Classification of radioactive waste

Radioactive waste is material in gaseous, liquid or solid form for which no use is intended, containing or contaminated with radioactive chemical elements (radioactive isotopes or radionuclides) in concentrations exceeding those established by regulatory bodies.

This waste can pose a risk to humans and the environment due to the ionising radiation emitted by the radionuclides it contains, and must therefore be controlled and managed safely. However, unlike other toxic waste generated by other industrial activities, the toxicity of radioactive waste decreases over time as the isotopes decay and transform into stable chemical elements.

From the perspective of the nature of the radioactive isotopes they contain, radioactive waste classification consists of the following categories:

• Very low-level waste.
• Low- and intermediate-level waste.
• High-level waste or spent fuel.
• Special waste.

The classification takes into account the initial activity of the waste and the half-life (the time required for half of the nuclei in an initial sample to decay) of the radionuclides it mainly contains, which may be short-lived or medium-lived, with a half-life of less than 30 years, or long-lived, with a half-life greater than this value.

Very low-level radioactive waste

RBBA can be defined as a subset of low- and intermediate-level waste that generally has specific activities between 1 and 100 becquerels per gram, reaching up to several thousand in the case of some radionuclides with low radiotoxicity or in small quantities.

Due to their low radioactive content, they require less stringent management requirements and are solid materials, generally scrap metal and debris, containing radioactive isotopes.

Most of this waste originates from the decommissioning of nuclear power plants and represents a very significant volume within the total low- and medium-level waste. Another source is mining and the manufacture of uranium concentrates containing radionuclides from the uranium-238 and thorium-232 decay chains, which generally have very long half-lives.

Low- and medium-level radioactive waste

RBMA waste is waste whose activity is mainly due to the presence of beta or gamma-emitting radionuclides with a short or medium half-life (less than 30 years) and whose long-lived radionuclide content is very low and limited.

For example, 95% of the radioactive waste generated in Spain is low and medium activity and usually comes from hospitals, research centres, industries and nuclear power plants.

High-level radioactive waste

RAWs are those that contain significant concentrations of long-lived alpha emitters, with a half-life of more than 30 years, which can generate heat due to radioactive decay, as their specific activity is high.

The main source of this waste is spent fuel unloaded from nuclear reactors. This group also includes other medium-level waste, which, due to its characteristics, cannot be managed under the conditions established at facilities such as the El Cabril Storage Centre in Hornachuelos, Córdoba (Spain).

For example, in Spain, this type of radioactive waste has so far been managed on site at the production facilities themselves, using spent fuel pools or dry containers in individual temporary storage facilities.

Special waste

They consist of nuclear fuel additives, neutron sources, intranuclear instrumentation, used or replaced components from the vessel system or reactor internals, generally metallic in nature, which, due to their radiological characteristics, cannot be managed at facilities such as the El Cabril Storage Centre located in Hornachuelos, Córdoba (Spain).

As long-lived radioactive waste with significant activity, its temporary and final management is approached in a similar way to that of high-level waste.