Guide to a longer life1 January 2002
The sixth PLIM + PLEX meeting was held in London, UK on 28-30 November, 2001. Conference chairman Myrddin Davies reports on the main issues.
September 11 continues to cast a shadow on the number of attending participants from particular countries at international meetings. Not only was this evident at the recent meetings including, for example, one on In-Service-Inspection and Structural Integrity held at Sevilla in November, but it was also true at this PLIM +PLEX meeting in London. Nevertheless, the programme for PLIM+PLEX 2001 was a full one (39 papers) so the ratio of attendees:number of papers has changed since the last meeting in Madrid, where there were more participants presenting about the same number of papers. For this London meeting there were 72 participants from 18 countries.
The meaning of life
The title of the meeting has changed from that used previously, but the acronym has stayed the same. PLEX now means 'Plant Licence Extension' instead of the now defunct 'Plant Life Extension', and which is more in keeping with the subject content. The subject continues to evolve and the papers at this meeting certainly captured the changing emphasis.
The current definition of PLIM+PLEX is an extension of the convergence of the recent IAEA and OECD-NEA views on the topic. Both organisations stress that meeting safety requirements is of paramount importance - without an operating licence there is no nuclear plant! But, with the recognition of that fundamental requirement to safeguard safety, PLIM+PLEX addresses the 'business aspects' of nuclear plants. The current scope of PLIM+PLEX is shown in the figure below, where it can be seen that 'plant life' encompasses the period from choice of plant through to eventual site clearance. In practical terms it covers the period that financial charges can be made against the plant project until its eventual return to a 'green-field' state.
'Operational life' is to do with the period when money is generated from the sale of electricity for servicing all the plant activities in the 'life cycle'. 'Operational' and 'plant life' have become confused over the years, but the 'plant life' covers that longer period to do with the time that financial charges can be made against the whole project. The 'operational period' is when money is being earned.
Usually, at the stage of 'start up' the economic features (amortisation period, cost - not price - of the electricity, load factors, design life and so on) are assessed from the assumed characteristics of the plant. But after operation - usually at about half-way in the 'design life' - the actual operating life of the plant is assessed using the operating experience and properties of the plant together with the actual cost of the plant, its running costs and the predicted revenue. In essence, the operating life is established and the income from predicted sales can be established. Until this stage, the operating life has not been defined. From this simple approach it can readily be seen that there is an incentive to maximise the operating life of the plant in order to optimise the revenue, to establish possible future charges and to reduce the periods and frequency of shutdowns and outages.
Interestingly, Russia is now attaching high priority to plant life management practice. Minatom has announced a basic conceptual approach which treats the whole plant from design through decommissioning as one continuous process. Except for the preliminary phase, this is similar to the approach outlined in the Figure.
The approach to operational life in the USA is different. While the overall principles are the same as in other nuclear countries, the 'licensed life' of 40 years seems to have been based on legal/economic criteria, but the period of operation - the 'design life' - was coincidentally the same. Since the early 1990s the possibility of renewing the operating licence became available to the plant owner. To continue operating, the plant the owner has had to demonstrate to the NRC that it can operate for 20 years beyond the 40 year licence period. (There is a trend for licencees to re-apply 20 years after the start of operation to give advance knowledge for planning, rather than delaying the licence application to a date just in advance of licence expiry which could leave the licencee with a large uncertainty.) The overall effect of a successful relicensing application is to increase the operational life.
The remaining major current issue to be addressed by nuclear power in the present world is its price competitiveness with fossil fuel electricity. PLIM can play a significant role in bringing about major cost reductions by removing the conservatisms usually implicitly included in the design approach to operational life.
From the commercial viewpoint of determining the cost of electricity the establishment of the duration of 'actual operational life' is crucially important, particularly if the period is significantly longer than that assumed at the outset of operation. Decreases in the number, duration and frequency of plant outages increases the plant availability and increases the efficiency. The time needed to carry out maintenance activities can also lead to shorter outages, increased availability and plant lifetime. Indeed, various feasibility studies and actual replacements show that nearly all components can be replaced. The role of efficient NDE and the use of modern mechanical test procedures can now allow safer and more precise evaluations of structural integrity to be assessed and, in turn, these improvements could also allow improved operating lifetimes. The use of component based inspections reduce down times and effort during outages.
Replacement and monitoring
The most significant reason for the replacement of 'internals' is the risk of unplanned repairs that may impose a plant shutdown of many months, while a planned replacement of the same component would have taken a week.
The Oskarshamn 1 BWR was subjected to a large repair and modernisation programme in 1993-95 when several cracks were found in the internals and other parts. Much effort was expended in optimising the design and manufacture for reducing the weld volume and to provide easy installation /inspection. The internals replacement was carried out in 1998. Åsa Henriksson from Westinghouse Atom described this preliminary work and went on to describe the replacement of the internals in Forsmark 1 and 2, owned by FKA. Replacement was justified by new inspection codes and expected future inspection costs. The internals consisted of a new core shroud and a new upper core support grid for each unit made from forgings of large ingots without structural welds in order to minimise cracking and reduce the amount of inspection. The removal and installation took 7.5 days and 9.5 days in Fosmark 1 and 2 respectively. This outstanding project will obviously save on the frequency and length of outages from these components in the future and increase the economic viability of these units.
