Sources, Processing, and Storage of Radioactive Waste

(Transcript of the lesson commentary.)

Radioactive waste and its sources

When they hear the term “radioactive waste”, most people associate it with waste from nuclear power generation. Very few people know, however, that most radioactive waste isn't created by nuclear power plants, but rather in completely different areas of human activity such as agriculture, industry, scientific research, and especially healthcare. These sectors become producers of institutional waste, which makes up up to 4/5 of all radioactive waste. It is important to realize that institutional radioactive waste is produced by all developed countries in the world 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 precisely due to its radioactivity and related negative effects 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 long-term isolation from the environment. On the other hand, a certain advantage of this waste is the fact that as opposed to toxic waste, which remains hazardous forever, the hazard posed by radioactive waste 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 leaks 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 nuclear power plant with a typical 1,000 MW reactor 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. In countries operating nuclear power plants, radioactive waste represents less than 1% of all toxic waste produced.

Classification of radioactive waste

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. It is primarily created in various sectors of industry and in healthcare. It requires no special shielding or handling and its radioactivity declines relatively quickly to natural background levels. Another 7% is designated as intermediate-level waste. It is primarily produced by the chemical industry and healthcare, but can also involve irradiated metal components used in nuclear power plants. Intermediate-level waste contains a higher percentage of radioisotopes and requires shielding. It is estimated to remain hazardous for hundreds to thousands of years.

The remaining 3% is high-level waste, primarily spent nuclear fuel. In the case of fuel reprocessing, the spent fuel itself is considered a raw material and high-level waste is only the concentrate that remains after 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. Clearly, every type of waste requires different handling, storage, processing, and final disposal. Radioactive waste is most often disposed of as a solid, because this makes handling easier, minimizes volume, increases radionuclide immobilization, and makes it easier to isolate it from the environment. Liquid radioactive waste is created, for example, in nuclear power plants when purifying the primary coolant or from solutions used for decontamination and maintenance. These are directly radioactive chemicals or liquids that contain dissolved radioactive substances. 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.

Processing of radioactive waste

The objective of processing 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, because the less waste needs to be stored, the lower the associated costs.

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. Radioactive or contaminated solids are placed in steel drums, which are then compacted into large pellets. This compaction process reduces volume by up to 80%. The volume of solid waste can also be reduced significantly through incineration. This process can transform large volumes of combustible low-level waste into a substantially small quantity of homogeneous ash.

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. Another specific immobilization process is vitrification, during which the waste is mixed with glass-forming materials and melted at a temperature of 1,200 °C to make borosilicate glass. This is how remnants from spent nuclear fuel re-processing are processed.

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 physical separation methods to eliminate solids suspended in the liquid. The most common such method is of course filtration, or centrifugal filtration. In certain cases solid radioactive components can be removed from suspension using chemical precipitation or flocculation.

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. Ion exchange resins remove unwanted radionuclide ions from the 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 liquid for 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 certain time. Because much of gaseous radioactive waste has a short half-life, during a short period of time (about ten half-lives) it is transformed into non-radioactive products and can be released into the atmosphere without any risk. Radioactive aerosols and other particulates are eliminated from gaseous waste prior to release via filtration, or can be separated using ion exchangers or chemically bound to an appropriate solid substance.

Storage and disposal of radioactive waste

The difference between storage and disposal of radioactive waste lies in the duration of radioactivity and options for further handling of the waste. While storage is temporary and further handling of radioactive waste and spent nuclear fuel is expected, permanent disposal applies only to radioactive waste, is permanent, and no removal of material from the repository is expected. On the contrary, a repository must ensure safe isolation of waste from the environment for the entire time that it is dangerous.

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 long-lived isotopes 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 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.