Acacia Naves

Long-term non-isothermal reactive transport model of compacted bentonite, concrete and corrosion products in a HLW repository in clay

Radioactive waste disposal in deep geological repositories envisages engineered barriers such as carbon-steel canisters, compacted bentonite and concrete liners. The stability and performance of the bentonite barrier could be affected by the corrosion products at the canister-bentonite interface and the hyper-alkaline conditions caused by the degradation of concrete at the bentonite-concrete interface. Additionally, the host clay formation could also be affected by the hyper-alkaline plume at the concrete-clay interface. Here we present a non-isothermal multicomponent reactive transport model of the long-term (1Ma) interactions of the compacted bentonite with the corrosion products of a carbon-steel canister and the concrete liner of the engineered barrier of a high-level radioactive waste repository in clay. Model results show that magnetite is the main corrosion product. Its precipitation reduces significantly the porosity of the bentonite near the canister. The degradation of the concrete liner leads to the precipitation of secondary minerals and the reduction of the porosity of the bentonite and the clay formation at their interfaces with the concrete liner. The reduction of the porosity becomes especially relevant at t=104years. The zones affected by pore clogging at the canister-bentonite and concrete-clay interfaces at 1Ma are approximately equal to 1 and 3.3cm thick, respectively. The hyper-alkaline front (pH>8.5) spreads 2.5cm into the clay formation after 1Ma. Our simulation results share the key features of the models reported by others for engineered barrier systems at similar chemical conditions, including: 1) Pore clogging at the canister-bentonite and concrete-clay interfaces; 2) Narrow alteration zones; and 3) Limited smectite dissolution after 1Ma.

Identifiability of diffusion and sorption parameters from in situ diffusion experiments by using simultaneously tracer dilution and claystone data

In situ diffusion experiments are performed in geological formations at underground research laboratories to overcome the limitations of laboratory diffusion experiments and investigate scale effects. Tracer concentrations are monitored at the injection interval during the experiment (dilution data) and measured from host rock samples around the injection interval at the end of the experiment (overcoring data). Diffusion and sorption parameters are derived from the inverse numerical modeling of the measured tracer data. The identifiability and the uncertainties of tritium and (22)Na(+) diffusion and sorption parameters are studied here by synthetic experiments having the same characteristics as the in situ diffusion and retention (DR) experiment performed on Opalinus Clay. Contrary to previous identifiability analyses of in situ diffusion experiments, which used either dilution or overcoring data at approximate locations, our analysis of the parameter identifiability relies simultaneously on dilution and overcoring data, accounts for the actual position of the overcoring samples in the claystone, uses realistic values of the standard deviation of the measurement errors, relies on model identification criteria to select the most appropriate hypothesis about the existence of a borehole disturbed zone and addresses the effect of errors in the location of the sampling profiles. The simultaneous use of dilution and overcoring data provides accurate parameter estimates in the presence of measurement errors, allows the identification of the right hypothesis about the borehole disturbed zone and diminishes other model uncertainties such as those caused by errors in the volume of the circulation system and the effective diffusion coefficient of the filter. The proper interpretation of the experiment requires the right hypothesis about the borehole disturbed zone. A wrong assumption leads to large estimation errors. The use of model identification criteria helps in the selection of the best model. Small errors in the depth of the overcoring samples lead to large parameter estimation errors. Therefore, attention should be paid to minimize the errors in positioning the depth of the samples. The results of the identifiability analysis do not depend on the particular realization of random numbers.

A single-site reactive transport model of Cs + for the in situ diffusion and retention (DR) experiment

In situ diffusion experiments are performed in underground research laboratories for understanding and quantifying radionuclide diffusion from underground radioactive waste repositories. The in situ diffusion and retention, DR, experiment was performed at the Mont Terri underground research laboratory, Switzerland, to characterize the diffusion and retention parameters of the Opalinus clay. Several tracers were injected instantaneously in the circulating artificial water and were then allowed to diffuse into the clay rock through two porous packed-off sections of a borehole drilled normal to the bedding of the clay formation. This paper presents a single-site multicomponent reactive transport model of Cs+, a tracer used in the DR experiment which sorbs onto Opalinus clay via cation exchange. The reactive transport model accounts for the diffusive-reactive transport of 11 primary species and 22 aqueous complexes, and the water–rock interactions for 5 cation exchange and 3 mineral dissolution/precipitation reactions. Most of the solutes except for Cs+ diffuse from the Opalinus clay formation into the injection interval because the concentrations in the initial Opalinus clay pore water are larger than those of the initial water in the circulation system. Calcite dissolves near the borehole while dolomite precipitates. Dissolved Cs+ sorbs by exchanging with Ca2+ in the exchange complex. The computed dilution curve of Cs+ in the circulating fluid is most sensitive to the effective diffusion, De, of the filter, the selectivity coefficient of Na+ to Cs+, KNa–Cs and De of the borehole disturbed zone. The apparent distribution coefficient of Cs+,

The DI-B in situ diffusion experiment at Mont Terri: Results and modeling

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Analysis of the parameter identifiability of the in situ diffusion and retention (DR) experiments

Kda, in the formation varies in space and time from 100 to 165 L/kg due to the temporal changes in the water chemistry in the formation. The results of a sensitivity run in which the initial chemical composition of the Opalinus pore water is the same as the initial chemical composition of the water in the circulation system show that the changes in

Identifiability of diffusion and retention parameters of anionic tracers from the diffusion and retention (DR) experiment

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