Nuclear fuel reliability - assessing competitiveness8 January 1999
Increasing competition throughout the world is forcing nuclear utilities to improve performance. Major efforts to increase fuel burn-up and reduce costs have had considerable impacts in recent years. But unforeseen problems threaten to undermine these improvements. by JOEL GINGOLD
Approaching the new millennium, the spectre of competition haunts every utility in the world and is a particular concern for those who operate nuclear power plants. One nation after another is deregulating its electric power market and the utility industry is in the throes of a revolutionary transformation. Only those generating plants with the lowest costs are guaranteed survival. Thus, the focus of all nuclear utilities has been the reduction of their operating expenses.
Many units are far more competitive than they were just a few years ago, but there is one factor which has not received sufficient attention – nuclear fuel and its impact on the economic performance of the plant.
Great strides have been made in the design of nuclear fuel assemblies and in the art of fuel management. Costs have been reduced through neutronically efficient fuel designs, fuel capable of greater exposure and more sophisticated analytical techniques for fuel and core design.
At the same time, uranium and enrichment prices have remained low and, in some instances, have declined. Today’s fabrication prices for advanced designs are often lower than prices bid several years ago for much less efficient fuel assemblies. This has resulted in a very favourable economic situation for nuclear plant operators.
But the bulk of these savings may be exhausted and future cost reductions could be much more difficult to achieve. In addition, there are serious fuel performance concerns which will have to be resolved to achieve a more efficient fuel cycle.
FOCUS ON THE FRONT END
The markets for nuclear fuel cycle commodities and services may offer opportunities for cost reductions in the contracting for fuel, but they are limited and are unlikely to produce savings in the long term.
The market for uranium concentrates remains volatile. It will undoubtedly have its ups and downs, and there will always be specific opportunities to obtain significant quantities of material at favourable prices. But one cannot count on substantial reductions in the near to mid-term, nor would it be wise to plan on always being able to find and consummate the unique low-priced contract. It is also possible, if not probable, that prices will rise. This is especially a concern if one considers the very significant role of Russia and the other republics of the former Soviet Union in the uranium market.
There may be greater opportunities for savings in the enrichment market, but these, too, may be limited to specific situations. The privatisation of the US Enrichment Corporation and the undoubted desire of its management, now free of government control, to maximise its return, could well impact on enrichment prices. The demise of the Louisiana Energy Services project cannot be considered a positive development for enrichment consumers. Prices have fallen in recent years, but are unlikely to fall much more and may even increase if one or more of the principal suppliers retreat from their quest for increased market share.
The price of fuel fabrication in the United States has probably reached its lower limit and prices have rebounded somewhat over the past few years. In Europe, a modestly declining price trend has been evident, although there is no guarantee that it will continue in the longer term.
Prices on both sides of the Atlantic will be affected by the inevitable consolidation in the fabrication market; the initial stages of such a consolidation are now apparent. The most significant example is the sale of Westinghouse Electric Company by CBS Corporation to Morrison-Knudsen and British Nuclear Fuels. Consolidation can only lead to increased prices as excess capacity is retired. Even if very attractive fabrication prices can be found and contracted for the long term, fabrication represents only a small fraction of total fuel cost and will not have a substantial impact on the total cost of generation.
Consequently while some savings may be achieved in the fuel cycle’s front end one cannot rely on them as part of any defined cost reduction programme.
Having accepted that savings in direct front-end fuel costs are limited at best, and recognising that the back end is fraught with even greater uncertainty, nuclear plant operators have turned to alternative approaches to lowering operating costs.
THE SEARCH FOR SAVINGS
Many utilities continue to reduce their operating and engineering staffs. They are doing their best to cope with the increasingly complex technical and commercial issues raised by the demands they are placing on the fuel with fewer and fewer people.
While their staffs are shrinking, the majority of utilities have considered increasing the energy output of the fuel, and many have already implemented such a strategy. Among the most popular approaches to expanded energy production have been:
• Increased discharge exposure of the fuel assemblies.
• Increased cycle length.
• Uprating the output of the unit.
Or some combination of these.
Economic analysis of these approaches shows that, despite the increased enrichment necessary to achieve greater assembly energy production, the reduction in the size of the reload batch needed to produce the desired core energy results in lower fuel costs in essentially every instance. An added benefit is that the number of spent assemblies discharged each cycle is reduced, lowering spent fuel management charges.
There has always been an assumption that the fuel is capable of coping with the resulting increase in duty. That reliability will not suffer to any significant degree. When analyses are made to justify increased burnup or higher plant output, the potential downside of such actions – the increased likelihood of fuel failure or other performance problems which could adversely impact on plant operation – is often given short shrift. In our attempt to minimise fuel costs, we may be creating problems whose economic impact will far overshadow any savings that may be realised.
