Kinetically Controlled Dissolution of UO 2 (s) Under Oxidizing Conditions. A Combined Dissolution-Oxidation Model.
- PDF / 316,673 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 37 Downloads / 227 Views
KINETICALLY CONTROLLED DISSOLUTION OF UO (s) UNDER OXIDIZING CONDITIONS. A COMBINED DISSOLUTION-OkIDATION MODEL.
IGNASI CASAS, J.GIMENEZ, V.MARTI, M.E.TORRERO AND J. DE PABLO Department of Chemical Engineering, Universitat Polit~cnica Catalunya E-08028 Barcelona, Spain.
ABSTRACT The release of uranium from three samples of UO (s) with different particle sizes (100-300 pm, 900-1100 pm, pellet) has been studied as a function of time. In all cases, the same pH, ionic medium, temperature, and oxygen partial pressure were used. Two distinctive trends were observed in the three cases, with a relatively fast initial uranium release followed, after 10-15 days, by a slower dissolution rate. The experiments were continued during 200 days; no change in the second dissolution rate was noticed. The uranium released as a function of time has been successfully fitted with a mathematical expression which combines an oxidation-dissolution mechanism. INTRODUCTION Oxidizing conditions should be considered in the near-field performance assesment of the final spent nuclear fuel repository due mainly to radiolysis of water. However, the effect of this radiolysis is still controversial [1]. In the presence of oxidizing species such as 02 and H 2 0 2 , the dissolution of unirradiated UO 2 [2] and spent fuel [3] shows a similar behavior. The uranium concentration in solution reaches a value which is approximately constant after an initial dissolution period. It is not clear if this concentration is controlled by solubility equilibria respective to an oxidized phase, or if it is governed by oxidation-dissolution mechanisms. On the other hand, the UO 2-matrix dissolution rate is a critical parameter in the assessment of the spent nuclear fuel stability under oxidizing conditions. Hence, several parameters which can modify the dissolution rate, such as pH, oxidizing species and complexing agents, have been studied. In this context, the pH seems not to affect significantly the dissolution rate in the range between 5 and 9 [4]. The presence of H0 [ 5 ] instead of oxygen and complexing agents like carbonates [6] increase the dissolution rate. Surface area is an important parameter to consider in order to compare the dissolution rates given by different authors is the surface area considered. Several approaches can be used to estimate the specific surface area such as BET measurements, calculations from the mesh size, and evaluations of the geometric area. The objective of this work is to study the UO 2-dissolution under oxidizing conditions, using different particle size and to compare the dissolution rates taking into account the specific surface areas determined by using BET measurements. At the same time, a mathematical model which includes an oxidationdissolution mechanism is developed in order to explain the experimental behaviour observed.
Mat. Res. Soc. Symp. Proc. Vol. 294, (c,1993 Materials Research Society
62
EXPERIMENTAL Unirradiated nuclear fuel pellets of UO 2 were crashed and sieved in order to obtain different particle sizes
Data Loading...
