The regulatory body establishes the nuclear safety principles and issues regulations on design; it needs to be able to evaluate the safety of the proposed design by reviewing and assessing the safety documentation (e. g. design basis, the safety analysis reports) and verifying the compliance of the design with regulatory requirements. The design basis is the range of conditions and events explicitly taken into account in the design of the nuclear installation, according to established criteria, such that the nuclear installation, through the planned operation of safety systems, can operate under these conditions and events without exceeding authorized limits. The design should be reviewed by the regulatory body considering the design basis accidents and design extended conditions. The design basis accidents (DBAs) are defined when key safety plant parameters do not exceed specified limits, with no or only minor radiological impacts, both on and off the site, and do not necessitate any off-site intervention mea­sures, and DBAs shall be analysed in a conservative manner. The design extended conditions (DECs) consider that the plant can be brought into a controlled state, the integrity of the containment is maintained (the con­tainment shall cope with core melt situation) and significant releases are practically eliminated. DECs may be analysed using a best-estimate approach. For those conditions that are not practically eliminated, design provisions shall be made such that only protective measures that are of limited scope in terms of area and time are necessary for the protection of the public, and sufficient time is available to implement these measures.

The IAEA established safety design requirements in 2000 (IAEA, 2000). The analysis of the operating experience and the comments by Member

States have recommended to revise the document. The revision has already been approved by the IAEA Committee of Safety Standards (CSS) but not yet published (IAEA, 2011b). Although the structure of the revised version is similar, it introduces some new concepts such as Design Extension Conditions, aimed at considering extreme circumstances.