A set of design extension conditions for a research reactor shall be derived for the purpose of enhancing the safety of the research reactor by enhancing its capabilities to withstand, without unacceptable radiological consequences, accidents that are either more severe than design basis accidents or that involve additional failures. The set of design extension conditions shall be derived on the basis of engineering judgement and by using a graded approach, deterministic assessments and complementary probabilistic assessments, as appropriate. The design extension conditions shall be used to identify the additional accident scenarios to be addressed in the design and to plan practicable provisions for the prevention of such accidents or mitigation of their consequences if they do occur.
6.64. An analysis of design extension conditions shall be performed24 to determine whether the potential radiological consequences would exceed those deemed acceptable by the relevant authority. The main technical objective of considering the design extension conditions is to provide assurance that the design of the facility is such as to prevent accident conditions beyond those considered design basis accident conditions, or to mitigate their consequences, as far as is reasonably practicable. This might require additional safety features for design extension conditions, or extension of the capability of safety systems to maintain the main safety functions, especially the confinement function25. These additional safety features for design extension conditions, or this extension of the capability of safety systems, shall be such as to ensure the capability for managing accident conditions in which there is a significant amount of radioactive material confined in the facility (including radioactive material resulting from degradation of the reactor core).
6.65. The design extension conditions shall be used to define the design specifications for safety features and for the design of all other items important to safety that are necessary for preventing such conditions from arising, or, if they do arise, for controlling them and mitigating their consequences. For existing research reactors, complementary safety reassessment shall be performed to determine whether there is a need for mitigatory measures or modifications of the facility to be implemented.
6.66. For subcritical assemblies, the likelihood of criticality shall be sufficiently remote to be considered a design extension condition. To ensure subcriticality, the design shall include safety provisions such as the use of only natural uranium
24 The analysis of design extension conditions could be performed by means of a best estimate approach (more stringent approaches may be used according to States’ requirements).
25 Confinement is the prevention or control of releases of radioactive material to the environment in operation or in accidents [8]. Confinement is a basic safety function that is required to be fulfilled in normal operational modes, for anticipated operational occurrences, in design basis accidents and, to the extent practicable, in selected design extension conditions.
The function of confinement is usually fulfilled by means of several barriers surrounding the main parts of a nuclear reactor that contain radioactive material. For a research reactor, the reactor building may be the ultimate barrier for ensuring confinement. Consideration may be given to the use of other structures (e.g. the reactor block in a fully enclosed research reactor) for providing confinement where this is technically feasible. For most designs of large nuclear reactor, a strong structure housing the reactor is the ultimate barrier providing confinement.
Such a structure is called the containment structure or simply the containment. The containment also protects the reactor against external events and provides radiation shielding in operational states and in accident conditions.
or limited amounts of fissile materials, or a fixed fuel/moderator ratio. If no such provisions can be provided, measures for mitigating the consequences shall be determined and implemented on the basis of safety analysis.
6.67. The analysis undertaken shall include identification of the safety features that are designed for use in, or that are capable of preventing or mitigating, events considered in the design extension conditions. These features:
(a) Shall be independent, to the extent practicable, of those used in more frequent accidents;
(b) Shall be capable of performing, to the extent practicable, in the environmental conditions pertaining to design extension conditions, as appropriate;
(c) Shall be reliable commensurate with the function that they are required to fulfil.
6.68. The design shall be such that the possibility of conditions arising that could lead to an early radioactive release or a large radioactive release26 is practically eliminated. The design shall be such that for design extension conditions, protective measures that are limited in terms of times and areas of application shall be sufficient for protection of the public, and sufficient time shall be available to take such measures.
Combinations of events and failures
6.69. Where the results of engineering judgement and deterministic safety assessments, complemented, as appropriate, by probabilistic safety assessments, indicate that combinations of postulated initiating events could lead to accident conditions, such combinations of events shall be considered to be design basis accidents or shall be included as part of design extension conditions, depending mainly on their likelihood of occurrence. Certain events might be consequences of other events, such as a flood following an earthquake. Such consequential effects shall be considered to be part of the original postulated initiating event.
26 An early radioactive release is a release for which off-site protective measures are necessary but are unlikely to be fully effective in due time. A large radioactive release is a release for which off-site protective measures limited in terms of times and areas of application are insufficient to protect people and the environment.