Reduction in outage time has been achieved in a number of plants that have been operating for some years through both technical and administrative improvements. Another factor has been the introduction of more computerised systems to aid in the planning and managing of outages. Technologies for improving light water operation and maintenance,

including outages, have been published in IAEA-TECDOC-1175 (2000). Figure 4.4 shows the global reduction in average planned outage time that has been achieved over the last decade.

The fuelling scheme clearly has an important influence on outage planning. For example, there is a general tendency towards longer fuel cycles. There are obvious requirements on the load design, the most important of which is safe operation, e. g. limits on shutdown margin, and ensuring negative moderator temperature reactivity coefficient. Once safety constraints have been satisfied, outages may need to be synchronised in order that there is a period of time between outages of different units at the same plant. Clearly outages should be avoided at times of peak demand, e. g. during winter periods, etc. Finally, outages should be planned in order to optimise fuel economy and cost.

Outages may be of different duration depending on the work planned. In-service rules may require complete in-service of the reactor vessel including complete unloading of the fuel, the removal of reactor internals, inspection of the reactor vessel at regular intervals, e. g. every 4 years. Shorter outages would be used to carry out a less ambitious programme. By way of example, in the Paks plant in Hungary, the new outage strategy includes outages of short (25-30 days), normal (30-35) and long (55-60 days) duration.

Another example (IAEA-TECDOC-1175, 2000) where a different technical approach has resulted in improvements of outage time concerns the Indian Point 2 reactor in the US. Here two safety systems modifications have been implemented, replacement of conventional hydrogen ignition systems with passive auto-catalytic hydrogen recombiners and the replacement of the conventional containment spray additive tank with baskets of tri-sodium phosphate. The original systems consisted of several hundred components which required significant maintenance and testing, the new passive systems require much less.

For the future, ways to reduce outage are being considered at the design stage, e. g. in the advanced European pressurised reactor (EPR) design. These include features such as improved accessibility of the reactor building during operation, and better logistics support including the need for special tools and availability of spare parts. Some of these approaches are also being considered for current plant.