The paper by Kim Wallin and Jussi Solin described the emerging use of the "Master Curve approach" which is used to measure fracture toughness directly and will probably replace the empirical overly conservative Charpy toughness approach that has been in use for so many years. This has been a major development. (This achievement has led to Professor Wallin being awarded the ASTM Irwin Gold Medal this autumn.) The advances in mechanical property determinations together with the improvements in non-destructive examination will reduce the constraints on plant life further and lead to an increased operating life.
A nuclear plant has about 5-10,000 valves. With the automated condition based monitoring system which makes use of computer based techniques (as described by Josef Sprehe from Framatome ANP), this leads to a large reduction in outages but a more comprehensive monitoring programme. This approach is coupled to optimisation of valve maintenance programmes. It is not surprising that this approach is making big inroads in its application to the world's nuclear plants.
Several papers commented on the effort and attention required in keeping records. In many cases concerning early plants there were inadequacies in this area, which indicated that the strategy for identifying and classifying records needed attention.
Garry Young from Entergy (who also chaired the second day of the meeting) reported that much progress had been achieved in the USA. Eight units had already received licence renewal, 14 plants have registered licence renewal applications with the NRC and 25 units have announced plans to submit applications for licence renewals in the next three years. This means that 40% of the 103 operating plants are publicly committed to licence renewal. It is predicted that 100 or more of the 103 operating plants will eventually seek licence renewal over the next 10-20 years. The original cost estimate in 1998 for licence renewal was $15million. In 2001 the estimate is $10-12 million due to increased efficiency. From 2002 a further decrease is expected - as much as 30%.
Besides this large increase of the operational lives of these plants, with the potential for corresponding reductions in the cost of electricity there has been the added benefit on the increased capacity becoming available. Without licence renewal more than three dozen plants would reach the end of their operating lives by 2015.
Licence renewal is seen as a "bridge to the future," which gives time to start considering a new build programme.
The other remarkable feature is the recent impact of the developments of nuclear plants on the cost of electricity from different fuels, which are shown in the table below. Once the amortisation life is met then a component of the cost of electricity is removed.
Tom Stokoe from BNFL (who also chaired the third day of the meeting) described the history of plant life management of the UK's Magnox plants. The Magnox stations were the first commercial plants to be constructed in the world and, therefore, were the first to address questions of modern safety standards and back-fitting. Reviews, which became known as Long Term Safety Reviews were initiated to establish a safety case for operation to 30 years. This experience led to the introduction and establishment of the Periodic Safety Reviews carried out at ten yearly intervals.
In 2000, a review of operating life factors and supporting plants was completed. The optimum date for the closure of each plant was decided and this coincided with the dates when the current PSRs expire.
Jean-Pierre Hutin from EdF, said that the current capital investment in plant in France was now about 50% amortised. Amongst the EdF ambitions for the future, the company intended to develop and play a major part in the energy business worldwide, so it is essential to keep the plants in operation as long as possible - to generate wealth. There would be no significant construction programme before 2010/2015. He concluded that, if operated safely and cost effectively, then the EdF units should last 40, 50, 60 years, or even more - presumably with corresponding reductions in the cost of electricity or in preparation for plant replacement programmes.
Reactor pressure vessel irradiation embrittlement was not seen as an issue in limiting plant operational life because the pressure vessel steels had benefited from lessons learnt from earlier programmes in other countries. RTNDT was expected to be less than 100ºC, or even lower with optimised fuel management. Fabrication defects in the key locations are now known because they have carried out 100% inspection of the core region. Fatigue of the reactor coolant system (RCS) is generally less than expected and, if necessary, repairs will be possible. Cast austeno-ferritic elbows in the RCS are carefully monitored for thermal embrittlement and, if necessary, could be replaced. Containment integrity is not an issue for 900MWe plants but inner wall tightness reinforcement may be necessary in some 1300MWe plants. Cables are not an issue and no major refurbishment of I&C is likely to be required before the third ten-year outages. The feasibility of replacing the pressuriser is to be studied, but is not seen as an issue at this time.
In the evaluation of reactor pressure vessel lifetimes in China, the China Guangdong Nuclear Power Corporation has concluded that the operational lifetime can be extended from 40 to 60 years and that, for newly projected RPVs, a 60 year lifetime is achievable.
The next step
After the 5th PLIM+PLEX meeting, the author commented that, in general, there was a convergence of views about the objectives of plant life management. While there is a strong convergence of understanding at an international level, the definition and objectives of PLIM have now become confused on a national level. The aim used to be in the pursuit of plant design life assurance but now, as plants are getting older, the main advantages of PLIM implementation is manifested in the achievement of longer operational life. Advantages to the current approach includes:
• Increased generation of electricity leading to increased revenue and a decrease in the cost of nuclear electricity.
• Increased capacity by avoiding premature shutdown of plant.
• Increased on-line (condition based) monitoring leading to shorter shutdowns and giving early warning of maintenance requirement.
• Better maintenance strategies.
• The identification of key components, their inspection and timely replacement.
• A higher quality of structural integrity assessment because of improved inspection and mechanical test data.
It is anticipated that this trend in the development of the
subject will continue. One foresees that the pressure on a nuclear plant is in the reduction in the price of the electricity it generates. As plants get older, more consideration will also be given to shutdown considerations and decommissioning. The reasons and procedure for closure would seem to be useful for a 'lessons learnt' session at a future meeting. In particular there was a lack of knowledge of the reasons for the Kozloduy closures - which were ascribed to European bank pressures.
We need electricity either to maintain or improve our living standards. Perhaps the environmental advantages of nuclear power will be important in industrial world terms, but the unit cost of generated electricity is a tangible measure on which its advantages are judged.
TablesCost of electricity in the USA in 1999