Modeling of the Effects of Radiolysis on UO 2 -dissolution Employing Recent Experimental Data
- PDF / 69,932 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 79 Downloads / 177 Views
0RGHOLQJRIWKH(IIHFWVRI5DGLRO\VLVRQ82GLVVROXWLRQ(PSOR\LQJ 5HFHQW([SHULPHQWDO'DWD Mats Jonsson, Fredrik Nielsen, Ella Ekeroth and Trygve E. Eriksen Department of Chemistry, Nuclear Chemistry, Royal Institute of Technology, SE-100 44 Stockholm, Sweden $%675$&7 This study examines the effect of water radiolysis on the dissolution of uranium dioxide. A model is created to describe the system of uranium dioxide fragments in water, and the production and reactions of radiolysis products (using recent kinetic data). The system is evaluated under different conditions using MAKSIMA-CHEMIST. Conditions examined include presence of carbonate in the water and effects of hydrogen. The simulations are compared to experimental results on spent fuel dissolution. Surprisingly, the simulated U(VI)-release agrees within a factor of three with the experimentally found U(VI)-release. The inhibiting effect of hydrogen is clearly demonstrated by the simulations. From the results of the simulations we are also able to conclude that the main inhibiting effect of H2 is the reaction with OH• and not the reduction of U(VI) to U(IV). ,1752'8&7,21 Spent nuclear fuel will according to the Swedish model, KBS-3, be stored in deep repositories with four barriers to prevent radionuclide release to the environment. The innermost barrier is the UO2-matrix of the fuel itself. In the event of a canister failure, whether caused by corrosion or mechanical wear, water may enter the containers and reach the spent fuel inside. The dissolution of uranium dioxide in water is a crucial safety issue. UO2 has low solubility in reducing ground water [1]. However, radiolysis of ground water will produce oxidants (and reductants) that alter the otherwise reducing conditions. Radiolysis of water produces both radicals and molecular products e.g. OH•, HO2•, H2O2, O2 (oxidants) and H•, eaq- and H2 (reductants) [2]. With carbonate present in the water, CO3•- will also be formed. A key question that has been discussed for several years is if and how the radiolysis products will increase the rate of dissolution of spent fuel. In numerous studies, spent fuel and radionuclide doped UO2 have been used to address this issue [3-6]. The main problem with these studies is that these systems are far too complex to draw conclusions concerning the reactivity of individual radiolysis products. This is a prerequisite for reliable theoretical modeling and long-term predictions of spent fuel dissolution. To circumvent this problem, a number of studies have been conducted on the reactivity of individual radiolysis products towards UO2. Most of these studies have been focused on the primary radiolysis product H2O2 and on O2 [7]. Based on the results of these studies, possible mechanisms of oxidative UO2 dissolution have been put forward. Recently, we studied the kinetics of UO2 oxidation using a number of one- and two-electron oxidants. Interestingly, the logarithm of the rate constant of oxidant consumption was found to depend linearly on the one-electron reduction p
Data Loading...
