The concept of defence in time is much less widely accepted. However, the components of defence in time are included in many publications related to operational safety. An excellent description of operational safety prin­ciples and practice is presented in the report Management of Operational Safety in Nuclear Power Plants, INSAG-13 (1999b).

Figure 10.4 illustrates the need for defence in time. The question of needed defence begins in the immediate present. We can presume that, at this time, all plant systems are performing perfectly, in accordance with the requirements of the operating licence and in accordance with the design

Prevention

Process systems

Mitigation

Safety systems

10.3 The defence in depth concept.

Fut

10.4 The need for defence in time.

intent. Now, as the time interval beyond this instant increases, uncertainties will arise with respect to the functionality of components and systems. The future is inherently uncertain. The direct question may be ‘Should we do inspection or maintenance operations of component or system “X” at this time, or can it wait until tomorrow?’ As time passes the overall uncertainty increases regarding the plant’s performance under both normal and poten­tial abnormal operating conditions — sometimes very rapidly. The answer, of course, is careful monitoring of all systems, inspection, and maintenance. These multi-faceted actions together constitute ‘defence in time’.

Obviously, the operating crew must carry the responsibility and authority for this aspect of safety defence. Infrastructure and methodologies for car­rying out these tasks must be established before plant first begins to operate, and must be continued for the whole lifetime of the plant.

An integral part of defence in time is regular examination, throughout the life of the plant, of events in the environment around the plant and to some extent events in the whole world that might reveal important short­comings or unappreciated advantages of the plant for which the operating crew is responsible. Revisions and upgrades may be initiated based on these regular examinations.