At the beginning of the nuclear energy era, several countries developed more or less independent standards in the attempt to define the acceptable level of risk, in terms of risk to the general public. The methods varied from simple dose limits within a specified frequency range (Hurst and Boyd, 1972) down to detailed listings of specific equipment that must be installed (Murley et al., 1991). These criteria have been refined over past decades; the present-day international standards for achieving a satisfactory level of power reactor safety can be found in IAEA publications (NS-R-1,2000 and 13 associated guides). These documents are not, in general, specific enough to serve as national standards for reactor licensing. Generally, each national regulatory group establishes a unique set of specific documents for licensing purposes. Most of these national regulations make reference to the higher — level IAEA documents. Within the community of large reactor owners there has been a major cooperative effort to establish inter-plant commu­nication as well as codes of ‘best practice’ to disseminate detailed plant operational and safety information to new owners. It is strongly recom­mended that a prospective new plant owner should join the appropriate group and to seek information and training from them. This in recognition of the large financial benefits that can be gained by doing this, as well as the fact that nuclear plant owners know that a serious accident anywhere in the world has immediate deleterious effects on all operating plants.

298 Infrastructure and methodologies for justification of NPPs Small power reactors and research reactors

Recently, IAEA has produced a separate Safety Requirements document (NS-R-4 and six associated guides) for small reactors. The Agency is prepar­ing one further guide entitled The Graded Approach. This guide will describe the unique acceptance criteria for this category of fission reactors. In the meantime, at least one national regulatory agency (Canada) has already produced such a guide showing details of the different levels of require­ments that may be applied in licensing of small reactors, as well as giving a definition of this reactor classification.