This article discusses improvement in nuclear power plant’s operational efficiencies in the USA in the past 40 years. The increase in nuclear generation has been achieved by a substantial increase in the overall capacity factor of the US plants from about 60% in 1980 to over 90% today. This large increase in capacity factor was achieved by reducing outages, having longer fuel cycles, using higher burnup fuel, and reducing unplanned outages and fuel failures. Combined with increases in power in various plants, this allowed nuclear power to maintain and increase its share of electricity generation. Such an increase in nuclear generation is the equivalent of having built 25–30 nuclear power plants during that period. The length of the planned outages has reduced from 106 days for an average operating plant in 1991 to 38 days in 2008. The fuel performance has also improved to a very high level over the last 20–30 years.


One of the reasons for the optimism for new nuclear plant construction progress in the USA is the significant increase in plant reliability and availability over the last 40 years. Insights on how the nuclear industry worked to improved the capacity factor of nuclear plants and ultimately reduce the cost to operate nuclear power plants are given in the following article.

The Experimental Breeder Reactor No. 1 (EBR-I) in Arco, Idaho produced the world's first electricity from nuclear technology in December 1951. In the next two decades, nuclear power demonstration plants and test reactors were built and operated in the U.S. and worldwide to bring nuclear technology to commercial acceptance. This nuclear power technology development was supported by the successful construction of the large number of U.S. Navy ships, particularly submarines, to nuclear reactor propulsion.

By the late 1960s, large nuclear power plants were being ordered, constructed, and placed in operation by the U.S. electric utilities at an increasing rate. In the early 1070's, the nuclear industry in the USA was just beginning to develop experience in the operation of nuclear power plants in the USA. The early U.S. commercial nuclear power plants were, by 1972, the least expensive sources of electricity. By 1978, more than 200 large nuclear power units were operating, under construction, or were awaiting construction permits. Two significant events caused the waning of nuclear power in the USA in the early 1980's. The first significant event was the nuclear accident that occurred at Three Mile Island (TMI-2) in 1979 and the second significant event was the severe recession that began in 1981. These events along with new regulatory requirements as a result of the TMI-2 event caused the cost of building the nuclear reactors to increase dramatically. Instead of the cheapest form of electricity, nuclear became the highest priced electricity to produce. Therefore many plants were cancelled and none were ordered for many years.

While the number of nuclear power plants in the United States has remained relatively constant for the past several decades, increasing only slightly in the last few years, (the last nuclear reactor to begin commercial operation, Watts Barr, came online in 1996) the percentage of nuclear power in the national energy mix has increased, as shown in the Figure 1 below (data from Energy Information Administration). The increase in nuclear generation has been achieved by a substantial increase in the overall capacity factor of the US plants from about 60% in 1980 to over 90% today. This great increase in capacity factor was achieved by reducing outages, having longer fuel cycles, using higher burnup fuel, reducing unplanned outages and fuel failures. Combined with increases in power in various plants (power uprates) this allowed nuclear to maintain and increase its share of electricity generation. Such an increase in nuclear generation is the equivalent of having built 25-30 nuclear power plants during that period. Clearly such gains are no longer available as the capacity factors cannot increase much more and new nuclear baseload capacity will be needed to maintain nuclear power percentage of electric generation. In addition, plant life extension allows current plants to continue operation past their original planned life cycle.


Figure 1 Percent of Total Electric Power Generated by Nuclear Power Plants in the US

Notable ways that the efficiencies of nuclear power plants were improved over the last 30 years are given in the following paragraphs.

