Can high levels of solar and wind coexist with nuclear generation on the grid?


Solar and nuclear: Can high levels of solar and wind coexist with nuclear generation on the grid? The answer is more complicated than you might think.

In 2015, Électricité de France put out a pamphlet titled Flexible nuclear generation to foster the development of renewable energy as one of its “50 Solutions for the Climate.” The pamphlet boasts of the flexibility of EDF’s nuclear fleet, showing a 1.3 GW nuclear power plant increasing and decreasing its output by 70% within 30 minutes.

The documentation centers on the Golfech plant, and shows these impressive feats of rapid ramping, with two cycles up and down within a 24 hour period. And while EDF noted that such cycling was prompted by daily changes in demand, it also stresses that such capabilities will be able to make nuclear power a good complement to the fluctuating output of wind and solar on the grid. However, in Northern Germany a situation has played out which challenges this line of reasoning. In February the Brokdorf nuclear power plant was taken offline after damage to its fuel rods was found. According to a local nuclear supervisory authority, the operation of the plant in “load-following” mode had contributed to unexpected oxidation of the rods. As of July, the plant was operating in “safe mode,” and politicians from Germany’s Green Party are calling on a Swiss reactor near the German border with similar problems to be shut down.

These two examples give very different accounts of whether or not nuclear power is suited to accompany high levels of wind and solar generation. The problem at Brokdorf comes as the U.S. nuclear industry is making a new push for subsidies for aging and uncompetitive plants, and as some U.S. energy journalists are promoting “flexible” nuclear as a solution to accompany high levels of renewable energy.

So can nuclear accompany high levels of renewable energy? This is a technical and economic question, and one that has ramifications for the future of the technology.

Wind, solar, and flexibility

Wind and solar are variable and intermittent. Their output varies not only seasonally, but from day to day and hour to hour. So does electricity demand, and the result is that a degree of flexibility has always been needed in power systems to adapt to changes in demand, including when people go to and from work, when factories shut on and off, and when weather leads large numbers of people to turn on and off air conditioning and electric heaters.

And while low-to-moderate amounts of solar typically reduce peak demand for electricity, much more flexibility is needed on power systems that have very high levels of wind and solar. In California, which currently gets more than 10% of its electricity from solar on an annual basis, other forms of generation need to supply an additional 10 GW of electricity over a three hour period to meet both the increase in evening demand and the fall-off in solar production when the sun goes down.

This means that come 5 p.m. gas-fired power plants across the state are increasing their output, which is called “ramping.” As the state puts more solar online to meet its 50% by 2030 renewable energy mandate, it will need even more ramping. Brendan Pierpont, an energy finance consultant at London’s Climate Policy Initiative, has worked on reports that model these details. He estimates that under a situation where 35% of California’s electricity comes from solar, during the most challenging day of the year, enough resources to meet roughly half of peak demand will need to ramp over a one hour period, and 80% during three hours.

California’s sole remaining nuclear power plant, the Diablo Canyon, does not ramp down during the middle of the day, despite frequent and recurring situations of negative prices, and even during days when a portion of the state’s solar output is curtailed on a system-wide basis. This is not unusual for the U.S. fleet, which typically runs 24/7 aside from times when individual reactors are shut down for maintenance and refueling.

Conditions are different in Europe. Nuclear power plants are regularly ramped up and down in France, to partially respond to the shift in electricity demand from day to night. Additionally, in other nations plants such as the Brokdorf facility are ramped to respond to fluctuations in wind and solar generation, although the vast majority of nuclear power plants are not.

Ramping, start-up and shut-down

The nuclear industry claims that all currently deployed boiling water reactors (BWR) and pressurized water reactors (PWR), which make up the entire nuclear fleet in the United States and the majority in Europe, can ramp quickly. The industry cites a ramping rate of 5% per minute between 50 – 100% of rated power, while EDF states that its reactors can ramp up to 80% up or down in 30 minutes, which may be due to changes in the control rods in French reactors.

Meanwhile, a 2010 report by Austria’s Ökologie Institut describes a mechanism whereby frequent ramping deforms the plastic on control rods, with potential cracking if the power increase is too large. In the case of the Brokdorf plant, safety inspectors attributed accelerated oxidation of the plant’s rods to ramping.

But whatever the full extent of the impact on control rods, this is only one of the components subject to increased wear. Ökologie Institut’s NPP Output Flexibility states that a reasonable conclusion is that ramping wears out entire nuclear power plants faster. “One can indeed assume that because of frequent load-following cycles, thermal stresses, fatigue, and mechanical constraints, flexible [nuclear power plants] NPP are likely to age quicker than those operating at base load,” reads NPP Output Flexibility.


Regardless of who is closer to the truth as to the impact of ramping on the life of plants, it is clear that there are economic impacts to ramping nuclear plants. Some of these come from the increased wear. EDF, as cited by Ökologie Institut, is reported to have found that running plants at partial load increases unscheduled outages, at a cost of several million euros.

However, the economic consequences of ramping are not confined to wear on the plants. Compared to other forms of generation, particularly gas plants, nuclear power plants are characterized by high up-front costs and low fuel costs, with a mid-range of capital costs six times as high as that of CCGT (and an even greater differential for other gas turbines). Ramping down affects the profitability of projects, and lengthens the timelines for paying off initial costs. The economic impact of reduced output is more severe for nuclear power than for fossil fuel generation.

This is not a trivial matter. Other than “peaking” gas plants and diesel generation, new nuclear power plants are already the most expensive form of conventional generation per unit of electricity, with consultancy Lazard estimating a levelized cost of electricity of $97-136 per megawatt hour. In fact, the record of nuclear reactor construction start dates shows that the fall in global nuclear construction preceded the Chernobyl Disaster, and writers such as Craig Morris have concluded that it was cost, not safety concerns, which prompted the global move away from nuclear in the 1980s. If nuclear power plants are expected to produce less power and generate lower revenues, this further undermines an already precarious position.

Oil and water

Within certain ranges and limitations, nuclear power plants can ramp quickly. Whether this is sufficient for the needs of the energy transition is unclear, but it is possible that such plants could accommodate moderate levels of solar by ramping down in the morning and up in the evening, as part of a larger portfolio of different types of generation. For very high levels of PV, the picture is less clear, and there is no track record to go on.

There are strong reasons why nuclear power plant owners will work against plants being operated regularly in this manner, and the economics of building new nuclear power plants for load following are dismal. The nuclear industry has been forthcoming about the fact that there is an economic disadvantage to ramping nuclear generation. There are clearly technical issues as well, which at least further undermine the economics of ramping nuclear power plants, and may add additional safety concerns.

While developed nations should prioritize rapid decarbonization over short-term costs, there is not now and never will be an unlimited amount of money to pour into this problem. The nuclear industry knows this, and as such the attempt to cast nuclear power plants as a suitable accompaniment to high levels of wind and solar is ultimately a desperate act by an industry which is in severe crisis in both Europe and the United States.

Nuclear reactors may be able to ramp (within limitations), but ultimately nuclear is fighting for space on the grid with wind and solar. As such the building of new nuclear power plants, and in some cases the extension of licenses for old ones, can limit the transition to renewable energy