Transport of Oxidants and Radionuclides Through a Clay Barrier

摘要

The mass transfer rate for oxidants to, and radionuclides from a capsule in a repository has been computed. The capsule which is 0.75 m in diameter is surrounded by Montmorillonite clay. The hole is 1.5 m in diameter. For one capsule about 1220g copper will corrode due to oxygen corrosion in 10 000 years. If the fissures in the rock nearest the hole are filled with clay, the corrosion will decrease significantly. This is valid for a case where the ground water is in equilibrium with oxygen of 0.2 bar pressure (normal air pressure). Measurements of the oxygen content in ground water at large depths show a more than 1 000 times smaller values. The transport rate will then be correspondingly smaller. Corrosion due to sulfate/sulfide corrosion may reach some 590 g in the same time if there is 10 mg/l of the least abundant component. The radionuclides exp 90 Sr, exp 137 Cs, exp 241 Am and exp 243 Am will decay totally in the clay barriers. exp 240 Pu will be seriously hindered. The total dissolution of the uranium oxide in a capsule takes at least 1.8 million years. Nuclides with high solubilities decrease in about 2 000 years to half their original concentration. The sodium in the Montmorillonite clay in the fissures is exchanged for calcium in about 20 000 years. The exchange of the sodium in the clay in the hole takes millions of years. (Atomindex citation 09:405344)