Improving Planned and Unplanned Outages

The competitive environment for electricity generation has significant implications for nuclear power plant operations, including the efficient use of resources, efficient management of plant evolutions such as on-line maintenance, unplanned and planned outages. Nuclear power plant outage management is a key factor for good, safe and economic nuclear power plant performance which involves many aspects, which include utility administration, co-ordination of available resources, nuclear safety, regulatory and technical requirements and, all activities and work hazards, before and during the outage.1

Plant outages (planned and unplanned) are shutdowns in which activities are carried out while the unit is disconnected from the electrical grid. The outage is the period where significant resources are spent at the plant, while replacement power must be purchased to meet the utility's supply obligations, therefore it has a significant impact on unit availability and net income for the utility. A planned outage is an outage where the utility schedules an usually to replace fuel and has adequate time to plan resources and events during the outage to optimize execution of the outage to minimize cost and duration of the outage. An unplanned outage is probably one of the worst situations for a utility where a plant outage occurs due to an unplanned scram of the reactor or due to some technical, safety or regulatory reason that the plant has to stop operating at power. The utility does not have much time to plan and resources have to be mobilized quickly. Usually the unplanned outage does not require the movement of fuel; therefore it is typically short in duration.

Over the last 30 years the utilities has spent significant resources on eliminating unplanned outages such as increasing scram reliability, identifying root causes of the unplanned events and fixing them and training operators and maintenance in proper techniques to ensure reliable operation. Figure 2 shows the reduction of unplanned outages from about 9 to 3 from the time period of 1976-1979 to 1986-91. This reduction in unplanned outages represents a significant improvement in the nuclear plant reliability, cost and safety of nuclear power plants.


Figure 2 Unplanned Outages and Unplanned Scrams for 47 Sample US Plants2

Planned outage management is very complex since it integrates the plant directives, the coordination of available resources, safety, regulatory and technical requirements and, all activities and work before and during the outage.

Each plant develops its strategy for short term, middle term and long term outage planning. Extensive efforts are usually directed towards detailed and comprehensive preplanning to minimize outage duration, avoid outage extensions, ensure future safe and reliable plant operation and minimize personnel radiation exposures. Planning and preparation are important phases in the optimization of the outage duration which should ensure safe, timely and successful execution of all activities in the outage. The post outage review provides important feedback for the optimization of the next outage planning, preparation and execution.

The fundamental basis 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 the different components, requirements of the Nuclear Regulatory Commission (NRC) and the electricity market affect duration and frequency of outages.

In the medium and long term planning, it has become a good practice to categorize the outages in three or four types with the objective to minimize the total outage time. The outages may be categorized into four different kinds:

  • Refueling only, which could be worked out in 7 to 10 days,

  • Refueling and standard maintenance, which could be worked out in 2 to 3 weeks,

  • Refueling and extended maintenance, which can last for one month,

  • Specific outage for major back fittings or plant modernization which could take more than one month.

In case the utility operates several nuclear power plants, a reference outage is defined as a generic outage including common activities to all outages. The reference outage could be for instance, a refueling and standard maintenance outage.

Outages can be optimized by different initiatives that can included the following:

  1. Ensure that all work that can be performed while on line is completed prior to the outage.

    Most US Utilities have a modification group composed of individuals from operations, maintenance, schedule and planning, design and system engineering, reactor engineering, nuclear licensing, quality control and assurance and contracts, which develop modification priorities (usually set by operations and maintenance) that are implemented while the plant is still on-line. Working together in a highly functional team fashion, the modification group has the clout to identify a modification that is needed to make the plant more reliable, ensure that the modification improves plant safety, ensure that necessary resources are provided, ensure necessary design engineering is performed in a timely manner, and that the modification is scheduled and planned in an optimum way. By performing this work on-line, the utility not only improves reliability and safety of the plant in a timely manner, but also eliminates work that may be required in the outage schedule, thereby shorting the length of the outages.

  2. Make sure that all maintenance activities that can be accomplished on-line are performed before the planned outage.

  3. Plan Plan Plan

    A successful outage always has a plan that was rehearsed and reviewed many times.

In the detailed planning and preparation, the following items should be considered:

  • Pre-outage milestones including planning, materials, schedule development, external services contracts, clearance preparation, ALARA reviews, design issues, regulatory issues, etc.

  • Outage duration for all 3 phases: shutdown, execution of work and startup.

