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Nuclear power plant performance and mechanical design — French experience
Nuclear Engineering and Design 92 (1986) 323-328 North-Holland, Amsterdam
323
NUCLEAR POWER PLANT PERFORMANCE AND MECHANICAL DESIGN FRENCH EXPERIENCE * Jacques L E C L E R C Q Senior Vice-President and Group Executive, Nuclear and Fossil Generation, Electricit~ de France, 3 Rue de Messine, 75008 Paris, France
Received August 1985
The fact that users of nuclear boilers of proved types are no longer troubled by the mechanical structure behaviour of reactors is due to the sustained efforts of construction engineers during the past ten or twenty years to perfect, and incorporate in their equipment studies, the latest developments which have emerged from the expert engineering community of the world, which you represent here today. This is another way of saying how priveleged I feel to be able to give you some observations from the viewpoint of an operator in charge of the French nuclear facilities. My presentation is divided into five parts. Firstly, two parts will be devoted to showing how and why the past twenty years have enabled us to achieve our present results which can objectively be regarded as excellent. Thirdly, I would like to explain why we believe that it is reasonable to assume that our power stations will have an effective life of up to 40 years. In the fourth part of my presentation, I shall revert to more immediate preoccupations and tackle the subject of the desirable optimisation of in service inspection programmes in the light of what we have learnt from modern design studies. I will conclude by suggesting some directions in which further progress is still desirable.
1. Two decades of progress in mechanical design Twenty years ago, design rules for pressure vessels were restricted to the application of simple theoretical formulae and to the following of common sense design rules. This was the era of mechanical analysis, based on "design by formula". These methods were economical, and justified by experience within their particular field; they ensured that the equipment would provide a satisfactory safety margin in relation to what we now call primary loading, but did not take sufficient account of secondary stresses. Application of the conventional design rules does permit avoidance of excessive local stress points due to pressure. But, on the other hand, this technique offers little assurance of prolonged resistance to cyclic loading arising from temperature variations during operating transients. Moreover, there are no practical ways in industrial practice to take accurate account of alternat* Invited lecture for the Opening Plenary Session of the 8th International Conference on Structural Mechanics in Reactor Technology(SMiRT-8), Brussels, Belgium,August 19-23, 1985.
ing stresses. In fact, it still happens that conventional fuel burning equipment breaks down more quickly than we would have desired when departing from its base load operation to take an intensive part in load frequency control. The authors of section I I I of the A S M E code (1963) are to be congratulated for having realised the importance of encouraging far-seeing progress in design methods during the nineteen-sixties. By codifying in a practical manner contemporary theories of mechanical damage affecting the principal reactor components, they gave designers the basis for analysing potential structural damage taking account of the cyclic stresses which they might be liable to suffer during their life - a system which we can call "design by analysis". And the use of these new methods should encourage designers and operators to cooperate closely with a view to defining the foreseeable operating policy; that they should both be able to understand the problems involved is an excellent thing in itself because the outcome will be that no final analysis will be made before the plant is commissioned. This analysis is intimately bound up with structural behaviour and, as we will see later, may well be modified as a function of how loads evolve (load
0 0 2 9 - 5 4 9 3 / 8 6 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
324
J. Leclercq / Nuclear power plant performance and mechanical design
follow, remote control) or of the structural damage condition (life extension), or again, in the light of actual knowledge (in service inspection). Operators can only be delighted at this progress, which must result in increased reliability and safety. The assurance that heavy components which are not readily accessible on account of radiation, will withstand the forecast service conditions throughout their useful life and that no unacceptable mechanical damage will emerge, will largely compensate for the higher cost of more sophisticated design analyses. Even if these costs rise considerably, they can still be regarded as relatively modest in relation to expenses represented by extensive repair work under operating conditions on reactor vessels. In this way, engineers have made a major contribution to safety and reliability in nuclear power plant construction. However, they were not the only originators of progress in this field. The engineers undeniably deserve great credit for developments in the area of fracture mechanics, and for providing a better assessment of resistance margins in vessels in relation to brittle fracture caused by fatigue; but we must not forget the metallurgists and welders. They too deserve their share of credit for their contribution to improving resistance since the "heroic days" of heavy boiler-plate workshops in the fifties. Finally, although further advances in the knowledge of the subject are still to be desired, notably in the field where mechanical damage is compounded by corrosion, progress generally has been nonetheless impressive. In addition, it must be remembered that at the same time the dramatic development in computer facilities has resulted in spectacular increase in the practical means of making calculations. Without these modern tools, much of our theoretical knowledge could not be fully exploited, for example, in the field of seismic studies. We must add that in all industrial countries which have developed nuclear programmes, public authorities responsible for safety have made it compulsory to apply new design methods based on the analysis of resistance to damage of the principal reactor components and equipment. In France, the basic regulations are embodied in a ministerial Order dated 26 February 1974 which has been applied to all installations since Fessenheim. But it would still be desirable to supplement these basic regulations by a more detailed codifications system. For, as our experience grows wider and is confirmed by a mastery of industrial processes involved in construction of nuclear power stations, we have gradually evolved design and construction codes. Although our RCCM 1980 (Design and Construction
Rules J?>r Mechanical Components qf PWR nuclear islands) is fairly close to the original American model on which it is based within the field of engineering design, it nevertheless reflects our own experience in the application of this model, and certain more European preferences for materials, construction technology and nondestructive tests which we have adopted. The interest and constant support received from our administrative authorities in our efforts to rationalise rules and construction methods has been very helpful to us in our successful completion of our organisational tasks.
