Safety features and systems

The safety of a nuclear power plant is based on the multi-stage ´in-depth concept´, the aim of which is to ensure that barriers to the release of radioactivity are always in working order. The first stage is the planning and construction of equipment and functions at a nuclear power plant in accordance with high quality requirements and adequate safety margins. Secondly, it is assumed that equipment may nevertheless develop faults or that operators will make mistakes, and the plant is equipped with protective systems and equipment. In the event of a disruption they will endeavour to restore the plant to a safe state. The third stage of the in-depth concept consists of the safety systems, which provide for mitigating the effects of a possible accident.

The reliability of the functions that are important for safety is guaranteed by means of multiple parallel equipment and systems. The automation in the safety systems and the supply of electricity are kept isolated from the systems normally used by the plant. The systems for the most important safety functions must be able to work, even if any single piece of equipment in a system is inoperative and a piece of equipment that affects a safety function is at the same time removed from use for repair or maintenance.

The operational reliability of the safety functions is determined by reliability analyses. A plant is modelled in detail in probability-based safety assessments so that the operational dependencies of the systems are made clear. The analyses help in determining the effect of a plant´s different parts and functions on overall safety. The weak links in safety can be localised and repaired.

Attention to safety at nuclear power plants means the minimization of risk caused to the environment by the plant. The acronym ALARA is an expression often used in connection with radiation risk. It means operating in such a way that the radiation doses caused to both people at the plant and to the surrounding population are kept As Low As Reasonably Achievable. Correspondingly, safety is also observed under the SAHARA principle: Safety As High As Reasonably Achievable.