Preparing for old age3 April 2002
In 1987 Electricité de France commenced a lifetime programme to understand and anticipate ageing problems. The programme regularly reviews everything that could have an impact on the lifetime of nuclear plants, and aims to carry out the measures needed to achieve the expected lifetime. By Bernard Roche
As a result of the deregulation of electricity markets, many nuclear plants have had to improve their competitiveness. Provided that maintenance costs do not become excessive, life extension of existing units provides a practical means of continued electricity generation within a fiercely competitive marketplace. On the other hand, if the lifetime of an existing reactor should be shortened by about ten years, dismantling and renewal would be brought forward which would increase its costs by several billion euro.
For life extension to be successful, utilities must improve their knowledge of ageing mechanisms, demonstrate to safety authorities the feasibility of life extension (especially taking into account critical components) and operate existing units in an exemplary way.
The lifetime programme
Electricité de France's (EdF's) lifetime programme was set up to research and gather data on ageing phenomena in order to understand the effects that operating conditions have on components and their lifetimes.
This programme distinguishes between:
• Two non-replaceable components: the reactor vessel and the containment. A summary file related to the behaviour of 900MWe PWR vessels in service for at least 40 years is currently being reviewed by the French nuclear safety authority. For containments, the lifetime of at least 40 years has been globally accepted for the 900MWe series. Containments for the 1300MWe series must be monitored and solutions must be adapted for each individual case.
• Fully or partially replaceable components, sometimes involving expensive components such as steam generators (already replaced on seven units) or vessel heads (already replaced on 30 units).
Ambitious R&D programmes are being carried out in order to understand degradation mechanisms such as erosion, corrosion, fatigue, wear, thermal ageing, ageing under irradiation, and the dynamics of these mechanisms. Experimental programmes on real equipment have been carried out in order to confirm this work; thus an important programme has been initiated on the Chooz A power plant (300MWe) which was the first PWR built in France and which was shut down in 1991 after 24 years of operation.
Furthermore, operating experience of nuclear power stations located in other countries and older than French units is monitored attentively (cooperation efforts have been organised with the operators concerned). In particular, there are about a hundred PWR nuclear units in the USA with an average age of 10 years more than the average of EdF's units, providing a major source of operating experience about the technical life of equipment. The performances of most of these power stations are continuing to improve, which demonstrates that there are large operating margins in the life of these units. The recent licence renewals granted to Calvert Cliffs, Oconee and Arkansas Nuclear One for 20 additional years (beyond the initial 40-year licence) confirms this idea.
Based on current knowledge, 900MWe and 1300MWe units should be able to achieve their 40 year lifetime objectives, provided that appropriate operating, monitoring and maintenance conditions are adopted.
In this respect, it is worth noting that lifetime objectives have now been announced by other countries such as the USA, Japan, the United Kingdom and Spain. In Japan, leading operators have recently concluded that the life of PWRs could reach 60 years, provided that appropriate maintenance actions are carried out.
The main duty of utilities is to run nuclear plants safely and economically. The prospect of extending life of existing nuclear units remains as long as their competitiveness is proven. It is therefore absolutely necessary to keep track of any changes to the safety reference case. As the safety case evolves, this could lead to unacceptable increases in costs or to impractical backfitting operations (if the modifications affect major technical features of the original design).
In France, as in some other European countries, periodic safety reviews and inspections are performed on a 10-yearly basis. This process, agreed with the safety authorities, consists of three main parts:
• Clarification of the safety reference case (rules, criteria, applicable specifications, and so on).
• Evaluation of the differences between the real plant situation and the current licensing basis (the reference case), and creation of a list of essential modifications (some minor non-compliances with the reference case may not need to be corrected).
• Reassessment of the safety reference case by taking into account feedback of experience and the licensing basis of the most recent units.
This last point gives rise to the backfitting programme, and requires more detailed analyses, both qualitative and quantitative, of the key solutions. Backfitting strategy begins with the determination of whether or not the regulatory provisions are needed to maintain nuclear safety. If not, a backfitting programme must be worked out on the basis of a cost-benefit balance. Both deterministic and probabilistic approaches should be properly combined to achieve a more balanced modification programme. Factors to be considered are the feasibility of implementation, costs, risks, radiation doses, radwaste, and other aspects.
The safety re-evaluation for the two Fessenheim units and the four Bugey units, which are now the oldest operating reactors in France, began in 1987. Significant improvements were made in protection against fire and floods. In general, the safety of these units after the modifications is equivalent to the safety of the other CP1-CP2 900MWe units.
The ongoing safety reassessment of units in the CP1-CP2 series is based on operating history, accident studies and the results of probabilistic safety assessments. Technical modifications approved by the safety authorities will be carried out consistently in the second 10-year inspection for the CP1-CP2 series, the first of which has been completed at Tricastin 1. Furthermore, these inspections provide an opportunity to carry out a complementary actions programme in order to verify assumptions made about ageing effects in areas not inspected within the preventive maintenance programmes. The cost of this project will be about E1.5 billion for the thirty-four 900MWe units. The first of the second 10-year inspections for the 1300MWe series will start in about 2005. Obviously, preparation of the third 10-year inspections for 900MWe units will have an important influence on their lifetime. Therefore, good preparation of this inspection project is essential.
Utilities must also pay attention to the lessons learned from the life extension process that is being carried out in the USA. It seems that risk informed regulations - along with individual PSAs and intensive monitoring programmes - lead to less onerous safety backfitting programmes.
The fact that EdF's nuclear plants are highly standardised makes it necessary to have advance knowledge of major degradation that could affect the main components and to determine the most robust possible long-term renovation/replacement strategies.
"Exceptional maintenance" refers to all maintenance operations programmed nationally on a large number of power stations, usually carried out once during the lifetime of units and which have a significant cost and/or impact on availability. Exceptional maintenance actions (for example, the replacement of steam generators, vessel heads, control rod mechanisms, rewinding of some alternator stators) represent an annual expenditure of about E250-300 million, compared with systematic costs of E1.5 billion per year for routine maintenance.
The "anticipation of an exceptional maintenance" programme consists of identifying exceptional maintenance operations that could be carried out, and making sure that all the appropriate measures are taken to minimise the effect of their implementation on network performances. In particular, it is essential to avoid being obliged to carry out a large number of important operations in the same period of time. This programme periodically reviews design, manufacturing conditions and operating experience of the most sensitive components; identifies major degradations that could occur on these components; evaluates the potential consequences and suggests the most appropriate strategies to achieve a lifetime of at least 40 years.
This first step in a lifetime extension programme is to look at the question of the technical end of design life for various equipment items. The ageing of components depends on the conditions in which they are operated and maintained. Their impact on extension of the plant's lifetime depends on the difficulties raised by their replacement. For each component, its design, manufacture, operational history, maintenance policy as well as R&D issues must be taken into account.
Some components are identified as critical in view of the difficulty or cost of their replacement. These are: the reactor vessel, the main primary large diameter pipes, the steam generators, the primary pumps, the pressuriser, the control rod mechanisms, the vessel internals, the containment, the turbine, the generator, I&C, the electrical cables, and the cooling tower.
On the basis of current knowledge, no equipment problem should prevent nuclear plants from reaching and passing the 40-year mark. Although some vessels and containments will deserve special attention, it appears that, for PWR units operating under appropriate equipment, surveillance and maintenance conditions, a 40-50 year time span is a reasonable target.