It is now generally accepted that current nuclear power stations generate power very cheaply, assuming that they are operated at high standards. In the United States and Europe, the best plants generate electricity at a production (that is, fuel plus operations and maintenance) cost of below 2 US cents per kWh, which should guarantee a high level of profitability in any power market. This profitability explains not only the likelihood of plant operating lives being extended to 60 years and maybe beyond, but also the attraction to governments of taxing current nuclear generation. This has been seen recently most strongly in Germany, but overenthusiasm for taxation has also occurred in other European countries like Sweden and Belgium. Indeed, once a nuclear operator has been though the long period of expense in getting a plant into operation, it can look forward to many years where the facility is rather like an automated teller machine (ATM), spewing out large quantities of cash as well as electricity. The big question for new plants, however, is whether this benefit is sufficient to offset the heavy costs of capital investment in the early years.
The powerful economics of modern nuclear power plants essentially comes from the improvements to their operations made over the past 15-20 years. While the number of nuclear power plants in the world has remained relatively constant over this time, the percentage of nuclear power in the world energy mix has only begun to decline in the past few years, as China, India and other developing countries have begun to add substantially to world electricity generation. Maintaining the world nuclear share in the 14-15% range has been achieved by a substantial addition to nuclear generation, largely through increases in the overall capacity factors of nuclear plants. These were achieved by reducing the length of planned outages, having longer fuel cycles, using higher burnup fuel, and through reducing unplanned outages and fuel failures. This has been combined with increases in power levels at various plants.
In the United States, the increase in nuclear generation over the past 20 years is the equivalent of having built an additional 25-30 nuclear power plants during that period. Clearly such gains are no longer generally achievable; in many countries, capacity factors cannot increase much more, so new nuclear baseload capacity will be needed to maintain nuclear power’s share of electric generation. Plant life extension certainly allows current plants to continue operation past their original planned life cycle, but new plants are also now needed.
Plant outages (planned and unplanned) are shutdowns in which activities are carried out while the unit is disconnected from the electrical grid. An outage is a period where significant resources are spent at the plant, while replacement power must be purchased to meet the utility’s supply obligations. It therefore has a significant impact on unit availability and net income for the utility.
Planned outages to replace fuel are scheduled in advance, when there is adequate time to plan resources and events in order to optimize execution to minimize cost and duration. An unplanned outage is one of the worst situations for a utility, whether it occurs due to an unplanned scram of the reactor or for some technical, safety or regulatory reason. The utility does not have much time to plan so resources have to be mobilized quickly. Since the outage does not require the movement of fuel it is typically short in duration.
Over the last 30 years, utilities have spent significant resources on eliminating unplanned outages through increasing reliability, identifying root causes of any unplanned events and by fixing these through training operators in proper techniques to ensure reliable operation. In the United States, unplanned outages have been reduced by a factor of three since the early 1980s; this improvement is significant in terms of nuclear plant reliability, cost and safety.
Planned outage management is very complex since it integrates plant directives, the coordination of available resources, safety, regulatory and technical requirements, and all activities before and during the outage. Each plant develops its strategy for short-term, medium-term and long-term outage planning. Extensive efforts are usually directed towards detailed and comprehensive planning to minimize outage duration, avoid outage extensions, ensure safe and reliable future plant operation and minimize personnel radiation exposure. Planning and preparation are important phases in the optimization of an outage duration that should ensure safe, timely and successful execution of all activities. The post-outage review provides important feedback for the optimization of the next outage planning, preparation and execution stages.
The fundamental bases for outages during the lifetime of a nuclear power plant are strongly affected by plant design and layout. The choice of fuel cycle length, desired mode of operation, operational strategies, maintenance periods for different components, requirements of the regulator and the electricity market affect duration and frequency of outages.
In medium- and long-term planning, it has become a good practice to categorize the outages into different types to minimize the total outage time. Categories include refuelling only (which could take 7 to 10 days), refuelling and standard maintenance (which could be 2 to 3 weeks), refuelling and extended maintenance (which could last for a month) and specific outages for major backfitting or plant modernization (taking more than one month).
Techniques for optimized outages have developed to the extent to which a plateau has been reached for outage duration in many countries since the fuel reload and required maintenance are generally of fixed length. But average lengths have been reduced substantially, from 106 days in 1991 to 38 days in 2008 in the USA, for example.
Fuel failures in operating nuclear power stations can lead to a power derate to protect the fuel from more failures, or even plant shutdown. In all cases, fuel failures lead to higher radiation levels in the plant. The cost associated with the loss of power is obvious, but the higher radiation levels can lead to maintenance and operation issues that will raise costs and possibly lower the unit’s capacity factor.
