1INTERNATIONAL ATOMIC ENERGY AGENCY, Design of Reactor Containment Systems for Nuclear Power Plants, IAEA Safety Standards Series No. NS-G-1.10, IAEA, Vienna (2004).

2The phrase ‘plant design envelope’ is used to refer, in a simplified way, to all conditions postulated in the design of a nuclear power plant.

3‘Well defined and accepted’ generally means either widely accepted by regulatory bodies or proposed by international organizations.

4These conditions should be analysed during the identification of situations to be practically eliminated, even though their consequences can generally be mitigated with implementation of reasonable technical means.

5In accordance with international practices, the pressure retaining boundary of components that are part of the reactor coolant pressure boundary should be designed and manufactured in compliance with the highest codes and standards defined by the industry for nuclear applications, except for parts of the reactor coolant pressure boundary whose failure would result in leakage that can be compensated by the normal water make-up system.

6In this context, ‘design value’ is the value of the leakage rate established as a target of the design and used in the safety analysis to determine the radioactive releases under design pressure and design temperature.

7An ‘SL-2 earthquake’ denotes the level of ground motion associated with the maximum earthquake to be considered for design, often denoted as the ‘safe shutdown earthquake’.

8Examples of such limits that are applied in States are 0.25–0.5% overall leakage of the contained mass of free gas and steam per day at design pressure for steel containments or concrete containments with a steel liner, and 1.0–1.5% per day overall leakage for prestressed concrete containments without a steel liner.

9A ‘closed loop’ is a piping or ducting system that penetrates the containment envelope and that is designed to form a closed circuit either inside or outside the containment, or inside and outside the containment in operational states and in accident conditions.

10An ‘automatic valve’ is a valve or damper that can be actuated either by the protection system or by other instrumentation and control without action by the operator, or by the process medium itself. For example, certain types of check valve are considered automatic valves. A ‘normally closed valve’ is a valve that is closed under active administrative control (e.g. being locked closed or continuously monitored to show that the valve is in the closed position) except for intermittent opening for specific purposes such as monitoring, testing or sampling. A ‘remotely operated valve’ is a valve or damper that can be actuated by an operator from the control room and in some cases from the supplementary control points.

11‘Annulus’ indicates the free volume between the two walls of the containment.

12Such an event might be a break occurring outside the containment of a pipe crossing the containment and carrying radioactive material, or for the failure of an interface between two associated systems (e.g. rupture of a heat exchanger tube of the component cooling water system) that leads to a release of radioactive material from a system inside the containment to a system outside.