Flexible nuclear power sounds like a contradiction in terms, at least in the United States.

In the United States, it seems to be an article of faith that a nuclear power plant has to run flat out around the clock, but in several places, most notably France and Germany, nuclear power plants have been operated flexibly for years.

It can be done, but “the issue is that nuclear power plants weren’t designed to do that in the United States,” said Jim Riley, senior technical advisor for nuclear operations at the Nuclear Energy Institute, an industry group that develops policy on issues related to nuclear energy.

In recent years, however, the business model that calls for nuclear power plants to run at full capacity has collided with other trends in the power sector, and nuclear power plants are suffering the fallout.

In the past several months, even more nuclear plant closures were announced. Exelon said it would close its Quad Cities and Clinton plants in Illinois. Pacific Gas and Electric is closing the Diablo Canyon plant, and Omaha Public Power plans to close its Fort Calhoun plant. Meanwhile, analyst Julien Dumoulin-Smith at UBS said that Xcel Energy’s Prairie Island plant in Minnesota and Entergy’s Palisades plant in Michigan are at risk for closure. Overall, analysts say about 10% of the U.S. nuclear fleet is at risk of early retirement.

One of the biggest problems facing owners of nuclear plants is that it is difficult for them to operate profitably in markets where low-priced natural gas sets the marginal cost of power and where a growing number of renewable resources with zero marginal costs are showing up.

Those changes in market fundamentals have translated into a binary decision for many nuclear operators: Continue to operate with losses or shut down.

Faced with those options, many plant owners are not looking at changing how their plants operate but at changing the regulatory environment in which they operate in order to get paid for the value they create by producing zero emission power and providing jobs and tax payments to the local economy.

This summer, New York regulators threw a lifeline to nuclear plants when they approved a clean energy standard that includes subsidies for nuclear plants. In Illinois, where Exelon has two troubled nuclear plants, the utility company is lobbying for similar assurances from the legislature.

What is often overlooked in these decisions is the option of moving away from the long-held practice that nuclear plants must operate as close to 100% of output as they can. Moving to flexible operation “is possible, but it is not considered practical,” said Scott Burnell, a public affairs officer at the Nuclear Regulatory Commission.

There are, in fact, many hurdles to operating a nuclear plant flexibly, but there are several examples that demonstrate that it can be done.

Overseas experience

France derives about 75% of its electricity from nuclear power. With such a large proportion of its fleet fueled by uranium, nuclear power plants there have had little choice but to provide the ramping services needed for grid stability.

Over the past three decades, French nuclear operators have learned to operate nuclear plants anywhere from 20% to 100% of capacity for the first 65% of the fuel cycle and from about 85% of capacity and upwards for the remainder of the cycle.

Germany has also adopted flexible operation of nuclear plants, and it provides a case that is closer in some respects to the challenges facing the U.S.

Germany decided to phase out its nuclear plants in the wake of the 2011 Fukushima nuclear disaster in Japan and now derives only about 16% of its electricity from nuclear power while renewable power provides an ever greater share of power supplies, briefly surpassing 95% in May 2016.

Germany’s utilities have learned to adapt the operation of their nuclear plants to this new reality. RWE is able to run its reactors in load-following mode at rates of between 50% and 100% of nominal power with a gradient of 63 MW/minute. At a higher capacity factor, 80% to 100% for instance, a ramp speed of 126 MW/min can be achieved.

A 2011 report from the OECD's Nuclear Energy Agency illustrated how a number of German nuclear plants follow load on a typical day:

According to a report from French nuclear firm Areva, the experience clearly confirms that these nuclear power plants are able to operate flexibly and, additionally, that there are no safety concerns related to routinely operating in a load-following mode. The report goes on to say that not only can nuclear plants run in load following mode, they often can do so better than coal and combined-cycle gas turbines and are only surpassed by gas turbines.