2. Improvements in mechanical design have contributed significantly to outstanding performance of French nuclear plants The mechanical behaviour of equipment in operation, which has benefited from design progress already mentioned, is generally remarkable and it can therefore be said that mechanical analysis is one of the factors which have contributed to improvement in performance. I would like to quote a few figures to illustrate this point, taking the French PWR nuclear power stations, which I naturally know best, as an example: As at 1st August 1985, 33 units had been connected to the grid since the commissioning of Fessenheim 1, at the beginning of 1977, including 31 of 900 MW and 2 of 1300 MW. 4 new units, including 3 of 1300 MW installations, are being started and to be connected to the grid during the second half of 1985. The availability factor see fig. 1 for the plants in commercial operation was 83% in 1984 with an un-
17%
,~AILABILITY ~
,__
f UNSCHEDULED
~ 83%
AVAILABILITY FACTOR
-
5%
SCHEDULED ~
Fig. 1. 1984 900 MW PWR availability.
12%
J. Leclercq / Nuclear power plant performance and mechanical design
325
1984 1974 OTHER
NUCLEAR
OTHER OIL
180 TWh
310 TWh
Fig. 2. French domestic electrical output.
scheduled outage rate of 5~. During the very cold weather which occurred last winter, availability was 90%. It should be confirmed in 1985 because our running average over the last twelve months ending in July 1985 was approximately 83%, i.e. the level observed in 1984. In 1984, the contribution of nuclear power to French domestic electrical output was 59% and the part played by nuclear installations during the past ten years is shown in fig. 2. It should reach 75% in 1990. This dominant nuclear contribution to the French electric system compels the PWR units to participate extensively to load, follow under entirely satisfactory techni-
cal and economical conditions, because it must be emphasised that the qualities of flexibility of these PWR units equipped with the new "grey" pilot mode are better than those provided by conventional fuel-burning units (variation of power of 5% of the rated capacity per minute). This situation of nuclear load modulation, a daily example of which can be seen in fig. 3, obviously means that all the available energy is at certain times not produced, but the cost of a base load nuclear k w h is still very much lower than if produced by coal, which would only be comparable over an annual load duration of 2 to 3000 h. This operating flexibility and economy represents
MWe
900
4S0, : : " : : : : : : ' ~ : : ' ~ ~ : ~ . ' . : : . : i . . . ]
Apeil13th,83
Fig. 3. Typical daily load cycling program.
~:.'.".."::::
Ap¢il141h,83
J
AprillSIh, 83
326
J. Leclercq / Nuclear power plant performance and mechanical destgn
the first fruits of our efforts undertaken in collaboration with the principal constructor and the Atomic Energy Commission in the field of fuel and monitoring mechanisms. But it would not have been possible to make this claim without mechanical analysis demonstration to prove that the principal boiler components would be able to withstand the stress represented by the many additional transients involved in this new mode of operation.
