1INTERNATIONAL ATOMIC ENERGY AGENCY, Deterministic Safety Analysis for Nuclear Power Plants, IAEA Safety Standards Series No. SSG-2, IAEA, Vienna (2009).

2The application and the establishment of such arrangements are beyond the scope of this Safety Guide. Requirements regarding these arrangements are established in IAEA Safety Standards Series No. GSR Part 7, Preparedness and Response for a Nuclear or Radiological Emergency [8]; and recommendations are provided in IAEA Safety Standards Series Nos GS-G2.1, Arrangements for Preparedness for a Nuclear or Radiological Emergency [9], and GSG-2, Criteria for Use in Preparedness and Response for a Nuclear or Radiological Emergency [10].

3In this Safety Guide, the term ‘control and limitation systems’ refers not only to the instrumentation systems for control and limitation of the plant variables but also to the systems for normal operation and those for anticipated operational occurrences actuated by them.

4In some States, these categories of design extension conditions are denoted respectively as ‘design extension conditions A’ (without significant fuel degradation) and ‘design extension conditions B’ (with core melting).

5Although equipment qualification is outside the scope of this Safety Guide, it is understood that typical equipment qualification programmes for design extension conditions with core melting might not always be applicable and an assessment of the operability of structures, systems and components is acceptable. The term ‘survivability assessment’ is used in some States for such an assessment.

6The ‘fundamental safety functions’ are also called ‘main safety functions’ [3].

7See further guidance in IAEA Safety Standards Series Nos NS-G-1.5, External Events Excluding Earthquakes in the Design of Nuclear Power Plants [16], NS-G-1.7, Protection against Internal Fires and Explosions in the Design of Nuclear Power Plants [17], and NS-G-1.11, Protection against Internal Hazards other than Fires and Explosions in the Design of Nuclear Power Plants [18].

8These conditions need to be analysed during the identification of situations to be ‘practically eliminated’. Nevertheless, consequences from para. 3.56(c)(i) and (ii) could generally be mitigated with the implementation of reasonable technical means.

9In this Safety Guide, a ‘surrogate variable’ is a measurable variable that provides an indirect measure of another variable that cannot be directly measured.

10 A ‘cliff edge effect’ is defined in the IAEA Safety Glossary [3] as “An instance of severely abnormal conditions caused by an abrupt transition from one status of a facility to another following a small deviation in a parameter or a small variation in an input value.” The term ‘parameter’ in this definition can be interpreted in a broad sense as any plant physical variable, design aspect, equipment condition or magnitude of a hazard that can influence equipment or plant performance.

11Aleatory uncertainty is uncertainty inherent in a phenomenon and is of relevance for events or phenomena that occur in a random manner such as random failures of items of equipment. Epistemic uncertainty is uncertainty attributable to incomplete knowledge about a phenomenon, which affects the ability to model it [3].

12The terms ‘conservative methods’ and ‘conservative analysis’ are to be understood to refer to any of Options 1–3 from Table 1, Section 2, and para. 2.14.

13Current practice in some States is that credit is given in the safety analysis for the availability of non-permanent equipment after, for example, 8 hours for equipment stored on the site or 72 hours for equipment stored off the site.

14Current practice in some States is that credit is given in the safety analysis for the availability of non-permanent equipment after, for example, 8 hours for equipment stored on the site or 72 hours for equipment stored off the site.