Life management at Loviisa23 December 1999
Fortum now hopes that the Loviisa plants can meet a 45-year lifetime.
The delivery contract specified a 30-year lifetime for the Loviisa plant, in Finland, but there is conservatism in these figures. Today the predicted economical lifetime for Loviisa is more than 45 years.
Plant owner and operator Fortum employs three decision levels for its plant life management programme: yearly, over the 5-10 year licensing period, and at plant lifetime level.
At the yearly level, consideration is given to preventive maintenance during operation and annual outages, and to the scope and timing of changes in the operating routine.
Plant management emphasises the importance of gentle treatment of all equipment during normal operating states, limiting thermal and hydraulic transient loads and maintaining water chemistry parameters.
Decisions taken over the 5-10 year licensing period are often the most important economic decisions. These include programmes for active and passive components, optimising maintenance, and long-term modification programmes. The current 10-year operating licences of the Loviisa units will expire at the end of 2007. Following favourable cooling water conditions and a comprehensive modernisation programme between 1996-98, the output of the Loviisa units has been increased from 440 MWe to 510 MWe.
Loviisa is developing a systematic programme for plant life management that will combine ageing evaluations with long-term planning. So far, the system in Loviisa is based on predicted lifetimes for the critical components, but annealing of the reactor pressure vessel may offer other strategies.
Plant computers in both units and the plant simulator have been replaced. New systems have been introduced to strengthen old plant designs, including additional emergency feed water systems, improved fire protection, modifications to the emergency cooling systems and operator support systems.
Decision-making on life management takes into account conservative safety analysis, new safety guidance from the regulators, economic factors, resources and public opinion. The most difficult part of the life management programme is timing and targetting resources. Decision-making inside the utility can be supported with tools such as probabilistic safety assessments, but risk-based maintenance or lifetime management have not been seen as useful tools for decision making in the nuclear plant environment.
The lifetime of the latest instrumentation & control systems is shorter than the lifetime of a modern nuclear plant, but complicated licensing processes and regulations prevent the use of next-generation I&C.
It is possible to manage the life of an I&C system by changing similar components from time to time, and a number of pressure transmitters, containment penetrations and some valve actuators with their cables inside the containment have been replaced. Sometimes the entire system must be replaced, whether because some components are no longer available or because there is a new generation of products.
The economic lifetime of the electrical components is not limiting the life of Loviisa and all main components, including the generators, are economically changeable. So far, only the ageing of the electrical cables and the diesel engines need cause concern.
The electrical cables inside the containment are qualified for 40 years of operation, but the ageing potential of silicon cables was detected during qualification tests, and the silicon rubber-insulated cables of safety-related circuits inside the containment were replaced in 1985.
Since then cable ageing has been detected in hot environments. A condition monitoring programme was established in 1998 and modifications have been made to the ventilation systems to improve conditions inside the containment.
Diesel engines have not been considered for replacement, but each has undergone a complete overhaul in an off-site factory. The last was overhauled in 1998.
Loviisa has some 140 valves, classified as loss of coolant accident (LOCA) or main steam line break valves. Between 1988 and 1996 Fortum carried out several projects to qualify the electric actuators as accident-classified. Some of the old valves did not fulfill strict sizing principles, and the valves, or parts of them, were replaced.
A survey of thermally loaded locations and loading mechanisms considered leaking valves and pipe sections with missing insulation, using a new data collection system focusing on locations where temperature stratification is expected. The locations are typically horizontal pipe sections with very low or no flow, or locations close to the T-junctions with a temperature difference. Inspection and analysis will show where thermal stresses caused by stratification layers could exist. A more systematic survey of possible mixing and stratification areas is going on.
Steam generators are critical components for Loviisa. They are difficult to replace, due to their location deep inside the containment, surrounded by thick concrete structures with narrow openings. The steam generators are in a good condition but their carbon steel feedwater manifolds are strongly eroded. They cannot be replaced because they are located deep inside the steam generator between the tube banks. As a remedy, a new austenitic steel manifold construction has been developed, to be placed above the tube banks. The useless old manifold construction will be left in place.
The cover plates of the primary collectors of VVER-440 steam generators are heavily stressed and a generic problem. A renovation project is under way.
The main coolant pumps may also become critical in the long term. They are the only ones of their kind, and their Finnish supplier has lost much of its pump know-how. It would be very difficult to replace them with pumps from another supplier.
A control programme was developed in 1982-1983 to manage erosion-corrosion in the secondary system. Because flow control orifices were not included in the programme a guillotine pipe break of the feed water system occurred in 1990 at unit 1. Despite widening the scope of the control programme and a heavy inspection programme, a second guillotine break occurred at Loviisa 2. Self-assessment revealed shortcomings in the inspection procedures and in interpretation of the results. To tackle the erosion-corrosion several options have been implemented: changing to low alloy or stainless steel piping and materials; exchanging components; and improving inspection techniques. One important feature was changing the secondary water chemistry from neutral to alkaline at Loviisa 2 in 1994 and at Loviisa 1 in 1995.
Control rod drives were not replaced after five years, as recommended by the manufacturer. Instead, there is a programme of periodic inspections and overhaul, which has indicated that some components should be changed periodically. Some new control rod drive systems have been acquired and installed, to permit overhaul of drives during power operation, but so far none of the original drives have been removed from service.
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