3. For the future, progress which will make it possible to give power stations an effective life of 40 years Our light water nuclear power stations have thus attained an operating efficiency and safety and reliability level of very high order. The equipment is still relatively untried, being barely 9 years old in the USA, 8 years old in Germany, 7 years old in Japan and 4 years old in France. The question arises as to how long it can be operated without encountering a major problem and how we should prepare for the future. A major economic factor is involved in view of the high investment concerned, and the difficulty in some countries of undertaking new projects. As far as mechanical damage resistance is concerned, we think firstly that the main structural components of our nuclear boilers are unlikely to cause any serious trouble within the 40 years of their design period, provided we keep a close and regular watch on the actual transients which they will be required to withstand. In fact, analyses made at design state which we have mentioned above, do not represent a valid guarantee of efficient performance unless it is certain that actual operation will match forecast operation. It is this which we refer to as "transient monitoring and book-keeping" and we have adopted this system for all our units since they became operational. In the initial stages, we conducted this book-keeping manually, making regular comparisons of actual pressure and temperature changes as recorded, with the design postulated curves. Then, by gradual stages, we automated this monitoring process. The experience was equally rewarding for us and for the designer. It enabled us to see that some parts of the circuit were obliged to withstand thermal transients which were more numerous and severe than anticipated. We were therefore able to prepare for the future by taking minor corrective action including slight changes in certain operating procedures and adjustments. We therefore followed from the outset the recommendations now advanced by the "life extension" experts; and
in fact we believe that our efforts in this direction are of primary importance for a concrete interpretation of the "equipment is designed to last for 40 years" formula, The resistance of PWR reactor vessels to core-fuel radiation has recently raised delicate problems and the chemical composition of certain selected materials has not always been very satisfactory from this viewpoint In France, we were fortunate in the sense that our metallurgists had for some time imposed very strict limitations on the copper content of steels for forgeability and welding reasons. As the final transition temperature of our vessels RTND r (Reference Transient Nil Ductility Temperature see fig. 4) - would hardly ever exceed an average of 50°C which was confirmed by the initial results of the surveillance programmes, we believe that we have nothing to fear from this aspect: no practical element to shorten the effective life period of at least 40 years, and no serious operating restrictions. But nevertheless, some of the mechanical components +- the leakproof gaskets, pump or valve packings and bolted flanges - may well on a long-term basis, require more intensive maintenance than we might wish, but here we are concerned with parts which can always be readily replaced without involving unacceptable outage periods, provided that the maintenance is correctly scheduled. We attach a great deal of importance to maintenance programming, and we benefit in this respect and in other ways by the uniform structure of our nulear facilities, which gives us a natural advantage. Again looking at maintenance expenses, the greatest potential problem likely to arise on a more or less long-term basis, is the behaviour of tube bundles in the steam generators +. These thin-walled structures are required to withstand various types of wear and corrosion which can only be prevented by applying all kinds of
RTNDT °C
50
I
20 f t
I
-25
2 I
I 30
I I 40 YEARS OF SERVICE
FROM EXPERIENCE
Fig. 4. Reference Transient Nil Ductility Temperature RTND+1 for a typical French reactor vessel.
J. Leclercq / Nuclear power plant performance and mechanical design measures involving selection of materials, fabrication, stress-relief treatments and the chemistry of water; these measures are often incompatible with each other, and difficult compromises must be reached before suitable designs can be worked out. Nevertheless, there is no assurance that all the steam generators will have a 40-year life, even under ideal maintenance conditions, and it is probable that we will be obliged to replace some of these units. But we have already learnt several times from experience that replacement of primary parts of these units can be programmed and implemented on an industrial basis without unduly sacrificing neither availability nor economic balance of the power stations. Finally, our experience and reflection have led us to conclude that basically, the heaviest structures of our recent nuclear boilers, which are the most difficult to repair or replace, should be capable of operating over very long periods. We will therefore be able to use them as long as they remain capable of performing economically, without having to scrap them prematurely, as we have al been obliged to do occasionally in the past in the case of certain prototypes. This seems to me to be a valuable asset.