Fuel failures have been traced to several different causes, the most common of which are corrosion and crud, mechanical fretting wear (in which foreign material such as a piece of wire vibrates against the fuel rod surface), and pellet-cladding interaction (in which the cladding cracks under pressure from contact with the fuel pellets and the aggressive radioactive environment on the inside of the fuel rod).
The total number of fuel failures today is significantly lower than in past decades, but in the US, the number of fuel failures since 1990 has not markedly decreased. In 2006, the Institute of Nuclear Power Operations (INPO) set an ambitious goal to achieve zero fuel failures by 2010. In response, US nuclear owners and operators backed a fuel integrity initiative that emphasized the development of fuel reliability guidelines. In the first instance, INPO led the development of guidance documents summarizing current industry information to assist utilities in improving fuel integrity and performance. Continued emphasis on reducing fuel failures should pay a high dividend in the final cost evaluations of a nuclear power plant.
The notion of power uprates has been around in the United States since the late 1970s and is now becoming general practice worldwide.
Uprating nuclear units is costly and technically challenging but it has been proved that owners can potentially receive a fantastic return on investment. Those involved in uprate projects face many challenges and must plan carefully.
In the plant’s licence, regulators specify the maximum power level at which a commercial nuclear power plant may operate. This power level, along with other plant-specific parameters, forms the basis for the specific analyses that demonstrate that the facility can operate safely. The maximum allowed reactor thermal power appears in the plant license and can only be changed by request to the regulator.
Considering US experience, there are essentially three types of power uprates. Measurement uncertainty recapture (MUR) power uprates are small (less than 2%) and result from spare capacity due to measurement uncertainty when calculating reactor power. These are accomplished by adding high-precision feedwater flow measurement devices, since feedwater flow is used as a basis for reactor power in nearly all nuclear plants. Since the new power level is within the currently analysed limits, it requires little or no re-analysis and no modifications to nuclear safety systems.
The second type of power uprate is referred to as the stretch power uprate. Stretch uprates are typically 5% to 7%, and take advantage of the design margin that is inherent in the design and construction of most power plants. Typically stretch uprates are selected at a level where no changes are required to the plant nuclear safety systems and minimal changes, if any, are required for the Balance of Plant (BOP) side. This has made stretch uprate relatively easy and inexpensive to implement. A large percentage of the early power uprate requests were stretch uprates, but their potential is largely exhausted now in the US.
Finally, extended power uprates (EPUs) are typically greater than stretch power uprates; increases as high as 20% have been approved. There is typically a large amount of reanalysis required for these uprates and the engineering effort to support the EPU can be formidable. Since the power output after an EPU can be substantially higher, they are typically accomplished with major modifications to the BOP systems. A new high pressure turbine is usually required, while extended power uprates typically also require major changes to other systems. These modifications make extended power uprates projects large and difficult to manage, with project costs typically running into the hundreds of millions of dollars. However, even at these large project costs, they can still deliver more kilowatts per dollar than new build of similar power levels.
Each utility evaluates the potential for a power uprate based on the local economic environment in its sales territory. The potential for increased revenue is balanced by the cost of the uprate and an economic decision is reached based on the project’s return on investment. In most instances, this remains favourable even in economic downturns. This is particularly true in the case of large EPUs. For example, a 20% EPU at a 1,000 MWe power plant results in a 200 MWe gain. Even at project costs that approach $500 million, a $2,500/kW cost is still acceptable when compared to new build costs for nuclear or fossil plants. When incremental fuel costs and cost stability are accounted for, nuclear power uprates have a clear economic and environmental advantage. Utilities perform detailed cost/benefit analyses to ascertain the best power uprate category to pursue for their particular regulatory and financial situation. In the US, the NRC has approved 135 power uprate applications to date and the cumulative additional electric power from those approved since 1977 is about 5,700 megawatts. This is the equivalent of more than five large new reactors added to the grid.
It can be argued that it has been the onset of electricity sector deregulation and liberalization that has caused many of these nuclear plant operation achievements. It was once feared that many existing nuclear stations would close down in the new environment unless their performance could be improved. The improvement in capacity factors combined with power uprates has now prevented wholesale industry closures; the US is the best example. The practices and high standards reached there are gradually spreading to some other countries that still lag behind. These achievements are an important foundation of new nuclear build over the next decades as potential investors need the certainty that their very expensive assets can be operated intensively and produce the almost certain high returns to repay all the money invested.
Steve Kidd is deputy director general of the World Nuclear Association, where he has worked since 1995 (when it was still the Uranium Institute). Any views expressed are not necessarily those of the World Nuclear Association and/or its members.Related ArticlesENEC asks regulator to approve preparatory work at Braka
Nuclear power has followed oil and gas in becoming international. Major vendors are scattered across the Americas, Europe and Asia, and utilities around the world routinely share expertise and lessons learned in safety practices.