Defence-in-depth safety approach

Ensuring the safety in order to prevent reactor damage and harmful effects of radiation is done on several successive functional and structural levels that secure each other. This approach is called defence in depth. Ensuring safety can be separated into preventive, protective and mitigating levels.

Nuclear power plant equipment is divided into safety categories according to their significance to safety. The more important the category of a device is, the higher the quality requirements are for its design, manufacture and maintenance.

To achieve a high level of operation, attention is paid to quality assurance and in particular to the operations, procedures, training and instruction quality in the organization.

The first, i.e. the preventive level

The aim is to prevent deviations from the normal operating state of the plant by means of a high level of design and operation. High quality standards and adequate safety margins are applied on the design, manufacture, installation and maintenance of important equipment. In addition, the design aim is to use inherently stable solutions that correct abnormal circumstances. In particular, the reactor is designed in such a way that the inherent feedback will seek to prevent an uncontrolled increase of reactor power. This is based on the fact that a power increase will lead to a rise in reactor temperature, which in turn reduces power.

The second, i.e. the protective level

Although high quality standards are set for the design and operation of facilities, accidents and operating problems may still occur. Because of this, nuclear power plants are equipped with systems to detect problems and prevent their development into serious accidents. These systems ensure in particular reactor shutdown, cooling of the reactor core and removal of residual heat.

The third, i.e. the mitigating level

If the progression of the accident cannot be prevented with actions on the first and second level, it is still possible to mitigate its consequences. In this case, the most important thing is to ensure that the containment building remains intact and that the containment building systems are operational.

According to the defence in depth approach, the aim is to prevent accidents using good design, high quality and careful operations. However, if a fault or an accident occurs, safety systems are used in order to gain control over it. If this also fails, the goal is to mitigate the environmental impact of the accident as effectively as possible.

Share this page



  • Tomi Routamo / Deputy Director