  • Final scope of work/activities.

  • Outage schedule, including the main outage schedule and work and safety related schedules (separate schedules for systems, reactor, turbine, startup, etc.). Those schedules shall comply with the main outage schedule. For each activity in the critical path, a separate schedule is made.

  • Work packages, including work orders and permits, instructions and procedures, materials, spare parts, consumables, human and material resources, special tools, post maintenance testing and startup programs, etc.

By implementing the strategies discussed above, utilities have shortened the outage time significantly from 1990 to 2000. After 2000, a plateau has been reached for outage duration since the fuel reload and required maintenance are generally of fixed length. Until fuel reload and required maintenance time durations can be shorted, the 33 to 40 day outage time plateau will remain.

Figure 3 shows the improvements in shortening the average outage in the US. This decline from an average outage length of 106 days in 1991 to 38 days on 2008 represents a significant saving to the utilities and ultimately to the US consumer. Fuel Costs: This is the total annual cost associated with the “burnup” of nuclear fuel resulting from the operation of the unit. This cost is based upon the amortized costs associated with the purchasing of uranium, conversion, enrichment, and fabrication services along with storage and shipment costs, and inventory (including interest) charges less any expected salvage value.

Fuel Cost Improvements and Improved Reliability


Figure 3 U.S. Nuclear Refueling Outage Days3

For a typical 1,000 MWe Boiling Water Reactor (BWR) or Pressured Water Reactor (PWR), the approximate cost of fuel for one reload (replacing one third of the core) is about $40 million, based on an 18-month refueling cycle.4 The average fuel cost at a nuclear power plant in 2009 was 0.57 cents / kWh.

Utilities have strived to achieve minimum fuel cost associated with reload fuel by soliciting several fuel vendors to bid on the fuel reload. During this process, the utility will also solicit special cost saving features such as power uprate services and extended plant analyses. By soliciting these services through the fuel bid, the utilities will typically get the best price for these services. Because nuclear plants refuel every 18-24 months, they are not subject to fuel price volatility like natural gas and oil power plants.

Operations & Maintenance (O&M) Costs: This is the annual cost associated with the operation, maintenance, administration, and support of a nuclear power plant. Included are costs related to labor, material & supplies, contractor services, licensing fees, and miscellaneous costs such as employee expenses and regulatory fees. The average non-fuel O&M cost for a nuclear power plant in 2009 was 1.46 cents / kWh.

Production Costs: Production costs are the O&M and fuel costs at a power plant. Since 2001, nuclear power plants have achieved the lowest production costs between coal, natural gas and oil.4

Fuel Failures: Fuel failures in operating nuclear power stations can lead to a power derate of the plant to protect the fuel from more failures, the shutdown of the plant due to too many fuel failures and in all cases 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 cause higher cost for operation & maintenance and possibly a lower capacity factor for the unit.

Electric Power Research Institute (EPRI) has developed a series of guidelines to help eliminate fuel failures at nuclear power plants, with the aim of achieving Institute of Nuclear Power Operations’ (INPO) goal of zero fuel failures by 2010.5

Fuel failures have been traced to several different causes including crud/corrosion, debris, grid fretting, Pellet Cladding Interaction-Stress Corrosion Cracking (PCI-SCC) and unknown causes. The most common of these fuel failure causes are corrosion and crud, mechanical fretting wear (foreign material such as a piece of wire vibrating against the fuel rod surface), and pellet cladding interaction (PCI – stress buildup on the cladding due to contact with the fuel pellets and interaction with the aggressive radioactive environment on the inside of the fuel rod).

The total number of fuel failures, for both BWR and PWR plants combined, is significantly lower today than in past decades. However, while the industry has moved in the right direction, the number of fuel failures since 1990 has not markedly decreased (see Figure 4). This reduction in the number of fuel failures can be attributed to utilities being more conscientious about fuel failures and by the utilities applying INPO and EPRI guidance in their day to day operational activities in the nuclear power plant.


