Low- and intermediate-level radioactive wastes are buried in geological repositories. These repositories must isolate the nuclear waste from the biosphere as long as 100,000 years. For the storage of radioactive waste, the geological formations were used where water-soluble compounds have been accumulated for millions of years, such as salt mines, clay rocks, granite, and tuff. In these geological formations, fur­ther engineering barriers are constructed. The nuclear waste is placed into stainless steel or reinforced concrete containers and deposited inside the engineering barrier system. Only solid wastes are stored; liquid wastes are solidified by cementation or bitumen. The holes among the containers are filled with cement too.

There are some very important aspects to take into account when selecting a suitable environment for waste disposal. These are, for example, the hydrological properties of the geological environment, the corrosion and erosion of the engineer­ing barrier system, leaching, and migration of the radionuclide in the geological environment. In addition, the microbiological activity and the effects of radiolysis have to be considered.

Low — and intermediate-level radioactive wastes contain the technological wastes of nuclear energy production (clothing, paper, wood, ion exchange resins, plastics, contaminated tools, instruments, etc.). In the corrosion and microbiological degra­dation of these substances, gaseous compounds are released. The corrosion pro­duces hydrogen, while the microbiological processes transform the organic substances of the nuclear wastes into carbon dioxide or methane, depending on the redox conditions. The formation of carbon dioxide is less important because the anaerobic conditions are dominant in underground disposal. The gases can have unfavorable effects during storage. For example, the increasing pressure can push the radioactive gases and solutions into the environment. As a result of the cemen­tation, the pH of the pore solution is set above 12. This pH inhibits the corrosion of the containers and the microbiological activity, decreasing the rate of gas release.

The radiolysis of water (discussed in Section 13.4.2) also releases gases; how­ever, this reaction can be disregarded for the disposal of low — and intermediate- level nuclear waste.