4. The desire to optimise in service inspection on a short-term basis
In service inspection (I.S.I.) weighs very heavily on maintenance programmes in nuclear power stations. In the EDF, for example, we have already devoted much time and study to perfecting a system whereby the longest shut-down period, long enough for us to make the prescribed full inspection of the nuclear boiler, shall never exceed 70 days on average. By way of comparison, normal annual refueling shut-down periods need never take more than half this time (our best result being 26 days) and this period allows time for a fairly detailed non-destructive inspection programme (see fig. 5). As our I.S.I. obligations when operating require three long shut-down periods during the first 12 years of operation of a power station, the importance of this point can be readily appreciated. Thus, for 1984, the impact on availability which only takes account of initial full inspections, i.e., those made after one year of operation, is of the order of 2% (1% for all nuclear power stations at present costs about 200 million francs). There is no doubt that well-planned I.S.I. programmes have made an important contribution to safety and even to availability. But the development and perfecting of ideal programmes including all essential points
Unmmilability %
327
/
152
10.
p.
Fig. 5. 900 MW PWRs' scheduled outages. and only the essential points, requires much experience, thought and discrimination. We think that today we are still a long way from this ideal situation. In fact, we were too near to the starting period when we prepared our present programmes ten years ago. Our working procedures at that time were an extension of former prescribed traditions, dating from a time when design studies, non-destructive inspection means and in service observation were very much less sophisticated then they are today. We recall, for example, the time when a high pressure hydraulic test was an essential, if not the only way of obtaining a minimum guarantee of resistance for an operating unit. To act for the best while still retaining these former principles, we have increased the frequency and extent of our non-destructive inspections, at the same time improving methods which now include ultra-sonic techniques, eddy currents, acoustic methods, etc. We have applied the new techniques as they emerged, barely untried, from the research laboratories, and without really knowing how effective they were. We did as much as we possibly could and sometimes went even a little further "as far as was practicable". We now feel that the time has come to use our knowledge of the behaviour of modern nuclear boilers, of their few weak points and their many reliable components to optimise our programmes of in service inspection. It is not unreasonable to assume that we can streamline these programmes and make them more efficient at the same time.
J. Leclercq / Nuclear power plant performance and mechanical design
328
This implies that we will be able to take full advantage of all data available today: theoretical analysis reports, production inspection summaries, experimental programme results and examinations of equipment made during full inspections - and up to now we have made over 60 full examinations of primary circuits. We should profit from our experience of the in service behaviour of all this equipment and combine our experience with that of our colleagues throughout the world: in a word - make the best/use of all the knowledge we have acquired during the past ten years; so far, we have profited only from the more obvious factors. It will cost use a great deal of effort to reach this target of rationalising our maintenance and in service inspection programmes. We shall require well-documented evidence to convince the safety authorities. But as I have just emphasised - so much is at stake that the task deserves fullest cooperation from all power station operators.
5. Some
suggestions
for
Unavailabi % d 40.
~duled cheduled
30.
20_
10_
Fig. 6. 900 MW PWR unit outages.
progress
There is no doubt that the engineering of modern nuclear power stations deserves full credit for their availability, economy and safety. As we have seen, much valuable work has been achieved in the past 20 years. But there is still room for the thought that the "design by analysis" concept can be used to determine the mechanical behaviour of a structure but is no cure for a second-rate initial design or ill-advised selection of materials - both essential factors for the effective life of the installation. There is always room for improvement and progress advances continually. Without any claim for completeness, we venture to put forward the following few suggestions, prompted by the thoughts expressed above, from among many others which could be made: - that the basic improvements in engineering design methods which have resulted in increased safety and reliability (see fig. 6) should also be extended, at the price of necessary simplification, to the more conventional items which are the focus of accidental failures. that there should be an improved definition of the precision and sensitivity targets for non-destructive examination, the performance of which has grown faster than technological production possibilities. that codifiable knowledge will advance in fields where the effects of corrosion interfere with those set up by
purely mechanical stresses, because there is rarely an example of a nuclear structure having been scrapped under the effect of excessive mechanical damage alone. I have devoted most of this presentation to Light Water Reactors which are in the forefront of our resources for today and tomorrow. However, I would not like to end without making special mention of Fast Breeder Reactors which represent for the distant future a major element in world strategy for mastery of energy resources. The core fueling of the Creys-Malville power plant started on Saturday, July 20th; fueling is due to be completed at the end of August; criticality will occur during the first half of September and the power station will be connected to the grid at the beginning of 1986. This represents an important development stage for this reactor type. Another essential element in FBR industrialisation must be availability of design rules for these components and equipment at high temperatures. We have made an intensive study of this point over the past few years in order to integrate the important results of engineering developments during the past decade into industrial codes. From this viewpoint alone the ground has been prepared for a future where nuclear-generated electricity will be firmly established as primary among the principal power sources of the world.
