Flexible Operation of Nuclear Power Plants Ramps Up

A widespread misconception persists that nuclear plants can only function as inflexible baseload sources of power-and it's hurting prospects for the nuclear sector's role in the world's future power mix, which will increasingly be crowded by intermittent renewable energy forms, several experts say.

Existing nuclear plants and new designs can technically perform both frequency control and load-following operations, but owing to high upfront capital costs and relatively low fuel and operational costs when compared with fossil fuel-generating units, the majority of nuclear generators across the world generally consider operating nuclear power plants at full capacity-for as long as maintenance and refueling allows-as the best option.

However, there appears a "recent and increasing need" worldwide to operate nuclear plants flexibly, noted the International Atomic Energy Association (IAEA) in an April 2018 report surveying knowledge, feasibility, and challenges concerning non-baseload operation of nuclear power plants. Among key reasons it cites for this trend are: "large nuclear generating capacity relative to the total capacity, growth in renewable energy generation, and deregulation or structural changes of the electricity supply system and the electricity market during the long operating lifetime of a nuclear power plant. These necessitate technical and regulatory changes, and also operational, economic and financial rearrangements, to maintain the efficiency of capital investment," it said.

Flexibility Is a Growing Nuclear Trend

The decision to boost operational flexibility at nuclear plants is often complicated, the IAEA noted, however. "Technically, newly built nuclear power plants have an advantage in that the planning and design of a plant have generally had flexible operation in mind. However, these systems need to be validated during initial startup testing, and any limitations have to be determined at the beginning of operations. Additionally, the operating license application (safety case) could be developed to support flexible operation."

On the other hand, existing nuclear plants that have operated only in baseload mode may need to consider modifications to support frequency control and load-following operations that are dependent on plant design and extent of flexibility requirements. "Licensing changes may also be required, and existing operation and maintenance philosophies may need adjustment to support flexible operation. Commercially, the deviation from baseload operation has to be considered within the electricity market framework, to minimize, eliminate or compensate for the impacts of flexible operation on the efficient use of capital investment while serving the overall energy structure needs."

However, according to FORATOM, the Brussels-based trade association for the nuclear energy industry in Europe, the feat isn't impossible. In countries like France, Germany, Slovakia, the Czech Republic, Belgium, Finland, Switzerland, and Hungary, nuclear power plants (NPPs) have consistently demonstrated that they have actual and noticeable load-following and flexibility maneuvering capabilities. "When combining the different capabilities, power variations of up to 10,000 MW could be absorbed by German NPPs in 2010. In France, with an average of 2 reactors out of 3 available for load variations, the overall power adjustment capacity of the nuclear fleet equates to 21,000 MW (i.e. equivalent to the output of 21 reactors) in less than 30 minutes. In addition, it is also possible to disconnect units temporarily from the grid, and then restart them later. If kept in 'hot stand-by' mode, full load can then be resumed within a couple of hours," FORATOM said in a May 2018 position paper.

These reactors are compliant with European Utilities Requirements, a set of documents developed by European utilities in 2014 to harmonize design specifications for safe and reliable operation of future nuclear plants. "Actually, nuclear energy appears as being the only large scale, non-weather dependent low carbon technology that is capable" of demonstrating on a grand scale that it can contribute to the stability of the electrical system by adapting to changes in demand and balancing the intermittency of variable renewables, it said (Figure 1).

1. A 2010 comparison of German nuclear, newly built hard coal, and combined cycle gas turbine (CCGT) power plants' ability to handle load changes suggests nuclear power plants could ramp at a rate of 63 MW/min, which hard coal ( 26 MW/min) and CCGT ( 38 MW/min) couldn't match. Courtesy: Sustainable Nuclear Energy Technology Platform, Nuclear Energy Factsheets-Load Following Capabilities of Nuclear Power Plants, 2017

Flexibility Is Proven in France and Germany

According to the IAEA, the reason the French nuclear fleet-which today provides 75% of the country's power-is so markedly flexible is because in the 1970s, it "correctly anticipated" that nuclear power would have to broadly participate in balancing of generation and demand (Figure 2). At the time, the agency noted, demand changes were characterized by seasonal variations, and weekly and daily differences.

