IEC 61226:2020 pdf download – Nuclear power plants – Instrumentation, control and electrical power systems important to safety – Categorization of functions and classification of systems.
3.1 anticipated operational occurrence AOO deviation of an operational process from normal operation that is expected to occur at least once during the operating lifetime of a facility but which, in view of appropriate design provisions, does not cause any significant damage to items important to safety or lead to accident conditions Note 1 to entry: This note applies to the French language only. [SOURCE: IAEA Safety Glossary, 2018] 3.2 common cause failure CCF failures of two or more structures, systems or components due to a single specific event or cause [SOURCE: IAEA Safety Glossary, 2018] 3.3 controlled state plant state, following an anticipated operational occurrence or accident conditions, in which fulfilment of the fundamental safety functions can be ensured and which can be maintained for a time sufficient to implement provisions to reach a safe state Note 1 to entry: See 4 for the list of fundamental safety functions. Note 2 to entry: See 3.21 for definition of safe state. [SOURCE: IAEA Safety Glossary, 2018] 3.4 design basis accident DBA postulated accident leading to accident conditions for which a facility is designed in accordance with established design criteria and conservative methodology, and for which releases of radioactive material are kept within acceptable limits [SOURCE: IAEA Safety Glossary, 2018] 3.5 design extension conditions DEC postulated accident conditions that are not considered for design basis accidents, but that are considered in the design process of the facility in accordance with best estimate methodology, and for which releases of radioactive material are kept within acceptable limits Note 1 to entry: Design extension conditions comprise conditions in events without significant fuel degradation and conditions in events with melting of the reactor core.
Note 2 to entry: A set of design extension conditions shall be derived on the basis of engineering judgement, deterministic assessments and probabilistic assessments for the purpose of further improving the safety of the nuclear power plant by enhancing the plant’s capabilities to withstand, without unacceptable radiological consequences, accidents that are either more severe than design basis accidents or that involve additional failures. These design extension conditions shall be used to identify the additional accident scenarios to be addressed in the design and to plan practicable provisions for the prevention of such accidents or mitigation of their consequences.[IAEA SSR-2/1] [SOURCE: IAEA Safety Glossary, 2018] 3.6 diversity presence of two or more independent (redundant) systems or components to perform an identified function, where the different systems or components have different attributes so as to reduce the possibility of common cause failure, including common mode failure [SOURCE: IAEA Safety Glossary, 2018] 3.7 electrical/electronic/programmable electronic item E/E/PE item item based on electrical (E) and/or electronic (E) and/or programmable electronic (PE) technology [SOURCE: IEC 62138:2018, 3.15] 3.8 electrical power system system performing electrical power generation, transmission and distribution; performing supply functions to operate plant equipment (pumps, valves, heaters, etc.) and to I&C systems Note 1 to entry: An electrical system can integrate E/E/PE items to perform its internal electrical control and protection. [SOURCE: IEC CDV 63046:2019, 3.12] 3.9 equipment one or more parts of a system; an item of equipment is a single definable (and usually removable) element or part of a system [SOURCE: IEC 61513:2011, 3.16, modified – The notes to entry have been omitted.]
5.2 Background The principle of defence in depth is firmly established in the safety design basis of nuclear power plants. The fundamental idea that there should be several layers or lines of defence in the prevention of unsafe conditions, and that the prevention of unsafe conditions, before mitigation is needed, is always to be preferred. Because of the large number of functions that are required to operate a NPP and sustain its safety, a number that increases with the principle of defence in depth, it is important that the safety significance of each function is known. All the functions performed at the different levels of defence in depth are considered. IAEA Safety Requirements SSR-2/1 establishes the idea of classification of NPP systems according to their safety significance, and gives examples of the classification of the major systems of several types of NPPs. All structures, systems and components, including software for instrumentation and control (I&C), that are items important to safety, shall be first identified and then classified on the basis of their function and significance with regard to safety. They shall be designed, constructed and maintained such that their quality and reliability is commensurate with this classification.