323
NUCLEAR POWER PLANT PERFORMANCE AND MECHANICAL DESIGN FRENCH EXPERIENCE * Jacques L E C L E R C Q Senior Vice-President and Group Executive, Nuclear and Fossil Generation, Electricit~ de France, 3 Rue de Messine, 75008 Paris, France
Received August 1985
The fact that users of nuclear boilers of proved types are no longer troubled by the mechanical structure behaviour of reactors is due to the sustained efforts of construction engineers during the past ten or twenty years to perfect, and incorporate in their equipment studies, the latest developments which have emerged from the expert engineering community of the world, which you represent here today. This is another way of saying how priveleged I feel to be able to give you some observations from the viewpoint of an operator in charge of the French nuclear facilities. My presentation is divided into five parts. Firstly, two parts will be devoted to showing how and why the past twenty years have enabled us to achieve our present results which can objectively be regarded as excellent. Thirdly, I would like to explain why we believe that it is reasonable to assume that our power stations will have an effective life of up to 40 years. In the fourth part of my presentation, I shall revert to more immediate preoccupations and tackle the subject of the desirable optimisation of in service inspection programmes in the light of what we have learnt from modern design studies. I will conclude by suggesting some directions in which further progress is still desirable.
1. Two decades of progress in mechanical design Twenty years ago, design rules for pressure vessels were restricted to the application of simple theoretical formulae and to the following of common sense design rules. This was the era of mechanical analysis, based on "design by formula". These methods were economical, and justified by experience within their particular field; they ensured that the equipment would provide a satisfactory safety margin in relation to what we now call primary loading, but did not take sufficient account of secondary stresses. Application of the conventional design rules does permit avoidance of excessive local stress points due to pressure. But, on the other hand, this technique offers little assurance of prolonged resistance to cyclic loading arising from temperature variations during operating transients. Moreover, there are no practical ways in industrial practice to take accurate account of alternat* Invited lecture for the Opening Plenary Session of the 8th International Conference on Structural Mechanics in Reactor Technology(SMiRT-8), Brussels, Belgium,August 19-23, 1985.
ing stresses. In fact, it still happens that conventional fuel burning equipment breaks down more quickly than we would have desired when departing from its base load operation to take an intensive part in load frequency control. The authors of section I I I of the A S M E code (1963) are to be congratulated for having realised the importance of encouraging far-seeing progress in design methods during the nineteen-sixties. By codifying in a practical manner contemporary theories of mechanical damage affecting the principal reactor components, they gave designers the basis for analysing potential structural damage taking account of the cyclic stresses which they might be liable to suffer during their life - a system which we can call "design by analysis". And the use of these new methods should encourage designers and operators to cooperate closely with a view to defining the foreseeable operating policy; that they should both be able to understand the problems involved is an excellent thing in itself because the outcome will be that no final analysis will be made before the plant is commissioned. This analysis is intimately bound up with structural behaviour and, as we will see later, may well be modified as a function of how loads evolve (load
0 0 2 9 - 5 4 9 3 / 8 6 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
324
J. Leclercq / Nuclear power plant performance and mechanical design
follow, remote control) or of the structural damage condition (life extension), or again, in the light of actual knowledge (in service inspection). Operators can only be delighted at this progress, which must result in increased reliability and safety. The assurance that heavy components which are not readily accessible on account of radiation, will withstand the forecast service conditions throughout their useful life and that no unacceptable mechanical damage will emerge, will largely compensate for the higher cost of more sophisticated design analyses. Even if these costs rise considerably, they can still be regarded as relatively modest in relation to expenses represented by extensive repair work under operating conditions on reactor vessels. In this way, engineers have made a major contribution to safety and reliability in nuclear power plant construction. However, they were not the only originators of progress in this field. The engineers undeniably deserve great credit for developments in the area of fracture mechanics, and for providing a better assessment of resistance margins in vessels in relation to brittle fracture caused by fatigue; but we must not forget the metallurgists and welders. They too deserve their share of credit for their contribution to improving resistance since the "heroic days" of heavy boiler-plate workshops in the fifties. Finally, although further advances in the knowledge of the subject are still to be desired, notably in the field where mechanical damage is compounded by corrosion, progress generally has been nonetheless impressive. In addition, it must be remembered that at the same time the dramatic development in computer facilities has resulted in spectacular increase in the practical means of making calculations. Without these modern tools, much of our theoretical knowledge could not be fully exploited, for example, in the field of seismic studies. We must add that in all industrial countries which have developed nuclear programmes, public authorities responsible for safety have made it compulsory to apply new design methods based on the analysis of resistance to damage of the principal reactor components and equipment. In France, the basic regulations are embodied in a ministerial Order dated 26 February 1974 which has been applied to all installations since Fessenheim. But it would still be desirable to supplement these basic regulations by a more detailed codifications system. For, as our experience grows wider and is confirmed by a mastery of industrial processes involved in construction of nuclear power stations, we have gradually evolved design and construction codes. Although our RCCM 1980 (Design and Construction
Rules J?>r Mechanical Components qf PWR nuclear islands) is fairly close to the original American model on which it is based within the field of engineering design, it nevertheless reflects our own experience in the application of this model, and certain more European preferences for materials, construction technology and nondestructive tests which we have adopted. The interest and constant support received from our administrative authorities in our efforts to rationalise rules and construction methods has been very helpful to us in our successful completion of our organisational tasks.
