[1] D. C. WADE AND R. N. HILL, The design rationale of the IFR, Prog,. Nucl. Energy, 31 (1997) 13-42.

[2] PLANCHON, H. P., SACKETT, J. I., GOLDEN, G. H., SEVY, R. H., Implications of the EBR-II Inherent Safety Demonstration Test, Nucl. Eng. Des. 101, (1987) 75.

TERMS USED

Small and medium sized reactors (SMRs)

According to the classification currently used by the IAEA, small reactors are reactors with an equivalent electrical power output of less than 300 MW, medium sized reactors have an equivalent electrical power output of between 300 and 700 MW [1].

Small reactors without on-site refuelling

According to the definition given in Ref. [1], small reactors without on-site refuelling are reactors designed for infrequent replacement of well-contained fuel cassette(s) in a manner that prohibits clandestine diversion of nuclear fuel material.

Safety related terms

Definitions from IAEA safety standards

The format used to describe passive safety design options for SMRs — provided in Appendix 3 and used in Annexes I-X — contributed by Member States, was developed reflecting definitions used in IAEA Safety Standards Series No. NS-R-1 Safety of Nuclear Power Plants: Design [2]:

Active component: A component of which function depends on an external input such as actuation, mechanical movement or supply of power.

Passive component: A component of which function does not depend on an external input such as actuation, mechanical movement or supply of power.

Plant equipment: (see Fig. 1).

Safety system: A system important to safety, provided to ensure safe shutdown of the reactor or residual heat removal from the core, or to limit the consequences of anticipated operational occurrences and design basis accidents.

Plant equipment

Items[27] important to safety

Items not important to safety

Safety related items

FIG. 1. Plant equipment [2].

beyond design basis accidents

(a) Accident conditions which are not explicitly considered design basis accidents but which they encompass;

(b) Beyond design basis accidents without significant core degradation.

FIG. 2. Plant states [2].

Protection system: A system which monitors the operation of a reactor and which, on sensing an abnormal condition, automatically initiates actions to prevent an unsafe or potentially unsafe condition.

Plant states: (see Fig. 2).

Normal operation: Operation within specified operational limits and conditions.

Postulated initiating event: An event identified during design as capable of leading to anticipated operational occurrences or accident conditions.

Anticipated operational occurrence: An operational process deviating from normal operation which is expected to occur at least once during the operating lifetime of a facility but which, with appropriate design provisions, does not cause any significant damage to items important to safety or lead to accident conditions.

Accident conditions: Deviations from normal operation more severe than anticipated operational occurrences, including design basis accidents and severe accidents.

Design basis accident: Accident conditions against which a nuclear power plant is designed according to established design criteria, and for which the damage to the fuel and the release of radioactive material are kept within authorized limits.

Severe accidents: Accident conditions more severe than a design basis accident and involving significant core degradation.

Ultimate heat sink: A medium to which residual heat can always be transferred, even if all other means of removing the heat have been lost or are insufficient.

Single failure: A failure which results in the loss of capability of a component to perform its intended safety function(s), and any consequential failure(s) which result from it.

Common cause failure: Failure of two or more structures, systems or components due to a single specific event or cause.

Safety function: A specific purpose that must be accomplished for safety.