Safety in the design of nuclear facilities, and especially nuclear power plants, is founded on the basic principle of defence in depth, which in summary is based on:

1. Prevention of deviations from the normal functioning, that is, making the functioning of nuclear power plants stable.

2. Control and protection systems to detect and interrupt deviations from normal functioning conditions and to prevent operational incidents that might occur from escalating into accidents.

3. Safeguard systems, together with their appropriate procedures, to prevent reactor core damage and the releasing of radioactive material to the environment. It is assumed that, although improbable, certain operational incidents may not be prevented by the above measures, therefore additional measures are added to prevent damage occurring in the reactor core and the releasing of radioactive material to the environment.

4. Adoption of complementary measures and the management of accidents to control plant conditions, prevent the progression of the accident and mitigate the consequences of accidents beyond the design base.

5. Application of emergency plans including various measures to protect persons and to mitigate the consequences of significant releases of radioactive materials.

The assessment by the Spanish Nuclear Safety Council of the suitable compliance with these principles is the basis for granting the construction permits first and then the operating permits for Spanish nuclear power plants.