Management and final disposal of radioactive waste

Radioactive waste is usually managed following a principle “concentrate and stop, prevent from outspread”. The principle is based on the fact that waste (radionuclides contained in the waste) is concentrated and stored in isolation to prevent it from spreading out in the environment and to preclude the possibility of danger posed to humans and the environment. Waste is concentrated to reduce its volume and isolated. This principle applies to both solid and liquid waste. Furthermore, waste should be stored in isolation long enough until the majority of radionuclides within them decay (become stable nuclei) and are no longer dangerous.

In some cases, when concentration is not possible or expedient, the above mentioned principle is not applied. Waste containing only short-lived radionuclides may be stored (preserved) until radionuclides within them decay. Normally, it is sufficient if waste is stored for the period of 10 half-lives. During that period the activity decreases about 1000 times. For example, the half-time of ¹³¹I which has been quite commonly used in medicine is 8 days. To have the activity of this source decreased 1000 times, this will require 80 days.

Radioactive waste management involves application of strict safety requirements, accounting and control. The waste of each type is managed and disposed of in a different manner.

The type of a repository used for final disposal of radioactive waste is directly determined by the geological conditions and specific necessities of the country (the amount of waste, its characteristics). Repositories are designed so that exposure of radionuclides to the environment (biosphere) does not exceed the permissible threshold values, irradiance of repository personnel and members of general public is reduced to a minimum and need for post-closure surveillance of the repository is minimal. These objectives may be achieved by coordinating technical measures with concrete features of the site.

For many decades short-lived low-level radioactive waste has been disposed of in mere holes in the ground. Simple near-surface repositories with no engineered barriers are ditches dug out in the clayey impervious soil. Such a method of waste disposal may only be considered safe if all radionuclides within them decay in the repository during the foreseen period of its surveillance (usually 100 to 300 years). Examples of repositories of this type may be found in the USA or South Africa.

An overall tendency has been observed that over time multiple-barrier engineered systems have been trusted more than a single, often natural, barrier. These waste disposal systems often consist of concrete vaults, gaps between packages, fillers, rainwater drainage systems. Complex devices with long-lived reinforced concrete barriers used for waste disposal safeguard the radioactive waste better than the mere natural environment. They may be used reliably for final disposal of higher-level radioactive waste.