Figure 4 Number of US Fuel Failures Since 1980

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 will pay a high dividends in the final cost evaluations of a nuclear power plant.

Power Uprates

The NRC regulates the maximum power level at which a commercial nuclear power plant may operate through the plants’ license. 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, or technical specifications, and is commonly referred to as the Current Licensed Thermal Power (CLTP). Since this power level appears in the plant license, it can only be changed by a License Amendment Request (LAR) that must be approved by the NRC prior to implementation. This process of requesting operation at thermal power levels above the current licensed power level is referred to as a Power Uprate. The notion of power uprates has been around since the late 1970's. In fact, the first power uprate applied for and granted by the NRC occurred at Calvert Cliffs in 1977 and 1978. Since then the NRC has reviewed and approved some 135 license amendments for operation at power above the original licensed thermal power (OLTP).6

There are three types of power uprates, which are described below.

Measurement uncertainty recapture (MUR) power uprates: These increases in license power take advantage of the requirement in 10CRF50 Appendix K that all safety analysis must be performed at 102% of licensed reactor power. The additional 2% in thermal power accounts for measurement uncertainty when calculating reactor power. Since the 2% requirement appears in 10CFR50 Appendix K, this uprate is sometimes referred to as an Appendix K uprate.

An MUR is 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. An MUR always results in an uprate of less than 2% of thermal power, since it is impossible to eliminate all measurement imprecision, and is typically adds 1.5% to 1.7% of CLTP. Since the new power level is within the currently analysed limits, it requires little or no reanalysis and no modifications to nuclear safety systems. Also, because of the relatively minor change in power, an MUR is usually well within the capabilities of the Balance Of Plant (BOP) systems and requires little or no changes to those systems. Therefore, it is usually accomplished for little effort and low cost.

Stretch power uprates: The second type of power uprate is referred to as the stretch power uprate. Stretch uprates are typically 5% to 7% of CLTP. Stretch uprates take advantage of the design margin that is inherent in the design and construction of most power plants. Typically a stretch uprate was selected at a level where no changes were required to the plant nuclear safety systems and minimal changes, if any, were required for the BOP side. This made the stretch uprate relatively easy and inexpensive to implement. Since the power level is higher than an MUR, the system evaluation required for a stretch uprate is more complex.

A large percentage of the early power requests were stretch uprates. There are currently no requests for stretch uprates on the docket and very few expected requests for stretch uprate. The desire for additional nuclear power capacity has replaced the stretch uprate with the last and more aggressive form of power uprate, the extended power uprate. In fact, several plants that already operate at stretch levels are pursuing or have been granted extended uprates. This is due to the favorable cost benefit for increased power at these facilities.

Extended power uprates (EPU): Extended power uprates are typically greater than stretch power uprates and have been approved for increases as high as 20 percent. These uprates may still be within the original design limitations of the nuclear safety systems and require little or no modification to those systems. 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 of after an EPU is be substantially higher, they are typically accomplished with major modifications to the Balance of Plant (BOP) systems. A new High Pressure turbine is usually required. Extended power uprates typically also require major changes to condensate, feedwater,feedwater heating, and electrical generation systems. These modifications make extended power uprates projects large and difficult to manage. Project costs can typically run in the hundreds of million dollars. However, even at these large project costs, they 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 total 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 projects return on investment. In most instances, the Return on Investment (ROI) for a power uprate remains favorable even in the current economic downturn. 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 per KW cost is still acceptable when compared to new build for 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. The NRC has approved 135 power uprate application todate.7

The cumulative additional electric power from all power uprates approved since 1977 is about 5,700 megawatts, which is the equivalent of more than five large new reactors added to the grid. The NRC currently has 16 power uprate applications under review, comprising a total of about 1,145 megawatts of electric power. The NRC expects to receive 39 new power uprate applications in the next five years for a total of about 2,400 megawatts of additional electric power output.Uprating nuclear units is costly and technically challenging but it has been proven that owners can potentially receive a fantastic ROI with their uprate projects. The industry is looking for ways to reduce costs and project time without compromising quality and safety. Careful planning and common sense strategies must be put in place by all involved in uprate projects and to do that, many challenges must be faced:7