2.French utility EDF began making its nuclear plants more "maneuverable" in the 1980s, and today it says a 1,300-MW reactor can increase or decrease its output by 900 MW within about 30 minutes. "This is made possible by EDF's unrivalled industrial expertise and specific capabilities," it said. This photo shows EDF's two-unit (each 900 MW) Saint-Laurent plant two hours away from Paris. Courtesy: EDF

"Due to the French energy mix specifics, the ilectriciti(C) de France (EDF) nuclear fleet was designed to provide load following and full ancillary services (primary and secondary reserves), mainly due to a large demand consumption pattern with high seasonal variations." But as the country's nuclear program has matured and its energy mix shifted, France has also embarked on improved programs to accomplish rapid load following-from 100% rated thermal power (RTP) to 30% RTP-frequency control (7% RTP), and rapid (up to 5% RTP/minute) return to full power, all with minimal reactor trips while maintaining stable power at various power levels. That required upgrades to nuclear plants with additional plant modifications.

As EDF told POWER, these included modifications to the mechanical design (pertaining to the surge line, spray line, and charging line); pressurizer control loops; control rod mechanisms; and introducing a new core control mode. Among challenges EDF faced were constantly matching power generated by reactor cores to power extracted from steam generators, or vice versa, as well as managing new stresses on components, such as the control rod drive mechanism, the control rods, and mechanical structures of the reactor coolant pressure boundary. However, EDF said the transition to flexible operations had little impact on maintenance costs and a low impact on unplanned capability load factors, and it stressed that its success hinged on appropriate designs and well-trained operators.

Load-following capabilities were also a "built-in" feature for new nuclear plants constructed in Germany as early as the 1970s, and German plant designs-including pressurized water reactors (PWRs) and boiling water reactors (BWRs)-considered and incorporated features to compensate for load changes over a large power range and a fast gradient (up to 5% rated electrical output [REO]/min, or, for some designs, 10% REO/min).

Flexibility at Nuclear Plants In Other Countries

In Belgium, where nuclear power plants now participate in automatic primary frequency control within 2.5% RTP (except at the end of a fuel cycle and in cases of fuel leakage), feasibility studies are underway to allow more extensive load reductions and ultimately to increase the maximum allowable power decrease from the current limit of 25% to 50% RTP. Ontario, Canada, meanwhile, recently introduced a requirement that allows nuclear generators to decrease output during periods of high generation and low demand. "To meet these requirements, selected nuclear generating units maintain reactor (thermal) power at 100% RTP, while plant operators reduce generator (electrical) output by up to 300 MW(e) for short periods. Excess steam is directed (dumped) to the condenser steam bypass system (SBS) to provide flexible operation in response to the independent electricity system operator requests during periods of surplus baseload generation," noted the IAEA.

In the Czech Republic, where nuclear still provides about 60% of baseload generation, modifications to the Dukovany nuclear power plant in 1996-including to reactor and turbine control systems-enable plant operation during extreme frequency deviations. And in Russia, where all VVER nuclear power plants that are either operational or under construction have the capacity to provide various degrees of flexible operation, since the mid-2000s, the country's grid system operator UPS has asked nuclear plants to ensure modes of frequency regulation and load following as its nuclear capacity increases. "Although the nuclear generation was only 16.5% of the total generation in the entire Russian Federation, the electricity from nuclear generation was supplying nearly one third (31.5%) of the UPS European region grid and approached 40% in the UPS European region's Centre sub-region. However, Russia's "equipment and systems are designed for operations with capacity flexible regulation modes 100a'50a'100% RTP in [VVER-TOI] and 100a'80a'100% RTP in NPP-2006 designs," noted the IAEA.

In South Korea, though flexible nuclear plant operation has not been a concern (and existing regulations do not allow for extended flexible operation), nuclear plants are capable of performing uninterrupted operations in case of, for example, load rejection at any power level, loss of a feedwater pump, or turbogenerator runback to house load, but they also can perform "limited" power cycling. "For example, the OPR-1000 design can perform a daily load cycle with typical 100-50a'100% RTP in 14a'2a'6a'2 hour patterns for up to 90% of the cycle length. Similarly, CANDU plants can be operated in frequency control mode without major design changes, as their normal operation mode is 'reactor follows turbine,' " noted the IAEA.

