1Factors relevant in determining the safety significance of items important to safety are set out in para. 5.34 in Ref. [2], and for convenience are reproduced in para. 2.2 of this Safety Guide.

2According to the IAEA Safety Glossary [6], the functions formerly named ‘fundamental safety functions’ are now named ‘main safety functions’. In quotations from IAEA safety standards, the term ‘fundamental safety function’ is to be understood to mean ‘main safety function’.

3For the purpose of this Safety Guide, a function is defined as any action performed by a single SSC or a set of SSCs.

4As stated in para. 5.9 of Ref. [2], “The postulated initiating events used for developing the performance requirements for the items important to safety in the overall safety assessment and the detailed analysis of the plant shall be grouped into a specified number of representative event sequences that identify bounding cases and that provide the basis for the design and the operational limits for items important to safety.”

5The possibility of certain conditions occurring is considered to have been ‘practically eliminated’ if it is physically impossible for the conditions to occur, or if the conditions can be considered with a high level of confidence to be extremely unlikely to arise.

6See Requirements 15 and 19 in Ref. [2].

7Definitions are provided in Ref. [2].

8Expert groups providing engineering judgement should include knowledgeable personnel from the operating organization of the plant, and personnel with skills and expertise in probabilistic safety assessment, safety analysis, plant operation, design engineering and systems engineering.