The plant operating envelope is agreed between the licensee and the regulator as part of the plant safety case. It is usually defined (Pershagen, 1989) by a set of rules and guidelines to ensure safe operation of the plant but also containing some degree of flexibility to enable the plant to operate in an optimal way. The degree of optimisation or the operating margins that can be achieved must be compliant with these rules and guidelines.

They include technical specifications, Table 4.2, which define bounding values for key safety-related parameters. If exceeded, the plant would need to shut down and the regulator would require a full investigation before operation could restart. There are requirements on the functioning of safety systems and components in order that the conditions of plant operation are met. If not all these requirements are met a reduced mode

Bounding values for the safety parameters and reporting arrangements to safety authorities if limits are exceeded

Allowable conditions for plant operation, including systems availability — how operations must be restricted if such systems functions are not in place

Specification and schedule for testing and inspection of components and systems — restrictions, if testing is not carried out or functionality is impaired

Rules for both normal and abnormal operation — reporting procedures for operational events and design modifications

Pershagen (1989).

of plant operation may be imposed. Conversely, a more optimised mode of operation may require more stringent performance of the systems and components, possibly a need for plant modifications. Similarly, the degree of optimisation that can be achieved may depend on the outcomes of inspection and testing programmes. Finally, any change in operating conditions must meet the rules for both normal and fault conditions.

The operating rules cover all plant states from start-up to shut-down and in all modes of plant operation. These are documented in detail and may be updated in the light of new experience on changes in plant, e. g. modifications.