2. Improvements in mechanical design have contributed significantly to outstanding performance of French nuclear plants The mechanical behaviour of equipment in operation, which has benefited from design progress already mentioned, is generally remarkable and it can therefore be said that mechanical analysis is one of the factors which have contributed to improvement in performance. I would like to quote a few figures to illustrate this point, taking the French PWR nuclear power stations, which I naturally know best, as an example: As at 1st August 1985, 33 units had been connected to the grid since the commissioning of Fessenheim 1, at the beginning of 1977, including 31 of 900 MW and 2 of 1300 MW. 4 new units, including 3 of 1300 MW installations, are being started and to be connected to the grid during the second half of 1985. The availability factor see fig. 1 for the plants in commercial operation was 83% in 1984 with an un-
17%
,~AILABILITY ~
,__
f UNSCHEDULED
~ 83%
AVAILABILITY FACTOR
-
5%
SCHEDULED ~
Fig. 1. 1984 900 MW PWR availability.
12%
J. Leclercq / Nuclear power plant performance and mechanical design
325
1984 1974 OTHER
NUCLEAR
OTHER OIL
180 TWh
310 TWh
Fig. 2. French domestic electrical output.
scheduled outage rate of 5~. During the very cold weather which occurred last winter, availability was 90%. It should be confirmed in 1985 because our running average over the last twelve months ending in July 1985 was approximately 83%, i.e. the level observed in 1984. In 1984, the contribution of nuclear power to French domestic electrical output was 59% and the part played by nuclear installations during the past ten years is shown in fig. 2. It should reach 75% in 1990. This dominant nuclear contribution to the French electric system compels the PWR units to participate extensively to load, follow under entirely satisfactory techni-
cal and economical conditions, because it must be emphasised that the qualities of flexibility of these PWR units equipped with the new "grey" pilot mode are better than those provided by conventional fuel-burning units (variation of power of 5% of the rated capacity per minute). This situation of nuclear load modulation, a daily example of which can be seen in fig. 3, obviously means that all the available energy is at certain times not produced, but the cost of a base load nuclear k w h is still very much lower than if produced by coal, which would only be comparable over an annual load duration of 2 to 3000 h. This operating flexibility and economy represents
MWe
900
4S0, : : " : : : : : : ' ~ : : ' ~ ~ : ~ . ' . : : . : i . . . ]
Apeil13th,83
Fig. 3. Typical daily load cycling program.
~:.'.".."::::
Ap¢il141h,83
J
AprillSIh, 83
326
J. Leclercq / Nuclear power plant performance and mechanical destgn
the first fruits of our efforts undertaken in collaboration with the principal constructor and the Atomic Energy Commission in the field of fuel and monitoring mechanisms. But it would not have been possible to make this claim without mechanical analysis demonstration to prove that the principal boiler components would be able to withstand the stress represented by the many additional transients involved in this new mode of operation.
