What is Nuclear Safety ?

Definition 

Nuclear Safety is Safety applied to all activities involving radioactive material or ionizing radiation, notably: 

• all types of nuclear reactors (Nuclear Power Plants — NPP, research reactors, propulsion reactors, etc.) ; 
• nuclear fuel cycle facilities (upstream and downstream) ; 
• hot laboratories and particle accelerators ; 
• Nuclear waste storage units ; 
• all types of nuclear facilities in decommissioning stage ; 
• all types of transport of radioactive material ; 
• activities (including medical) involving radioactive sources. 

Thus, from the Safety Glossary of the IAEA (International Atomic Energy Agency), Nuclear Safety is the achievement of proper operating conditions, prevention of accidents or mitigation of accident consequences, resulting in protection of workers, the public and the environment from undue radiation hazards (Terminology used in Nuclear Safety and Radiation Protection, 2007 Edition). 

In nuclear industry, safety is distinguished from two other important and complementary concepts: 

• Radiation Protection; 
• Safeguards of nuclear material (protection against nuclear proliferation). 

After defining the objective of Nuclear Safety, we will focus on the particular approach of Defense in Depth and end with the Fundamental Safety Principles of the IAEA, which will then lead us to the Frame and Regulatory Requirements for nuclear safety solutions. 

Nuclear Safety general objective

According to the Fundamental Safety Principles of the IAEA, the fundamental safety objective is to protect people and the environment from harmful effects of ionizing radiation. 

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 to: 

1. control the radiation exposure of people and the release of radioactive material to the environment; 
2. 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; 
3. 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. 

Eventually, it applies to all operating conditions, from normal operation to the management of radiological emergencies. 

Thus, nuclear consulting companies need to consider safety for all nuclear power consulting. 

Defence in Depth

Definition 

Defence in Depth is originally a military concept. 

Against a given threat, since no single defence is infaillible, the idea of Defence in Depth is to stack several independent levels of defence.
Each level is designed to prevent so far as possible: 

• proceeding to the next level of defence; 
• the consequences of previous level failure. 

The International Nuclear Safety Group (INSAG) of the IAEA has settled Defence in Depth for Nuclear Safety in its report Defence in Depth in Nuclear Safety (INSAG 10, 1996).
Defence in Depth in Nuclear Safety, thus structured in five levels, applies to all stages of design and operation of nuclear reactors and facilities. 

The training courses given by SureDyna include a complete presentation of Defense in Depth as well as its applications to design and operation of nuclear reactors and nuclear facilities. 

Meanwhile, three basic levels of Defense in Depth should be remembered:  

1. Prevention 
2. Protection 
3. Mitigation of consequences 

Example of application

The best-known application of the Defense in Depth concept is the three containment barriers of radioactive materials around the core of a nuclear reactor: 

1. Fuel cladding; 
2. Reactor Primary Coolant boundary; 
3. Containment building. 

However, although this example of defence in depth is excellent and rightly the most cited, it must still be considered as an instance of application of the concept of Defence depth, not as Defense in Depth itself.

Indeed, the barriers do not match the levels of Defense in Depth and should not be mistaken for them. 

General application

According to the different levels, the Defense in Depth approach defines categories of normal or abnormal operation situations (incidents and accidents) with associated means of design and operation (redundant equipment in case of failure, backup devices, specific procedures, etc.).
The requirements with regard to a given situation are all the more strong as this situation is likely to occur more frequently, as shown on the so-called “Farmer” diagram.

In any case, the levels of Défense in Depth must be independent in order to respect the Single Failure Criterion:
A single event (incident) of a given category shall not generate an event of a higher category without other events occurring independently. 

Safety Principles

The IAEA has defined ten Fundamental Safety Principles:  

1. The prime responsibility for safety must rest with the person or organization responsible for facilities and activities that give rise to radiation risks 
2. An effective legal and governmental framework for safety, including an independent regulatory body, must be established and sustained 
3. 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 
4. Facilities and activities that give rise to radiation risks must yield an overall benefit 
5. Protection must be optimized to provide the highest level of safety that can reasonably be achieved 
6. Measures for controlling radiation risks must ensure that no individual bears an unacceptable risk of harm 
7. People and the environment, present and future, must be protected against radiation risks 
8. All practical efforts must be made to prevent and mitigate nuclear or radiation accidents 
9. Arrangements must be made for emergency preparedness and response for nuclear or radiation incidents 
10. Protective actions to reduce existing or unregulated radiation risks must be justified and optimized