Plant Life Extension

The Nuclear Regulatory Commission (NRC) is the government agency established in 1974 to be responsible for regulation of the nuclear industry, notably reactors, fuel cycle facilities, materials and wastes (as well as other civil uses of nuclear materials).8

In an historic move, the NRC in March 2000 renewed the operating licenses of the two-unit Calvert Cliffs nuclear power plant for an additional 20 years. The applications to NRC and procedures for such renewals, with public meetings and thorough safety review, are exhaustive. The original 40-year licenses for the 1970s plants were due to expire before 2020, and the 20-year extension to these dates means that major refurbishing, such as replacement of steam generators, can be justified.

As of the end of 2009 the NRC had extended the licenses of 59 reactors, over half of the US total. The NRC is considering license renewal applications for further units, with more applications expected by 2013. In all, about 90 reactors are likely to have 60-year lifetimes, with owners undertaking major capital works to upgrade them at around 30-40 years.

Extended lifetime from 40 to 60 years will add 20 years of operational abilities for all of these power plants at very little additional cost. This represents a significant saving to the US consumer.

Improved Maintenance and Surveillance Procedures and Processes

The NRC has a new oversight and assessment process for nuclear plants. Having defined what is needed to ensure safety, the NRC now has a better-structured process to achieve it, replacing complex and onerous procedures which had little bearing on safety. The new approach yields publicly-accessible information on the performance of plants in 19 key areas (14 indicators on plant safety, two on radiation safety and three on security). Performance against each indicator is reported quarterly on the NRC web site according to whether it is normal, attracting regulatory oversight, provoking regulatory action, or unacceptable (in which case the plant would probably be shut down).

On the industry side, the Institute of Nuclear Power Operations (INPO) was formed after the Three Mile Island accident in 1979. A number of US industry leaders recognized that the industry must do a better job of policing itself to ensure that such an event should never happen again. INPO was formed to establish standards of performance against which individual plants could be regularly measured. An inspection of each member plant is typically performed every 18 to 24 months.

It is difficult to quantify the amount of improved reliability that these improved procedures and processes have provided, but the improved understanding of the operation and maintenance of the plant has been enormous.


The increase in nuclear generation has been achieved by a substantial increase in the overall capacity factor of the US plants from about 60% in 1980 to over 90% today. This large increase in capacity factor was achieved by reducing outages, having longer fuel cycles, using higher burnup fuel, reducing unplanned outages and fuel failures. Combined with increases in power in various plants (power uprates) this allowed nuclear to maintain and increase its share of electricity generation. Such an increase in nuclear generation is the equivalent of having built 25-30 nuclear power plants during that period.

The reduced length of the planned outages from 106 days for an average operating plant in 1991 to 38 days in 2008 and the reduced number of unplanned outages. Figure 2 shows the reduction of unplanned outages from about 9 to 3 from the time period of 1976-1979 to 1986-91, The reduction in planned outage length and the number of unplanned outages represents a significant improvement in the nuclear plant reliability, cost and safety of nuclear power plants.

Additionally, power uprate, which allowed plants to operate at a higher power, and power plant life extension, which extended the operating life of a power plant beyond 40 years allowed more electrical power to be generated at a reduce total production cost. Also, fuel performance has improved to a very high level over the last 20-30 years.

These initiatives improved the overall performance of nuclear power in the US and has provided adequate justifications to building more reactors in the USA.


IAEA-TECDOC-1315, “Nuclear power plant outage optimisation strategy”, October 2002


“Learning by Accident? Reductions in the Risk of Unplanned Outages in the U.S. Nuclear Power Plants After Three Mile Island”, Journal of Risks and Uncertainty, 13-175-198, (1996).


WNA, World Nuclear Association,