In Ontario, the Bruce nuclear plant is often in load-following mode to accommodate the influx of wind power. Each of Bruce’s eight, 760-MW units can operate as low as 300 MW. The Bruce plant uses CANDU reactors, and the design allows them to bypass the steam turbines and dump steam to the condenser, lowering the electrical output even though there is no change of status to the reactor.

The ramp rate of a CANDU unit using steam bypass is up to 10% of full power per minute, a higher rate than that of a combined-cycle gas turbine, which can ramp down at about 5% of full power per minute.

In the Pacific Northwest, the 1,170-MW Columbia nuclear plant has operated flexibly for years, but it does not follow load. It shapes load to adapt to spring conditions when snow melt could cause spillover at the hydroelectric plants run by the Bonneville Power Authority.

Columbia’s load shaping is done in response to requests from BPA, and the plant operator must receive a minimum 12-hour notice prior to ramping down power to 85%. For a 65% reduction, 48-hour notice is required, and for a full shutdown, 72 hours. Generally, the plant’s operators adjust reactor recirculation flow to reduce power output to 85% and adjust control rods to reduce power output to 65%.

Technical challenges

By and large, the rest of the U.S. nuclear fleet operates nearly flat out. Nuclear operators, in fact, frequently cite their high capacity factors in their earnings reports. It is obvious, though, that there is nothing in the design of a nuclear plant that prevents flexible operation. Experts say that most nuclear plants are built with that capability. But that is not to say there are no technical challenges to operating a nuclear plant flexibly.

At the very least, operating any plant flexibly puts more wear and tear on parts such as valves and pumps; but for a nuclear plant, the issues are more complex.

Ramping a conventional thermal plant is done by reducing the flow of fuel. One way of ramping down of nuclear plant is by adjusting the fuel rods in the reactor core. France has done a lot of work on that method, devising modified fuel rods, such as “gray” rods and working with different configurations of rod placement.

In a pressurized water reactor the power level can be changed by moving the control rods and by changing the concentration of the boric acid, a neutron absorber, in the primary coolant. But changing the boron concentration is a slow process, and it has environmental implications. In boiling water reactors, ramping can be done by changing the coolant flow rate and/or by moving the control rods.

A less complex approach would be to dump steam as the Bruce plant in Ontario does. By design, all the nuclear plants in the U.S. are capable of ramping down by dumping steam. Ramping by dumping steam allows the reactor to remain at full power, and avoids the potential for xenon buildup.

Xenon is a byproduct of nuclear fission, and it is “neutron poison,” as one nuclear expert put it. Xenon absorbs neutrons, so the nuclear reaction has to be stepped up to compensate for its production. At full throttle, operators are able to reach an equilibrium between output and xenon production, but if a plant is ramped up or down too quickly, xenon might be produced more quickly than it can be burned off. The result is a reactor might not be able to be restarted.

At the Columbia plant, cycling it to 85% of output, back up to 100% and down again to 85% can require as many as 17 different reactivity manipulations using recirculation flow and control rod movement.

Xenon production can be managed – the French and Germans have learned to do it – but it is one of the challenges to moving to flexible operation.

The other challenge in the U.S. is regulatory. There could be many issues, but the chief regulatory challenge appears to lie in the fact that U.S. rules require that nuclear plant operators must by licensed by the NRC. So, unlike a gas turbine in the PJM Interconnection, for instance, a nuclear operator cannot be controlled by the grid operator.

It is possible to work around that restriction by having the grid operator communicate with the nuclear operator, who would still be in charge of the plant, as is done at the Columbia plant. But it adds a layer of complexity to flexible operation that is not encountered with non-nuclear plants. For instance, it would greatly complicate using a nuclear plant for minute to minute frequency regulation.

Economics matter

The biggest challenge to operating flexibly, however, is not regulatory or technical. “Technically nuclear plants can operate in a very flexible manner,” said Edward Kee, owner and principal consultant at Nuclear Economics Consulting Group. “The reason they don’t is economic. They have very large capital costs and fixed operating costs.”

