Chapter 2: Introduction to Nuclear Safety

Safety Functions

As discussed, there are three fundamental safety functions which must be fulfilled:

• 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.

Safety functions are subdivisions of the fundamental safety functions including those necessary to prevent accident conditions or escalation of accident conditions and those necessary to mitigate the consequences of accident conditions. They can be accomplished, as appropriate, using systems, components or structures provided for normal operation, those provided to prevent AOOs from leading to accident conditions or those provided to mitigate the consequences of accident conditions, and also with prepared staff actions.

The number of safety functions is usually limited at the conceptual stage but grows when the design is in development. Table 1-2 provides an example of a list of safety functions that may be used at an early design stage:

Example of a list of safety functions
Table 1-2: Example of a list of safety functions used at an early design stage (from TECDOC-1787).

The safety functions are intended as a basis for determining whether a structure, system or component performs or contributes to one or more safety functions. They are also intended to provide a basis for assigning an appropriate gradation of importance to safety structures, systems and components that contribute to the various safety functions.

TECDOC-1787 Read more → SRS-46, Appendix 1 Read more →

Plant equipment is subdivided in items important to safety and items not important to safety (where an item is a structure, system or component). Items are important to safety if their failure could lead to on-site or off-site radiation exposure. For more information, see items important to safety.

A safety system is a system important to safety, provided to ensure the safe shutdown of the reactor or the residual heat removal from the core, or to limit the consequences of anticipated operational occurrences and design basis accidents, For more information, see safety system.

Hence, safety systems provide or support the safety functions during DBA and limit their consequences within authorized limits. For example, the ECCS keeps releases below prescribed limits during and after a design basis LOCA. An example of design limits which are derived from the release limit are the maximum fuel cladding temperature and oxidation during a DBA.

In order to have more stringent requirements for structures, systems and components (SSCs) that are designed to perform one or more safety functions, these SSC are classified for safety. Where a higher class is related to more stringent design requirements. This subject is further treated in Subchapter 3.1.4.

Redundancy and diversity

A number of safety functions is designed for redundancy, i.e. at least one more safety system is capable to provide the requested safety function.

If two or more systems or components are redundant in performing an identified function, where the different systems or components have different attributes, they are called 'diverse'.

Examples of such attributes are: different operating conditions, different working principles or different design teams (which provide functional diversity), and different sizes of equipment, different manufacturers, and types of equipment that use different physical methods.

Diversity reduces the probability of common cause failures (these are failures of two or more structures, systems and components due to a single specific event or cause).

The design of equipment shall take due account of the potential for common cause failures of items important to safety, to determine how the concepts of diversity, redundancy, physical separation and functional independence have to be applied to achieve the necessary reliability.

More information on redundancy and diversity is available in SSR- 2/1 (Rev.1), Requirement 24, 27. 52, 58, 62, Read more →

Design requirements for the systems performing safety functions are further elaborated in Subchapter 3.1.4.