CHAPTER 2: Introduction to Nuclear Safety

Fundamental Safety Principles 

Safety objective and safety principles

The fundamental safety objective is to protect people and the environment from harmful effects of ionizing radiation.

This fundamental safety objective of protecting people — individually and collectively — and the environment has to be achieved without unduly limiting the operation of facilities or the conduct of activities that give rise to radiation risks. To ensure that facilities are operated and activities conducted so as to achieve the highest standards of safety that can reasonably be achieved, measures have to be taken:

a) To control the radiation exposure of people and the release of radioactive material to the environment;
b) To restrict the likelihood of events that might lead to a loss of control over a nuclear reactor core, nuclear chain reaction, radioactive source or any other source of radiation;
c) To mitigate the consequences of such events if they were to occur.

The fundamental safety objective applies for all facilities and activities, and for all stages over the lifetime of a facility or radiation source, including planning, siting, design, manufacturing, construction, commissioning and operation, as well as decommissioning and closure. This includes the associated transport of radioactive material and management of radioactive waste.

Ten safety principles have been formulated, on the basis of which safety requirements are developed and safety measures are to be implemented in order to achieve the fundamental safety objective. The safety principles form a set that is applicable in its entirety; although in practice different principles may be more or less important in relation to particular circumstances, the appropriate application of all relevant principles is required.

Principle 1: Responsibility for safety.
The prime responsibility for safety must rest with the person or organization responsible for facilities and activities that give rise to radiation risks.

Principle 2: Role of government.
An effective legal and governmental framework for safety, including an independent regulatory body, must be established and sustained.

Principle 3: Leadership and management for safety.
Effective leadership and management for safety must be established and sustained in organizations concerned with, and facilities and activities that give rise to, radiation risks.

Principle 4: Justification of facilities and activities.
Facilities and activities that give rise to radiation risks must yield an overall benefit.

Principle 5: Optimization of protection.
Protection must be optimized to provide the highest level of safety that can reasonably be achieved.

Principle 6: Limitation of risks to individuals.
Measures for controlling radiation risks must ensure that no individual bears an unacceptable risk of harm.

Principle 7: Protection of present and future generations.
People and the environment, present and future, must be protected against radiation risks.

Principle 8: Prevention of accidents.
All practical efforts must be made to prevent and mitigate nuclear or radiation accidents.

Principle 9: Emergency preparedness and response.
Arrangements must be made for emergency preparedness and response for nuclear or radiation incidents.

Principle 10: Protective actions to reduce existing or unregulated radiation risks.
Protective actions to reduce existing or unregulated radiation risks must be justified and optimized.

See SF-1, para 3.1 to 3.40 for further information. Read more →

Defence-in-depth
The primary means of preventing and mitigating the consequences of accidents is ‘defence in depth’. Defence in depth is implemented primarily through the combination of a number of consecutive and independent levels of protection that would have to fail before harmful effects could be caused to people or to the environment. If one level of protection or barrier were to fail, the subsequent level or barrier would be available.

When properly implemented, defence in depth ensures that no single technical, human or organizational failure could lead to harmful effects, and that the combinations of failures that could give rise to significant harmful effects are of very low probability.

The independent effectiveness of the different levels of defence is a necessary element of defence in depth.

The concept of defence-in-depth is further elaborated in Subchapter 2.2.

Safety functions
The concept of defence-in-depth is structured around a series of safety functions, derived from the three fundamental safety functions, which are (SSR-2/1 (Rev. 1), Req. 4):

• Control of reactivity;
• Removal of heat from the reactor and from the fuel store;
• Confinement of radioactive material, shielding against radiation and control of planned radioactive releases, as well as limitation of accidental radioactive releases.


As SSR-2/1 (Rev.1), para 4.2 states: Means of monitoring the status of the plant shall be provided for ensuring that the required safety functions are fulfilled.

Safety functions and their applications are further discussed in Subchapter 2.3.

Initiating events

Nuclear power plants need to be designed against a number of postulated initiating events. These are selected on the basis of their anticipated frequency and/or possible consequences.

Further discussion on the initiating events is presented in Subchapter 3.1.1.

Single Failure Criterion

A single failure is a failure that results in the loss of capability of a system or component to perform its intended safety function(s) and any consequential failure(s) that result from it. The single failure criterion is a criterion (or requirement) applied to a system such that it must be capable of performing its task in the presence of any single failure.

The single failure criterion shall be applied to each safety group incorporated in the plant design.

In most reactor designs, the following systems are designed against single failure:

• Reactor shutdown;
• Residual heat removal from the core;
• Emergency core cooling;
• Containment isolation;
• Containment heat removal.

SSR-2/1 (Rev. 1), Requirement 25. Read more →