Most U.S. nuclear plant were built so long ago that they have paid off their debt, though they can still incur capital cost of regulatory compliance for things such as updating equipment for license extensions. Generally speaking, nuclear plants have higher fixed operating costs than conventional fossil-fuel plants. In round numbers it costs about $35.5/MWh to operate a nuclear plant.

In wholesale markets where low natural gas prices often set the price of electricity, prices frequently fall below that level. Even more damaging, prices can turn negative.

Negative power prices can have several causes, but one of the chief contributors is wind power that earns a federal tax credit to produce electricity. Even if prices are negative, wind farms can still earn money. But conventional generation, especially nuclear plants that can’t ramp down quickly, end up paying if they continue to run.

That means that during negative price events, some nuclear plants are not only losing money, they are putting even more negative pressure on prices.

That is one of the toughest problems nuclear operators are trying to solve, and it could get worse as more renewable penetration continues to increase.

In this environment, some nuclear operators have begun to take a serious look at flexible operation. In September 2015, the Nuclear Energy Institute held a brainstorming session on the flexible operation of nuclear plants. And in 2014, the Electric Power Research Institute, an industry research group funded by electric utilities, issued a report laying on the flexible operation of nuclear plants.

“It is not a new conversation,” Sherry Bernhoft, program manager for long-term operations at EPRI’s nuclear sector and project manager for the report, said.

“The grid is changing; it is becoming very dynamic, and some members are evaluating this option. It is one of the tools that can help them manage their grid.”

In fact, market sources say Exelon is beginning to operate some of its nuclear plants serving the western PJM region flexibly.

Exelon was not able to respond to a request for comment by press time.

The Argonne National Laboratory is also in the midst of a study that models flexible nuclear operation using data representative for the Southwest United States. According to preliminary results from the study, in a scenario with wind and solar power penetration of 22%, it would be economic to operate nuclear plants flexibly, said Jesse Jenkins, a graduate student and researcher at the Massachusetts Institute of Technology, who is working on the study.

The study models a scenario with and without the federal production tax credit (PTC) for wind power and finds flexible operation is more valuable with the PTC, but that it is still valuable without the PTC.

The model includes a modest change in costs to operate a nuclear plant flexibly, but improves system costs by a couple of percentage points and increases profits for the nuclear plants by a couple of percentage points while reducing the curtailment of renewable resources. “It is a win, win, win,” said Jenkins.

One of the critical considerations in assessing the flexible operation of a nuclear plant is fuel costs. The treatment of fuel costs is very different at nuclear plant than at a conventional fossil fuel plant.

Nuclear fuel is relatively cheap on a per British thermal unit basis, but because it is bought in advance and can last for years, fuel costs for a nuclear plant are a fixed cost. That is not the case at a conventional plant where fuel is a variable cost and can comprise up to 75% of all costs. At a nuclear plant fuel is between 15% and 20% of costs.

Unlike a coal or gas plant, the fuel cost savings from ramping down a nuclear plant are minimal. So the operating costs remain the same and the income declines. “If all costs are fixed, you do not save money by lowering output, you only lose money,” said Kee.

Being able to ramp down a nuclear plant may mean that it would lose less money under certain circumstances. But in most circumstances ramping a nuclear plant up and down would be a money losing proposition.

But given where wholesale markets are heading and the market forces that nuclear plants are facing, flexible operation could be a tool that could enable operators to extend the life of some nuclear plants, especially plants operating in regions where prices turn negative. In those circumstances, flexible operation “could mean a nuclear plant could make more money by operating less,” Jenkins said.

Correction: A previous version of this post misstated the purpose of the Electric Power Research Institute. It is a research group for electric utilities, not their trade group. It also misstated how ramping by dumping steam leaves potential for xenon buildup. It avoids xenon buildup.