The majority of the gaseous radioactive waste is short-lived. All that is needed is to wait sufficiently long until it decays to stable elements that can be then released into the atmosphere by vents or smokestacks.

Radioactive waste must be safely isolated from the biosphere for the whole time it might be dangerous. However, the waste must be processed. Its state or composition must be altered in such a way that it is easier to reduce its volume (facilitating its storage), leading to a simpler separation of the radionuclides from the non-radioactive substances. This is important primarily for economic reasons because the less waste must be stored, the lower the associated cost. Lowering the volume is important especially for low-level waste: its volume is large, but it contains only a limited amount of radioactive isotopes.

In certain cases, the opposite approach to concentration and storage may be adopted. In the case of radioisotopes occurring also in nature (e.g. tritium, carbon ¹⁴C, etc.), they can be diluted to a concentration that is lower than the natural concentration and released into the environment. Such release of radionuclides is very strictly regulated.

Evaporation

Evaporation is the most effective method to reduce the volume of liquid waste. The resulting product is in the form of radioactive salts containing all the radionuclides; it is homogeneous and is easily handled as solid waste.

Filtration, centrifugal filtration

Methods of physical separation are used to separate out the solid components dispersed in a liquid. Following filtration or centrifugal filtration, evaporation is often employed to eliminate excess liquids.

Similar to water treatment plants, for concentrating of liquid radioactive waste the following methods based on evaporation, flocculation, coagulation, filtration or using suitable microorganisms, are used.

Ion exchange

This method is used to treat the water from a nuclear reactor’s primary circuit. Ion exchangers are porous polymer substances whose basic skeleton is electrically charged (most often synthetic organic substances primarily on the basis of styrene, polyacrylate, phenol formaldehyde resins, etc.) that binds ions with a higher affinity to the ion exchanger. Spent ion changers are handled as solid radioactive waste.

Fixation in solids

Following the processing by one or more of the procedures described above, the concentrated waste is fixed into a more solid form. A frequently used method is cementing, i.e., fixing in cement. Another possibility is bituminization (fixing in a substance similar to bitumen). This method is more costly, but because it is a hot process, additional water is evaporated from the waste making it even more concentrated. If neither cementing nor bituminization is suitable, fixation in plastics may be used (polyester, epoxide…) although this is more expensive.

Radioisotopes that are commonly found in Nature do not need to be concentrated or stored. Dilution to a concentration below the naturally found concentration and subsequent release into the environment is sufficient.

Chemical precipitation and flocculation

Using chemical agents, radioactive components come out of suspension in the form of solid flock or flakes and the clarifying chemicals used may be then filtered out from the suspension. The degree of separation is in this case relatively low and thus this chemical procedure is usually combined with other procedures.

Some liquid waste (e.g. tritium) is diluted to a concentration below the naturally occurring concentration and is then released into the environment.

Non-combustible low-level solid waste is often pressed into ordinary 200-liter barrels. If the barrel containing waste is also pressed, the resulting super compact pellet may be then stored in a container and deposited in storage.

Solid waste is very often fixed in cement or bitumen. After processing, concentrated radioactive waste is fixed in a solid matrix and placed into steel, concrete, or copper flasks that are then deposited in a suitable repository.

Vitrification

This procedure is often used to handle the remains from the reprocessing of spent nuclear fuel but it could also be used for any crushed radioactive waste. The waste is mixed with glass-forming additives and melted at 1,200 °C to make silicone or borosilicate glass. Sometimes, the glass is coated with lead to improve its thermal conductivity. This glass is very durable and the waste is well fixed in it. Glass is one of the most resistant materials against leaching. Vitrification is thus a suitable procedure to process waste with a long half-life.