A NOTE OF CAUTION
Over recent years, a number of plants have suffered deratings, or even premature termination of an operating cycle, due to unanticipated problems associated with the fuel. Many of these problems can be attributed, at least in part, to increased duty.
They include such concerns as: Excessive corrosion High levels of so-called “shadow corrosion”, in which excessive cladding corrosion appeared under the grids, have affected some high power BWRs in Europe. While the cause has been attributed to a combination of coolant water chemistry and material performance, and has been addressed, the mechanism is not yet fully understood.
Among PWRs, a number of fuel rods were found to be failed in their initial cycle of operation in one US unit. The rods, having operated at relatively high power throughout the cycle, had corroded significantly. While the incident is still under investigation, it appears that a form of localised corrosion was at least a significant contributor to the failures.
Significant secondary degradation of the cladding following a primary failure In a number of BWRs in the US and Europe, an initial fuel rod failure from debris, weld defects etc has resulted in substantial secondary degradation of the cladding up to and including splits of almost the entire length of the rod. To limit this phenomenon, many BWR operators are suppressing the power in the failed assemblies and employing more conservative post-failure operating strategies. Some are shutting down the plant in mid-cycle to repair or replace the failed assemblies. This has increased costs and restricted operating flexibility.
The great majority of the affected fuel rods contained zirconium barriers to protect against pellet-clad interaction related failures, and it is generally accepted that the barrier is implicated in the degradation and splitting of the cladding. The compositions of some claddings and barriers have been modified, but there is not yet enough operating experience to declare the problem finally solved. This has led some BWR operators to consider purchasing fuel without a barrier and accepting the resultant manoeuvring restrictions and loss of capacity factor rather than run the risk of a severely degraded fuel rod.
Incomplete rod insertion (IRI) In a number of US and European PWRs, control rods failed to fully insert upon shutdown. The cause was distortion of the guide tubes due, principally, to high compressive loads on the assembly during operation. Because of its potential safety implications, this problem attracted the attention of several national regulatory authorities. In Belgium, utilities were required to perform rod drop tests at various times during the cycle, sometimes involving shutdown of the unit for mid-cycle tests.
Fuel failure due to grid-rod fretting PWRs in many nations have experienced grid-rod fretting failures. While the causes are varied, they generally appear to stem from an incomplete understanding of the local flow patterns in the reactor and the performance of the grid and grid spring materials. This was exacerbated, in some instances, by inadequate testing of new designs before they were brought to market. A number of design changes have been successfully instituted to resolve the problem. However, recent failures in “corrected” fuel assemblies suggest that a full understanding may not yet have been achieved.
Axial offset anomalies (AOA) Some PWRs have experienced axial offset anomalies. In these cases, boron compounds in the coolant precipitated out on crud deposited on the upper portions of the fuel rods. This results in a disproportionate shift in power production to the lower part of the core and can result in approaching regulatory limits such that plant operation is adversely affected. Although there has only been one reported instance of a significant plant derating due to AOA, a number of other units have restricted their manoeuvring to ensure that they remain within their technical specification limits.
These and similar events have been common enough that they cannot be dismissed as isolated incidents. They also raise serious questions regarding fuel reliability under the conditions resulting from contemporary reactor operating strategies.
Most disturbing is that despite all of the design, testing and analysis most of these phenomena came as complete surprises. And, even after they arose, the causes in some cases are still not fully understood.
This has led some reactor operators to return to a more conservative operating strategy and to wait until more operating experience is available before increasing the duty on their fuel.
While the previous discussion may appear to focus on technical concerns, it is intimately related to the competitiveness of nuclear power plants. Very simply, these kinds of fuel problems increase the cost of plant operation. Not only does the operator suffer a direct increase in the unit cost of electricity production through reduced generation and the need to purchase replacement power, but he must also bear the ancillary costs of such factors as:
• Loss of the remaining energy potential of the failed fuel assemblies.
• Identification, packaging and handling of failed assemblies.
• Increased contamination of the primary system.
As blasphemous as it may sound for an industry with over three decades of experience, we may simply not understand as much about the operation of the reactor and the behaviour of the fuel and its constituent materials as we thought we did. Many of the concerns are related to the performance of the fuel materials and to the water chemistry of the operating plants. While there is extensive literature on the characteristics of zirconium alloys, our knowledge is far from complete. This is especially so when one considers the severe duty to which the fuel is subject in its contemporary applications.
One of the principal factors that limits the maximum exposure of the fuel is corrosion. Corrosion behaviour is a science in itself, but a number of corrosion phenomena are not yet fully understood and there is a significant complement of art in the science of corrosion. This is evident from some of the unforeseen corrosion problems which have recently arisen and for which a definitive cause has not yet been determined.
In addition, it appears evident that our ability to predict local flow patterns in various areas of the core, especially on the periphery, may also be less precise than formerly believed.