U.S. Nuclear Generators Are Looking Into Flexibility as an Option

Finally, even in the U.S., where economic factors have historically driven baseload operation, load cycling (reduced load and extended time at reduced power operation) has been used, even if infrequently, in response to plant issues, to extend the cycle, or to follow seasonal demand. Columbia nuclear plant, a BWR in Washington, for example, communicates frequently with the ISO to plan power output based on weather, river flows, and forecasted load demands.

Now, however, assailed by market changes, including from the proliferation of ever-more competitive wind and solar, and cheaper and more efficient natural gas power, many U.S. nuclear plants are beginning to encounter periods of zero or negative power prices if they cannot reduce power as requested by grid operators, and flexible operation is appearing more attractive. Yet, according to experts, before nuclear generators resort to flexible operations, the economics must be sound.

Efforts on that front are underway. Last June, researchers from the Department of Energy's Argonne National Laboratory and Massachusetts Institute of Technology told the Federal Energy Regulatory Commission that by boosting flexibility, nuclear generators could actually both lower power system operating costs and increase revenues, while significantly reducing curtailment of renewables. The researchers' published studies analyzed planned load following, frequency regulation, spinning reserve, and dynamic price-responsive operations as modes of flexible operations. They also considered an assortment of technical constraints for light water reactors. These include control rod movement (insertion into the core to reduce power output and withdrawal to increase it); thermal and mechanical stresses, including fuel cladding cracking failures; and coolant temperature and pressures, which could put stress on other components, and result in longer-term changes in the equilibrium concentration of Xenon 135, a powerful neutron absorber; and fuel burn-up, which could affect maneuverability.

The Quest for Good Economics

The IAEA, too, recently conducted an economic study to quantify the cost-revenue aspects for flexible nuclear operations based on a large-scale country-level power plant dispatching model. It concluded that "technologies with high capital costs and low fuel costs " could experience significant adverse economic impacts if the costs of providing flexible services are not internalized within the energy system." Specifically, it noted, flexible operations could ramp up operational and maintenance costs, and unplanned outages, and it suggested "market arrangements" should be considered to allow plants to recover those costs. Notably, "The change in cycling costs at the plant level is not proportional to the needs of flexibility. Several factors such as age, vintage, design, maintenance activities and past cycling affect the plant level cycling costs. Moreover, the frequency and intensity (rate of change and magnitude of change) of future flexibility requirements will have a direct impact on plant cycling costs."

Significantly, the report says, it is apparent that generators in deregulated markets will need to assess profitability based on net benefits at a system level because there may not be a benefit at the plant level. On the other hand, in some member countries nuclear generators could face a "large financial penalty" for not being able to operate flexibly, and that could mean policymakers must incentivize owners to install retrofits to provide flexibility to bolster system benefits.

Nuclear Vendors Gearing Up for Ramp Up in Flexibility

Riding on the trend for now, nuclear plant designers, equipment makers, and fuel vendors are reportedly providing nuclear generators with additional guidance and procedures to address technical restrictions, as well as with guidelines concerning fuel conditioning and deconditioning. Westinghouse, for example, has put out an extended reduced power operation guideline to U.S. generators providing information regarding additional monitoring and limitations associated with operation of the fuel at less than full power for periods that exceed two weeks for Westinghouse and Combustion Engineering designs utilizing Westinghouse-designed fuel.

Meanwhile, Framatome, the French nuclear equipment and fuel technology conglomerate, is also banking on expanding demand for flexible operation solutions. The company is actively marketing services, such as feasibility studies, and promoting plant upgrades at PWRs and BWRs that would precisely cater to grid demand and a plant's individual capabilities. Upgrades include the Advanced Load Following Control (ALFC) system for PWRs, which it says "ensures fully automated flexible load-following operation without any manual intervention"; optimization or operation modes to minimize cumulated usage factors; and rapid boron monitoring to support steep load-following ramp increases.

According to Craig Ranson, senior vice president of Framatome's Installed Base America division, discussions about flexibility options have been ongoing with U.S. customers for five or six years. "But what I've noticed over the last year or so is much more serious talking about it-where they're actually wanting to sit down and really understand what's needed to get to a flexible operation state, what the timeline looks like, and what changes are needed." Ranson said the primary reason the nuclear generators appear to be assessing flexibility is simply to have options. "They're looking at how nuclear can bring even more value to the grid by having some flexible capability," he said.

-Sonal Patel is a POWER associate editor.

Updated (April 1): Adds Craig Ranson's comments.

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