Sources, Processing, and Storage of Radioactive Waste

(Transcript of the video commentary.)

When they hear the term “radioactive waste”, most people associate it with nuclear power. Very few people know, however, that most radioactive waste isn’t created by nuclear power plants, but rather as institutional waste in agriculture, industry, scientific research, and especially healthcare. It is important to realize that institutional radioactive waste, which is up to 4/5 of all radioactive waste, is produced by all developed countries regardless of their nuclear energy policies. And when they produce it, they of course have to dispose of it safely.

Radioactive waste is hazardous for humans and the entire biosphere due to the negative effects of radiation on living things. Such waste must be handled very cautiously and the proper attention must be paid to its storage and possible processing. The main objective of activities related to radioactive waste is its long-term isolation from the environment. However, a certain advantage of this waste is the fact that the danger it poses declines over time, and after a certain amount of time has passed, it stops being hazardous and can be handled like ordinary waste.

The medical sector is the largest producer of institutional radioactive waste. Radioisotopes are used for various diagnostic and treatment purposes, and everything from the sources of therapeutic radiation as such to the equipment that comes into contact with these products becomes radioactive waste. In industry and agriculture, radioactive isotopes are most often used as markers to help detect leakage or to precisely determine the absorption of important substances.

In the nuclear power sector, the greatest amount of waste is created during the processing of uranium ore. However, even though the separated slag contains about 75% of the original radioactivity of the uranium ore, it is not considered radioactive waste, which is primarily created during nuclear power plant operation and decommissioning. In this case, radioactive waste involves radioisotopes separated from coolant, used filters or decontamination solutions, and after the nuclear power plant has been decommissioned, also structures and equipment that have become radioactive.

Spent nuclear fuel is a special type of highly radioactive waste. A typical nuclear power plant produces around 20 m3 of highly radioactive and potentially hazardous waste on an annual basis. All handling of this waste requires the use of sophisticated technologies and adherence to safety rules.

Radioactive waste is most often classified according to three key characteristics: radioactivity level, form or phase, and the predominant radionuclides present. Various combinations of these parameters require various ways of handling the waste and specific systems for its storage and processing. Radioactivity and radionuclide content also have an effect on how the waste is finally stored in radioactive waste storage facilities.

Radioactive waste is classified as low-level, intermediate-level, and high-level waste depending on its radioactivity level. About 90% of all radioactive waste consists of low-level waste, created primarily by industry and healthcare. It requires no special shielding or handling and its radioactivity declines relatively quickly to natural background levels. Another 7% consists of medium-level waste from the chemical industry and healthcare, or of irradiated metal components used in nuclear power plants. Intermediate-level waste contains a higher percentage of radioisotopes, requires shielding, and can remain dangerous for hundreds to thousands of years.

The remaining 3% is high-level waste, primarily spent nuclear fuel and concentrates created during its reprocessing. High-level waste usually produces large quantities of heat due to radionuclide decay, and hence requires cooling, shielding and long-term isolation from the environment. It remains hazardous for tens of thousands to millions of years.

Radioactive waste may be in solid, liquid, or gaseous form. Each form of waste required different handling and different storage and processing systems. Radioactive waste is most often disposed of as a solid, because this makes handling easier, minimizes volume and increases radionuclide immobilization. Liquid radioactive waste is created, for example, in nuclear power plants when purifying the primary coolant or from solutions used for decontamination and maintenance. Gaseous radioactive waste is usually created during waste processing, such as incineration. The most significant gaseous waste includes radon, iodine, and xenon.

Depending on how it was created, waste contains different amounts of radionuclides with short and long half-lives. Isotopes with a short half-life (up to 30 years) often emit intense gamma radiation, are harder to handle, and require better shielding, but their activity declines quickly. Isotopes with a long half-life (up to millions of years) often emit alpha and beta radiation, which is easily blocked, but remain radioactive for a very long time.

The objective of processing of radioactive waste is to modify its properties, composition, or form to make it safer and more suitable for storage. For economic reasons, technologies that change its volume are also important, especially for low-level waste, which is produced in great quantities.

Solid radioactive waste is most often processed by compaction, incineration, and immobilization in solid materials. The simplest way to reduce the volume of solid waste is compaction, where radioactive or contaminated solid materials are placed in steel drums, which are then compacted into large pellets. The volume of solid waste can also be reduced significantly through incineration.

After compression and incineration, the waste is often immobilized in steel or concrete containers that are then placed in an appropriate repository. The best-known methods for immobilizing radioactive substances in a solid matrix are cementation and bituminization. A specific immobilization process is vitrification, during which the waste is mixed with glass-forming materials and melted into borosilicate glass.

When processing liquid radioactive waste, the most effective volume reduction method is simple evaporation. The result is a mixture of radioactive salts containing all the original radionuclides. The evaporation itself may be preceded by precipitation of radioactive components and physical separation of particulates suspended in the liquid.

Ion exchange is used to treat water from a nuclear reactor's primary circuit. The principle of this process consists of the flow of water through porous polymers called ion exchange resins, which bind ions with greater affinity to the ion exchange medium. In other words, ion exchange resins remove unwanted radionuclide ions from water. When the ion exchange resin is saturated, it is handled as radioactive waste.

After increasing the concentration and reducing the volume of liquid waste, it is appropriate to convert it into a solid to make storage safer. During the cementation process, this concentrate is used as the water phase of a concrete mixture. Another method for immobilizing radionuclides in a solid matrix are bituminization and immobilization in plastic.

The simplest way to process gaseous radioactive waste is to keep it in a suitable space for a duration of around ten half-lives. After most of the radionuclides have been transformed into non-radioactive products, gaseous waste can be released into the atmosphere without any risk.

When discussion the storage and disposal of radioactive waste, we must be aware that while storage is temporary and further handling of radioactive waste and spent nuclear fuel is expected, disposal in a repository applies only to radioactive waste, is permanent, and no removal of material from the repository is expected.

There are various types of repositories depending on the radioactivity and nature of the waste. For disposal of waste containing short-lived isotopes, a surface repository with barriers for several centuries is sufficient. Waste containing isotopes with a long half-life that will remain radioactive for tens of thousands to millions of years must be isolated in a deep repository that safely isolates the waste from the environment for a very long time.

Surface and sub-surface repositories are used throughout the world to dispose of low-level and some intermediate-level waste. These are usually isolated concrete structures sunk slightly below the surface or artificially created underground cavities near the surface. Such repositories must last several hundreds of years and require constant supervision. Once it is full, a surface repository is sealed and often covered with soil.

In the future, permanent deep repositories built several hundred meters underground in stable geologic formations will be used to store highly radioactive waste with a long half-life. The waste will be stored in special containers that along with other barriers will ensure safe isolation from the environment for thousands or even millions of years. No such deep repository is in operation yet, though suitable locations are being selected throughout the world and in some cases they are under construction.

This is because there are only two ways of safely dealing with spent nuclear fuel — either placing it in a repository straight away, or reprocessing it. And neither of these two options can be used without building a deep repository. Alas, no “waste-free” method of disposing of spent nuclear fuel exists, and likely never will.