Various performance indicators have been defined for measuring the success of a plant in terms of its availability to produce energy safely and economically. The maintenance of good availability depends primarily on the following (IAEA-TECDOC-1098, 1999): control of outage activities, reduction of unplanned outage, reduction of plant transients, improvement of thermal efficiency, good housekeeping of the facilities, minimising plant ageing and optimising staff utilisation.

Different agencies have put forward different performance indicators (IAEA-TECDOC — 1098, 1999; WANO, 2002) but they have much in common. Important indicators include, e. g. the energy availability factor (EAF) and the unit capacity factor (UCF) or cumulative EAF. These are included among the IAEA and its power reactor information system (PRIS) measures and WANO measures. The EAF is defined to be the ratio of

the actual energy generation (net) in a given period, as a percentage of the maximum energy that could have been produced by continuous operation. Figure 4.1 shows how the worldwide UCF (WANO) has steadily improved over the last decade. Precise definitions of the WANO measures are given in Table 3.1.

Unavailability factors are also considered by both agencies and others. The unavailability factor is usually broken down into planned (PUF) and unplanned energy unavailability factors (UUF) (IAEA-TECDOC-1098, 1999). In IAEA-TECDOC-1098 (1999), energy losses are considered to be planned if scheduled 4 weeks in advance. Planned energy losses include planned outages for refuelling, maintenance, testing, etc. under management control. Unplanned outages include not only unplanned outages requiring similar activities, but also for losses beyond the control of management. Figure 4.2 shows how the unplanned capability loss factor of WANO has steadily reduced over the last decade. Figure 4.3 shows a similar reduction in unplanned automatic scrams.

These improvements are attributed to improvement in plant maintenance management and through taking advantage of the benefit of experience.

Across the range of reactor types, the PWR, BWR and AGR units have kept a broadly constant level of performance in recent years. For VVER & RBMK units, there was a decrease in energy availability in the early 1990s, but there has been recovery more recently. The initial decrease was due to the implementation of back-fitting programmes and increase in other maintenance activities during this period.