The CEGB believes that to assist in the achievement of a satisfactory tec el of safety for an operating re­actor, it is desirable to set down safety criteria at the beginning of a project against which the developing design can be assessed. The criteria should take the form of specific basic targets for designers, but should not be such as to inhibit them from making legiti­mate balances between system configuration and plant standards to produce optimum and economic design solutions.

The general safety requirements for the early mag — nox and AGR nuclear power stations have been subject to a continuous process of improvement and refine­ment in the light of technical and scientific develop­ments as successive designs of reactors have been through the design process. In 1974 the CEGB de­veloped the present detailed design safety requirements in order to consolidate experience and worldwide de­velopments in the safety field, and to encourage the use of probabilistic techniques in safety assessments. The fundamental criteria are now set down in the CEGB’s Health and Safety Department’s (HSD) docu­ment ‘Design Safety Criteria’ (HS/R167/81 Revised, March 1982) which lays down the safety requirements for all types of nuclear power stations. A more de­tailed guide to designers and the implementation of the criteria is given in the CEGB’s Generation Develop­ment and Construction Division’s (GDCD) document ‘Design Safety Guidelines* for each reactor type (DSG2 for the PWR).

The Design Safety Criteria and Guidelines provide guidance on the important safety-related factors which need to be taken into account during design. Several of these are based on the concept of acceptable risk and are expressed in probability terms as design tar­gets for each reactor on a site, while others describe targets in qualitative or engineering terms.

The criteria fall into three broad groups. The first, setting out the fundamental or basic criteria, specifies the targets and methods of assessment for doses to operating staff and the public under normal operating conditions. The second deals with the assessment of faults and hazards; that is, faults originating within the plant itself and hazards (e. g., earthquakes) arising from outside it. The third group details the engineer­ing criteria specifying system reliability criteria and the requirements for segregation of plant, separation of functions, inspection, testing and monitoring, emer­gency control and operator actions. It also contains requirements for items such as the control of radio­active discharges and specifies a requirement for a comprehensive quality assurance programme with re­sults fully documented and retained.

The criteria are not station operational limits but targets for designers, so that if a particular reactor design does not meet the criteria in all respects, it does not automatically follow that the design is un­acceptable to the CEGB from a safety point of view.

Nonetheless, while there is some latitude in the criteria quoted for operator exposures, releases and dose, it is generally the case that these cannot be significantly exceeded if the general safety objectives are to be met and an acceptable design produced.