Safety features and systems

The safety of a nuclear power plant is based on a multi-level “defence in depth” approach. It is intended to ensure that the radioactivity release barriers are operable under all conditions.

The first stage is that the equipment and functions in a nuclear power plant are designed and constructed according to high quality requirements and sufficient safety margins. Secondly, it is assumed that equipment may nevertheless fail or users can nevertheless make mistakes, and the plant is equipped with protective systems and equipment. In case of a disturbance, the protection systems and equipment aim to restore the plant to a safe state. The third stage of defence-in-depth comprises the safety systems that are used to mitigate the consequences of a possible accident.

Several independent, parallel systems

The reliability of functions important for safety is ensured by means of several parallel components and systems. The automation and power supply for safety systems are separated from the plant’s systems for normal operation. The systems implementing the most important safety functions must be able to carry out their functions even though an individual component in any system would fail to operate and, additionally, any component affecting the safety function would be out of operation simultaneously due to repairs or maintenance.

The reliability of safety functions is determined by means of reliability analyses. Probabilistic safety analyses model the plant in detail in order to determine the functional dependencies of the systems. Reliability analyses help with determining the impact on overall safety of the plant’s different components and functions.

Ensuring the safety of a nuclear power plant means minimising the risk it causes to the environment. The expression ALARA is commonly used with radiation risks. It refers to acting in a manner where the radiation doses to the plant personnel and the surrounding population are kept As Low As Reasonably Achievable. A similar approach to safety is known as SAHARA: Safety is kept As High As Reasonably Achievable in all functions. Further information about radiation safety can be read on TVO's Radiation safety site.