High level radioactive waste is that containing appreciable concentrations of long-lived alpha-emitting radionuclides and/or beta/gamma emitters with a half-life greater than 30 years, which can generate heat due to their radioactive decay, given their high specific activity.

The National Radioactive Waste Company (Enresa) is responsible for managing radioactive waste in Spain, which it undertakes following the guidelines of the Sixth General Radioactive Waste Plan, a document approved by the government.

In nuclear power plants, radioactive waste is handled in accordance with the Radioactive Waste and Spent Fuel Management Plan, an official document of the facility whose contents are set out in a CSN Safety Guide on the contents and criteria for the preparation of the plans for managing radioactive waste in a nuclear facility.

As indicated in the definition, the high level radioactive waste contains radioactive isotopes with half-lives of more than 30 years. They may also emit heat and be active for thousands or tens of thousands of years.

This type of waste essentially consists of the spent fuel generated during the operation of the nuclear power plants in the form of fuel assemblies. Additionally, in Spain, there is a small quantity of materials from the reprocessing of the spent fuel from the Vandellós I power plant.

During the irradiation of the fuel in the reactor, high level radioactive isotopes are generated such as the uranium itself, plutonium and minor actinides and fission and activation products. The radiations emitted by the fuel during the decay time of these isotopes require the development of a management approach in accordance with the risks to the public, the environment and the future generations, in various stages:

• Initial storage. The spent fuel is stored for a few years in the nuclear power stations’ spent fuel pools to reduce the heat load.
• Intermediate storage. Medium or long term storage (between 20 and 60 years) in spent fuel pools or dry storage in casks in an Individualized Temporary Storage Facility (ATI) on the nuclear power plants sites that have these. It may also be stored in a Centralized Temporary Storage Facility (CTSF), independent of the nuclear power plants sites.
• Definitive disposal. Given the long life of this waste, Deep Geological Repositories (DGR) are the internationally accepted option for the final disposal of high level radioactive waste. The geological barrier is key to the multi-barrier design concept.

To reduce the volume of this waste and to re-use the fissionable material, other countries adopt closed fuel cycle strategies, such as the reprocessing of the spent fuel, which involves separating the uranium and plutonium in the fuel for re-use in new nuclear fission processes in nuclear power plants. The high level radioactive waste generated is vitrified for its storage. Currently, the so-called advanced closed fuel cycle is under development and involves the transmutation of the minor actinides (with a long life) and some fission products to reduce their activity and radio-toxicity before their storage.