Modelling UO 2 and Simfuel Leaching Behaviour in Granite Groundwater Under Oxidizing Conditions
- PDF / 330,595 Bytes
- 7 Pages / 381.06 x 606.06 pts Page_size
- 81 Downloads / 180 Views
ABSTRACT The Uranium concentration leaching behaviour for U02 and SIMFUEL in granite groundwater under oxidizing conditions is modelled. The model contemplates two terms: a source term and a loss term. The source term represents the bulk dissolution process and its upper limit is controlled by the solubility of the thermodynamically stable solid phase. The loss term emulates the solute removed due to groundwater flow into the repository. It can forecast the uranium steady state concentration when groundwater flow is considered. The model has been tested with three different types of leaching experiments, performed with U02 and SIMFUEL. Good agreement between experimental results and model predictions are found. INTRODUCTION Deep underground disposal is a generally accepted alternative solution for the nuclear waste problem. Long-term interactions between spent fuel and groundwater have to be understood in order to safely dispose the fuel in a final underground repository. The prediction of the waste form stability for assessing the safety of the repository requires theoretical understanding and the development of methods for extrapolating laboratory data to long periods time. The stability of the nuclear waste depends on its inherent properties, and on the geochemical and hydrological constraints. Experiments of leaching under repository conditions would permit extrapolating information from short-term laboratory trials to long periods of time. Confidence in predictions, essential for performance assessment studies, can only be established if models proposed for simulating spent fuel behavior, under repository conditions, can accurately describe laboratory experiments. The first kinetic studies of uranium leaching were carried out [1, 2] with the purpose of improving the extractive metallurgy for mined uranium ore. These studies in general were not adapted to allow creating a thermodynamic database; however, in some cases, they suggested dissolution mechanisms. Nowadays, there are kinetic models based on oxidation/dissolution mechanisms [3, 4] that try to predict U02 dissolution from experimental data, and also thermodynamic models that describe the control of the dissolution by different processes [5]. EXPERIMENTAL PROCEDURE Batch leaching tests have been carried out using powder of natural U02 and SIMFUEL (chemical analogue of irradiated fuel [6]) with different particle size distribution, 479
Mat. Res. Soc. Symp. Proc. Vol. 556 ©1999 Materials Research Society
50 - 100 and 100 - 315 pm in borosilicate vessels. The ratio of the solid surface area to leachant volume S/V for all of the experiments was of 3.000 m1 . Leachant volume used in whole cases was 200 ml. Table I shows the composition of synthetic granitic water used [7]. Before starting the leaching tests, all the solids were rinsed in order to eliminate fines that could be adhered to the powder, and to avoid the presence of organic residue [8]. Table I. Composition of synthetic granitic water used in the leaching experiments [7]. Species Concentration (
Data Loading...
