Energy & Climate Change

Nuclear power is frequently being used for load following in the electricity market in France and Germany. Photo by iStock

The role of nuclear power in a changing energy market

According to a persistent myth, nuclear power is only suitable for stable production of “baseload” electricity. However, nuclear power can also be used for load following, meaning it can react and adapt power output according to demand.

In my earlier article I discussed electricity markets and the possible future situation where solar PV and wind power shares in the electricity grid increase significantly. If, and when, the use of variable power production increases, the need for flexible power plants that can “load follow” demand, also increases.

Mostly we use hydro power for load following, if it is available. If not, we often use combustion-based generation such as natural gas turbines. But the use of natural gas and other combustion-based sources should decrease radically if we plan to reach our emission targets and mitigate climate change effectively.

What about nuclear power?

In the power grid, demand and production must stay in balance every moment. The persistent myth about nuclear power is that it can only be used for stable “baseload” production. From this, some people have drawn the conclusion that nuclear power isn’t compatible with variable wind- and solar power in the same power grid.

This conclusion has two faulty assumptions. First, there is a weird “master and slave” assumption. The problem with variable renewable energy sources – the fluctuation of their production – is projected onto other energy sources. Wind and solar power production are seen as “the masters” while the other sources are their servants, picking up the bill when the night is done.

This “master and slave” thinking leads to an even stranger follow-up situation, which I once heard during a talk with a representative of Greenpeace. The idea was that rather than to have some reliable baseload production in the mix with nuclear, it would be better to add more wind power on top of the existing wind power. This would lead to even more variable power production, which increases the costs of maintaining a functioning power grid considerably.

The figures below illustrate the situation. Both depict a situation where roughly the same amount of energy is produced during a period of two weeks. The combination of nuclear and wind power is reasonably stable, but the power production in the wind only option fluctuates between one and nine gigawatts. Adapting a power production profile like this to meet the demand of industry, businesses, services and households is much more complicated compared to a more stable power production solution.

Figure 1: Combined energy production of nuclear and wind power with a resolution of 3 minutes from the first two weeks of October 2019. Output fluctuates roughly between three and four gigawatts due to the variability of wind power in the mix.
Figure 2: Roughly the same amount of energy as in figure 1 produced with wind power only by multiplying wind power production output by six. The diagram is only an example, as in reality the production would be a bit more stable due to wind turbines spread more in different parts of Finland.


Nuclear can be used for load following

Another faulty assumption relates to the perceived inflexibility of nuclear power. Nuclear can be used for load following and is already used for this purpose in Germany and France as part of normal operations. Nuclear has a share of roughly 75% of France’s electricity production. When the demand goes down in the weekends, some reactors are simply shut down.

In the end, the feasibility of this comes down to cost. The average cost of nuclear electricity increases if the plant is closed or operated at a reduced output. The fixed costs of running the facility stays constant, while the saving in reduced nuclear fuel use are not significant. If we consider the low electricity consumer prices in France, this extra cost seems relatively reasonable. In addition, the electricity production in France is one of the cleanest among industrialized countries.

"The need for flexibility in power output has already grown as the production of variable renewable energy such as wind and solar has increased."

In Europe nuclear plants are licenced either for baseload production or for load following, and changing this status afterwards is not that simple. For example, in Finland all nuclear plants operate as baseload plants, whereas in France and Germany they are often licenced for load following.

New 3rd generation nuclear plants, such as the EPR of Olkiluoto starting its production soon and the VVER1200 being built in Hanhikivi, have even better load following capabilities than the older generation plants of the current fleet.

Small and flexible

Flexible load following has been a special consideration in the designing of the next generation of small modular reactors (SMR). Their size is optimal for adapting to quick changes in power production. For example, the US-based SMR developer NuScale has designed reactors which can adjust power output either by using control rods or with a turbine bypass which adjusts the amount of steam going into the turbine. This way the power output of the turbine can be adjusted very quickly. What happens to the bypassed steam depends on the design of the nuclear plant. For instance, the steam could be used in a district heating network if one is available.

In some molten salt reactors under development, such as those by the UK based Moltex and Canadian Terrestrial Energy, the power plants can store high temperature heat into molten salt. The same method is used in concentrating solar power plants. The energy stored in the molten salt can later be used to generate electricity during higher peaks of power demand. This enables flexible load following without much losses in power production, and the costs of load following remain low.


Figure 3. Flexible load following has been preliminarily modelled in the New England area power grid in the United States. In the model, approximately a ten percent share of flexible nuclear power was added to the NREL 2034 high renewable & low-cost gas -scenario which included a lot of wind and solar power in addition to cheap natural gas. The added flexible nuclear capacity is marked with a red circle. Source: LucidCatalyst Ltd.
Figure 4. The consequences of adding flexible nuclear production to the amount of energy produced. As can be seen, flexible nuclear replaces most of the natural gas production and its emissions. The production of renewable energy stays the same.

Is load following with nuclear economically feasible?

If lowering a power plants output doesn’t reduce its overall costs, there is no economic reason to reduce power below the maximum output. The only exception to this rule is a situation where the price of the electricity becomes negative, meaning that the producer of the electricity has to pay to produce electricity to the grid.

This is the case with wind, solar and nuclear. There are no overall cost savings even if production is reduced. However, if there is a need for this type of service, it should have some market value.

The need for flexibility in power output has already grown as the production of variable renewable energy such as wind and solar has increased. The generous subsidy policies have favoured them, leading to massive deployment and reduced costs for these technologies. Some countries have implemented different kinds of “capacity markets”, where gas or coal plants are paid for being “on call”, ready to ramp up production as needed.

As these fossil fuel plants are approaching the end of their operative lives, it is possible that in the future some nuclear plants can be used for load following. They could adjust their power production to meet demand peaks at least to a certain extent. Some of their production could be directed to some other valuable use, such as heat storage or the production of clean hydrogen. All this would reduce the costs of flexibility.

The question is, who pays the extra costs of added flexibility and on what grounds remains open? One option would be to use the “polluter pays” -principle, where the cost would be paid by the one who causes it. But finding and defining the cause of the cost is not necessarily all that simple.

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