3. For the future, progress which will make it possible to give power stations an effective life of 40 years Our light water nuclear power stations have thus attained an operating efficiency and safety and reliability level of very high order. The equipment is still relatively untried, being barely 9 years old in the USA, 8 years old in Germany, 7 years old in Japan and 4 years old in France. The question arises as to how long it can be operated without encountering a major problem and how we should prepare for the future. A major economic factor is involved in view of the high investment concerned, and the difficulty in some countries of undertaking new projects. As far as mechanical damage resistance is concerned, we think firstly that the main structural components of our nuclear boilers are unlikely to cause any serious trouble within the 40 years of their design period, provided we keep a close and regular watch on the actual transients which they will be required to withstand. In fact, analyses made at design state which we have mentioned above, do not represent a valid guarantee of efficient performance unless it is certain that actual operation will match forecast operation. It is this which we refer to as "transient monitoring and book-keeping" and we have adopted this system for all our units since they became operational. In the initial stages, we conducted this book-keeping manually, making regular comparisons of actual pressure and temperature changes as recorded, with the design postulated curves. Then, by gradual stages, we automated this monitoring process. The experience was equally rewarding for us and for the designer. It enabled us to see that some parts of the circuit were obliged to withstand thermal transients which were more numerous and severe than anticipated. We were therefore able to prepare for the future by taking minor corrective action including slight changes in certain operating procedures and adjustments. We therefore followed from the outset the recommendations now advanced by the "life extension" experts; and
in fact we believe that our efforts in this direction are of primary importance for a concrete interpretation of the "equipment is designed to last for 40 years" formula, The resistance of PWR reactor vessels to core-fuel radiation has recently raised delicate problems and the chemical composition of certain selected materials has not always been very satisfactory from this viewpoint In France, we were fortunate in the sense that our metallurgists had for some time imposed very strict limitations on the copper content of steels for forgeability and welding reasons. As the final transition temperature of our vessels RTND r (Reference Transient Nil Ductility Temperature see fig. 4) - would hardly ever exceed an average of 50°C which was confirmed by the initial results of the surveillance programmes, we believe that we have nothing to fear from this aspect: no practical element to shorten the effective life period of at least 40 years, and no serious operating restrictions. But nevertheless, some of the mechanical components +- the leakproof gaskets, pump or valve packings and bolted flanges - may well on a long-term basis, require more intensive maintenance than we might wish, but here we are concerned with parts which can always be readily replaced without involving unacceptable outage periods, provided that the maintenance is correctly scheduled. We attach a great deal of importance to maintenance programming, and we benefit in this respect and in other ways by the uniform structure of our nulear facilities, which gives us a natural advantage. Again looking at maintenance expenses, the greatest potential problem likely to arise on a more or less long-term basis, is the behaviour of tube bundles in the steam generators +. These thin-walled structures are required to withstand various types of wear and corrosion which can only be prevented by applying all kinds of
RTNDT °C
50
I
20 f t
I
-25
2 I
I 30
I I 40 YEARS OF SERVICE
FROM EXPERIENCE
Fig. 4. Reference Transient Nil Ductility Temperature RTND+1 for a typical French reactor vessel.
J. Leclercq / Nuclear power plant performance and mechanical design measures involving selection of materials, fabrication, stress-relief treatments and the chemistry of water; these measures are often incompatible with each other, and difficult compromises must be reached before suitable designs can be worked out. Nevertheless, there is no assurance that all the steam generators will have a 40-year life, even under ideal maintenance conditions, and it is probable that we will be obliged to replace some of these units. But we have already learnt several times from experience that replacement of primary parts of these units can be programmed and implemented on an industrial basis without unduly sacrificing neither availability nor economic balance of the power stations. Finally, our experience and reflection have led us to conclude that basically, the heaviest structures of our recent nuclear boilers, which are the most difficult to repair or replace, should be capable of operating over very long periods. We will therefore be able to use them as long as they remain capable of performing economically, without having to scrap them prematurely, as we have al been obliged to do occasionally in the past in the case of certain prototypes. This seems to me to be a valuable asset.
4. The desire to optimise in service inspection on a short-term basis
In service inspection (I.S.I.) weighs very heavily on maintenance programmes in nuclear power stations. In the EDF, for example, we have already devoted much time and study to perfecting a system whereby the longest shut-down period, long enough for us to make the prescribed full inspection of the nuclear boiler, shall never exceed 70 days on average. By way of comparison, normal annual refueling shut-down periods need never take more than half this time (our best result being 26 days) and this period allows time for a fairly detailed non-destructive inspection programme (see fig. 5). As our I.S.I. obligations when operating require three long shut-down periods during the first 12 years of operation of a power station, the importance of this point can be readily appreciated. Thus, for 1984, the impact on availability which only takes account of initial full inspections, i.e., those made after one year of operation, is of the order of 2% (1% for all nuclear power stations at present costs about 200 million francs). There is no doubt that well-planned I.S.I. programmes have made an important contribution to safety and even to availability. But the development and perfecting of ideal programmes including all essential points
Unmmilability %
327
/
152
10.
p.