We have also assumed that, through research and use of sophisticated analytical models, we can determine the conditions which will result in fuel failure and derive operating strategies ensuring that we operate with adequate margins. However, recent experience suggests we may not be certain of where performance limits are and may be approaching, or exceeding, these limits.
It is imperative that we know precisely where these limits are. Unlike commercial considerations or the restrictions of regulators, physical limits are not subject to alteration and manipulation. We simply cannot retain a clever lawyer to find a loophole in the laws of chemistry. We cannot lobby the legislature to repeal the laws of physics.
The most logical response to this dilemma is to increase our research and development efforts and find the answers to all of these questions. But life is not that simple.
COMPETITION AFFECTS VENDORS
Traditionally, the bulk of fuel research and development has been performed by the vendors of fuel assemblies. Responding to the demands of their customers, they developed fuel assemblies capable of the service required by nuclear plant operators. The price of the fuel reflected the need to invest in research so that the technology could continually advance.
However, faced with the substantial overcapacity in the market place, the vendors have cut prices in an attempt to gain, or even maintain, market share, while the advanced designs they have developed require fewer fuel assemblies to produce the same amount of energy. Their financial predicament is worsening as their older, higher priced contracts expire and are replaced with the lower priced agreements which resulted from the buyers’ market of the past several years. Revenues may well decline and profits are being squeezed.
To meet this challenge, the vendors have embarked on cost reduction programmes. One initiative has been a severe reduction in the expenditures for research and development. The efforts to immediately address and resolve today’s performance problems are also draining funds which could otherwise have been applied to the longer term research needed to get definitive answers to the open questions regarding fuel performance. In addition, the majority of the vendors have significantly reduced their staffs in the engineering office as well as on the shop floor, thereby affecting their ability to provide services to their customers.
Clearly, advances in analytical and communications technology have filled a portion of the personnel gap. But it is not clear that the industry can function effectively and efficiently without the direct attention of highly skilled, experienced people at all of its plants.
The fabrication industry is at a unique juncture. Many of the senior people at vendor and utility companies attended the birth of the commercial industry and have nurtured its development and growth through adolescence to maturity. They remember what happened twenty or thirty years ago. They have direct experience with the problems that arose in the past and how they were resolved. They recall exactly why things were done a particular way and why alternative approaches were rejected.
With early retirements representing a popular vehicle for downsizing at both utilities and vendors, large numbers of these people are being lost in a very short period. There are not only fewer people to carry the load, but those with the greatest experience are no longer available.
Nor does the future look particularly bright in the personnel area. A number of the younger nuclear scientists and engineers, concerned about the future, are leaving the industry for other pursuits and fewer of the better technically oriented students are opting for careers in the nuclear industry. It has become increasingly difficult to recruit and retain the top talent that both utilities and fuel suppliers need.
THE SEARCH FOR SOLUTIONS
Nuclear plant operators must review their specific situations and balance the benefits of lower fuel costs resulting from increased fuel duty against the risks of increased fuel failure. They must determine their individual levels of risk aversion and make their individual decisions.
In the longer term, there are a number of co-operative industry initiatives focused on answering the open questions on fuel performance. However their results will not be available for some years and, for some plants, this may be too late.
What then for the near term? Should plant operators continue to drive the fuel into unknown territory and hope for the best? Should operating limits be cut back to past levels? Should utilities refuse to venture past previously established safe boundaries and forgo potential economic benefits? It is not necessary that the industry adopt either extreme. By judicious attention to the design, manufacture and operation of the fuel, utilities and their vendors can do much to ensure reliable fuel performance while maximising the extraction of the fuel’s energy potential.
One approach to achieving this objective is implementation of a fuel reliability programme.
The overriding tenet of good fuel reliability is that it must be emphasised at every step of the process from the time a designer sits down at his console to develop a design improvement until the irradiated fuel is finally discharged from the reactor. In an era of tight budgets and reduced staffs, companies must cooperate to ensure that the reliable performance of the fuel is paramount. But, in the end, it is the plant operator who must take responsibility for ensuring reliability, for it is he who will bear the bulk of the burden if fuel failures occur.
Utilities must take positive action to preclude problems. They must exercise oversight of their vendors in the design, testing and manufacture of their fuel assemblies. They must ensure that they have expedient access to all relevant developments in the industry and that they analyse any problems which occur and minimise the likelihood that their plants will be adversely affected. And they must exercise appropriate care in the operation of their plants.
Some form of fuel reliability programme is essential to the long-term performance of every nuclear unit and its ability to remain competitive. Those who elect not to embark on such activities in the hope of achieving modest cost savings may well find that the subsequent cost of plant deratings, outages and maintenance due to fuel performance problems dwarfs those savings and, in the extreme, could render a plant non-competitive.
Through judicious attention to fuel design, manufacturing and operation, we can achieve our goals and keep our plants competitive. We can ensure a long and prosperous future for our industry.