1INTERNATIONAL ATOMIC ENERGY AGENCY, Protection against Internal Hazards other than Fires and Explosions in the Design of Nuclear Power Plants, IAEA Safety Standards Series No. NS‑G-1.11, IAEA, Vienna (2004).

2INTERNATIONAL ATOMIC ENERGY AGENCY, Protection against Internal Fires and Explosions in the Design of Nuclear Power Plants, IAEA Safety Standards Series No. NS‑G‑1.7, IAEA, Vienna (2004).

3For example, some combinations of hazards might involve external events that are not plausible in certain locations (e.g. sandstorms, blizzards). Therefore, it is not necessary or even feasible to prescribe a set of combined hazards that would be applicable to all sites.

4Spurious actuation of plant components (of the same type or combinations of different types of component) has the potential to place a given plant into an unsafe operating condition that might not be bounded by the plant’s safety analyses.

5A system or set of components can be divided into redundant ‘divisions’ to allow for the implementation and maintenance of physical, electrical and functional independence with respect to other redundant sets of components [8].

6High energy arcing faults are energetic or explosive electrical equipment faults characterized by a rapid release of energy in the form of heat, light, vaporized metal and pressure increase due to high current arcs between energized electrical conductors or between energized electrical components and neutral or ground. Such faults might also result in projectiles being ejected from the electrical component or cabinet of origin and result in fire.

7In some States, a high energy pipe is defined as a pipe with an internal operating pressure of more than 1.9 MPa or an operating temperature of more than 95°C in the case of water. In other States, these limits are 2.0 MPa and 100°C respectively. Other limits may apply for other fluids, for example gas at greater than atmospheric pressure.

8This approach is considered acceptable only in some States.

9Some States have defined ‘small’ as a pipe with a nominal diameter of 50 mm or less. In other States, pipes with nominal diameter of 25 mm or less are considered small.

10In the context of this Safety Guide, ‘global effects’ refers to possible effects across the entire site.

11This is applicable in States where the leak before break concept has been accepted.

12This subsection addresses water based flooding; however, the same considerations apply to other liquids on the site if they exist in sufficient quantities and locations that could cause a flood. Possible examples include fuel, chemicals and fire extinguishing materials.

13‘Conservative’ depends on whether failure of the door would be advantageous (e.g. by allowing water to flow away from SSCs important to safety) or disadvantageous (e.g. by allowing water to flow towards SSCs important to safety).

14The following cases are assessed in some States with realistic assumptions: drop of the reactor pressure vessel closure head on the reactor pressure vessel, drop of the reactor cavity cover slab on the reactor pressure vessel closure head (when the slabs above the vessel are removed) and drop of the reactor cavity cover slab on the reactor cavity floor slab.

15An exclusion zone is defined by the minimum distance permitted between the point of installation of an SSC important to safety and where portable sources of electromagnetic radiation are allowed to be activated.

16A performance based approach does not prescribe specific steps that have to be taken, but rather defines a desired outcome and clear, objective, and measurable criteria to determine whether that outcome has been reached. Various methods could be used, provided the desired outcome is reached.

17‘Simultaneously’ in this case does not mean that the hazards occur exactly at the same time but rather that the second hazard occurs before the effects of the previous hazard have been completely mitigated.