Fig. 5. 900 MW PWRs' scheduled outages. and only the essential points, requires much experience, thought and discrimination. We think that today we are still a long way from this ideal situation. In fact, we were too near to the starting period when we prepared our present programmes ten years ago. Our working procedures at that time were an extension of former prescribed traditions, dating from a time when design studies, non-destructive inspection means and in service observation were very much less sophisticated then they are today. We recall, for example, the time when a high pressure hydraulic test was an essential, if not the only way of obtaining a minimum guarantee of resistance for an operating unit. To act for the best while still retaining these former principles, we have increased the frequency and extent of our non-destructive inspections, at the same time improving methods which now include ultra-sonic techniques, eddy currents, acoustic methods, etc. We have applied the new techniques as they emerged, barely untried, from the research laboratories, and without really knowing how effective they were. We did as much as we possibly could and sometimes went even a little further "as far as was practicable". We now feel that the time has come to use our knowledge of the behaviour of modern nuclear boilers, of their few weak points and their many reliable components to optimise our programmes of in service inspection. It is not unreasonable to assume that we can streamline these programmes and make them more efficient at the same time.
J. Leclercq / Nuclear power plant performance and mechanical design
328
This implies that we will be able to take full advantage of all data available today: theoretical analysis reports, production inspection summaries, experimental programme results and examinations of equipment made during full inspections - and up to now we have made over 60 full examinations of primary circuits. We should profit from our experience of the in service behaviour of all this equipment and combine our experience with that of our colleagues throughout the world: in a word - make the best/use of all the knowledge we have acquired during the past ten years; so far, we have profited only from the more obvious factors. It will cost use a great deal of effort to reach this target of rationalising our maintenance and in service inspection programmes. We shall require well-documented evidence to convince the safety authorities. But as I have just emphasised - so much is at stake that the task deserves fullest cooperation from all power station operators.
5. Some
suggestions
for
Unavailabi % d 40.
~duled cheduled
30.
20_
10_
Fig. 6. 900 MW PWR unit outages.
progress
There is no doubt that the engineering of modern nuclear power stations deserves full credit for their availability, economy and safety. As we have seen, much valuable work has been achieved in the past 20 years. But there is still room for the thought that the "design by analysis" concept can be used to determine the mechanical behaviour of a structure but is no cure for a second-rate initial design or ill-advised selection of materials - both essential factors for the effective life of the installation. There is always room for improvement and progress advances continually. Without any claim for completeness, we venture to put forward the following few suggestions, prompted by the thoughts expressed above, from among many others which could be made: - that the basic improvements in engineering design methods which have resulted in increased safety and reliability (see fig. 6) should also be extended, at the price of necessary simplification, to the more conventional items which are the focus of accidental failures. that there should be an improved definition of the precision and sensitivity targets for non-destructive examination, the performance of which has grown faster than technological production possibilities. that codifiable knowledge will advance in fields where the effects of corrosion interfere with those set up by
purely mechanical stresses, because there is rarely an example of a nuclear structure having been scrapped under the effect of excessive mechanical damage alone. I have devoted most of this presentation to Light Water Reactors which are in the forefront of our resources for today and tomorrow. However, I would not like to end without making special mention of Fast Breeder Reactors which represent for the distant future a major element in world strategy for mastery of energy resources. The core fueling of the Creys-Malville power plant started on Saturday, July 20th; fueling is due to be completed at the end of August; criticality will occur during the first half of September and the power station will be connected to the grid at the beginning of 1986. This represents an important development stage for this reactor type. Another essential element in FBR industrialisation must be availability of design rules for these components and equipment at high temperatures. We have made an intensive study of this point over the past few years in order to integrate the important results of engineering developments during the past decade into industrial codes. From this viewpoint alone the ground has been prepared for a future where nuclear-generated electricity will be firmly established as primary among the principal power sources of the world.