partially water-saturated soils

Pierre MAZET, doctorate of University Joseph Fourier - Grenoble I, spéciality: sciences de la Terre et de l'Univers, 289 p., defended on the 27th october 2008

Document type > *Mémoire/HDR/Thesis

Keywords > reliability, strontium, accident situation, experimental facilities, modelling

Research Unit > IRSN/DEI/SARG/LETS

Authors > MAZET Pierre

Publication Date > 27/10/2008

Summary

This work is devoted to the migration of strontium in the surface soils of Chernobyl.
The reactive transport of 85Sr was studied on laboratory columns, focusing on the influence of transient unsaturated flow (cycles of infiltration and redistribution) associated with controlled geochemistry (constant concentrations of major elements and stable strontium in water).
An original experimental tool (gamma attenuation system) allows us to follow at the same time the variations of humidity of the soil and the migration of radionuclide, in a non-destroying and definite way.
First stage of this study concerned the implementation of the experimental tool to measure transient hydraulic events within the columns of sand. Several experiments of transport of 85Sr were then performed with different water condition (saturated, unsaturated, permanent and transient flow).
Experimental results were simulated using the computer codes HYDRUS-1D (phenomenological approach with partition coefficient Kd) and HYTEC (mechanistic geochemical/transport approach).
Confrontation between experience and modelling shows that, for our operating conditions, transfer of 85Sr can be predicted with an “operational” approach using : 1) simplified geochemical model with partition coefficient Kd concerning interactive reaction with the soil (Kd value determined independently on saturated column, with the same water geochemistry), 2) permanent saturated (or unsaturated) flow, taking into account the cumulated infiltrated water during unsaturated transient hydraulic events concerning hydrodynamic.
Generalization of these results (area of validity) suggests that the “cumulated infiltrated water + Kd” approach can be use, for controlled water geochemistry, when the numerical value of Kd is fairly strong (Kd>>1), and that it is insensitive to the value of the water content.
Moreover, the presence of immobile water (~10%) recorded with tritium transport, is undetectable with strontium. Explanation of this result is allocated to the different characteristic time residence implicated with tritium (short) and strontium (long). Consequence of this result is that modelling transport of strontium, for our operating conditions (for a series of infiltrations and drainages), doesn’t need to take into account physical kinetics of exchange between mobile and immobile water and reduce the effect of flow to convection and dispersion.
Geochemical modelling takes into account appropriate species in competition and uses "intrinsic" coefficient, independents of the variations of the water chemistry. More general, this approach can be used to test, for example, the influence of the concentration of isotopes or other species (stable strontium and calcium for our instance) on the transfer of radioactive Sr. This geochemical influence appears to be very strong on the mobility of strontium compared to the one from variation of water saturation in the porous media.

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Christophe Ardois, Thesis director

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