1In this Safety Guide the term ‘device’ (e.g. shutdown device or reactivity control device) is used to designate a part (regardless of its shape, material or purpose) that is inserted into the core (e.g. a control rod or a tube containing fluid for reactivity control). The term ‘device’ may include the drive mechanism for the parts. By contrast, the term ‘means’ (e.g. means of shutdown or means of reactivity control) is used to denote more broadly the functional aspect of the component or system.

2The objective of this recommendation is to avoid cladding failures caused by high cladding temperatures. In some designs, critical heat flux conditions during transients can be tolerated if it can be shown by other methods that the cladding temperatures do not exceed the acceptable limits.

3In BWRs, the pressure difference between the inside and outside of the boundary of the channel box may induce swelling of the box. This deformation, as well as fuel bowing, may consequently increase the local flux peaking factor.

4It is general practice for some reactor types to ensure that coolant additives do not cause the power coefficient of reactivity to become positive. (For further discussion of the coefficients of reactivity, see Appendix I.)

5For some designs, e.g. PHWRs, the unavailability of two shutdown rod devices with the highest reactivity worth is assumed for this case.

6The simplest common form of design for fail-safe shutdown allows the shutdown decices to be held above the core by active means. Provided that the guide structures for the shutdown devices are not obstructed, the devices will drop into the core under gravity in the event of a de-energization of the active means of holding them (e.g. a loss of current through a holding electromagnet).

7In some States an analysis of anticipated operational transients without scram is not required if two diverse and independent shutdown systems are provided, each of which is capable of quickly rendering the nuclear reactor subcritical from all operational states and accident conditions within the design basis.

8The IAEA is revising the requirements and guidance in the subject area of quality assurance as established in Safety Series No. 50-C/SG-Q (1996) in new safety standards on management systems for the safety of nuclear facilities and activities involving the use of ionizing radiation. The term ‘management system’ has been adopted in the revised standards instead of the terms ‘quality assurance’ and ‘quality assurance programme’. The new standards will integrate all the aspects of managing a nuclear facility, including the safety, health, environmental and quality requirements, into one coherent system.

9In certain types of reactor, moderator and coolant are separated in such a manner that a decrease in density by voiding in the coolant may lead to a positive reactivity effect.

10For BWRs, the coolant density coefficient of reactivity is expressed as a void coefficient of reactivity

11In several States the temperature coefficient of reactivity for the moderator and/or coolant for PWRs is allowed to be slightly positive.

12Research is in progress on the possible benefits of reducing the release of fission gas by changing the microstructure of the fuel.