Graphic of the Week: Can nuclear plants load-follow?

German gas and nuclear ramp-up on solar record day 7.7.2013

This week the graph shows hour by hour fluctuation in German energy generation from two very different energy sources, nuclear and gas. It details the production (as a percentage of daily maximum) of these generators on 7th July 2013, a day that saw record solar production and hence a great need to throttle back existing generators to ensure grid stability. As you can see from the graph, nuclear plants in fact throttled back – and then up again – more than gas turbine plants. This is remarkable, because the conventional wisdom says that nuclear plants are completely inflexible, while “flexible” gas turbines are the necessary component of renewable energy system. At least in this case, the conventional wisdom was proved completely wrong. Although the above graph shows “relative” changes, nuclear power also provided far more actual throttling (in megawatt hours) than gas plants: 100% nuclear represents the generation of 8676 MWh, while 100% gas represents 842 MWh. Sadly, the data is no longer easily or freely available, but I can provide the spreadsheet if requested.

As the graph above shows, nuclear power can and does “load-follow” as the load in the power grid fluctuates. By doing so, it helps balance out large fluctuations in the production of variable renewables. So, far from being the obstacle to renewable deployment as often claimed, nuclear power seems to be a quite critical part of it.

The claims that nuclear cannot load follow and is incompatible with renewables are mostly fiction based on outdated regulations. In France, nuclear load following is daily routine; in Germany, it became legal in 2010. It is true that nuclear plants are not run in optimum manner if they are used to load follow; for several reasons, both technical and economic, stable generation should be the goal. But load following is definitely possible and may not even be that costly: even though France even shuts down some reactors entirely for the weekends, the French enjoy one of the lowest electricity rates in Europe. And, it needs to be reminded, some of the cleanest electricity anywhere.

More on the topic of nuclear load following here:

ANS Nuclear Cafe: Responding to System Demand II: Extreme Scenarios

About J. M. Korhonen

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5 Responses to Graphic of the Week: Can nuclear plants load-follow?

  1. A bit late maybe… but here’s a comment anyway. The comparison with “flexible gas turbines” can’t really be made here. As you can see from the much lower daily production, there were only very few gas-fired power plants online that day, which is usually the case when net load is low. Those still online are most likely smaller CHP plants that cannot be ramped up and down at will, as they are driven by heat demand.

    What it shows is that nuclear can be flexible, which is true. You would, however, almost always ramp gas and coal power plants down before you curtail nuclear power, as it is easier and cheaper.

    You can take a look at the German demand and power mix at, where you can see behavior like this quite often. You will also notice the about 2 GW of gas power plants that are always running, even if coal (which is supposed to be cheaper) is ramping down. That’s the effect of CHP. Over the year, the peaking gas and coal plants provide much more flexibility than nuclear.

    Also, keep in mind that the German nuclear phase-out is tied to the amount of energy produced by each plant. Each time a plant is ramped down, its operation time is prolonged by a few hours. german nuclear plant operators might use load-following to extend their plants’ life time, still hoping political change ending the phase-out will come while the plants are still operational.

    • Peter-Philipp,

      thanks for your comment and apologies for the delayed reply. I believe you are correct in your assumption that the gas plants in the figure are “slow” CHP plants that cannot load follow as fast as those plants that purely generate electricity.

      You are also quite correct in your other points as well. It´s certainly true that one would wish to ramp gas and to less extent coal plants before nuclear plants (although the situation is complicated because older coal plants in particular are not very flexible due to design limitations). However, the primary goal of this post is simply to demonstrate that the oft-heard argument about nuclear power being fundamentally incapable of load following is simply not true.

      Of course, for those who really are interested in energy systems and are not just advocating against nuclear, this would be old news – France, for example, has been load following with nuclear plants for decades. Unfortunately, I still regularly encounter disinformation that claims nuclear plants simply cannot load follow.

      Thank you also for the comment regarding the particulars of German nuclear phase out. I didn´t think of that before but it clearly makes nuclear load following much more attractive to operators.

      • You are completely correct. There are a lot of valid points critics of nuclear power have – however, lack of ramping capability is not one of them.

        I just wanted to make a point about context – people are easy to jump the gun and claim that nukes are wilfully compared to “slow” gas power plants here to detract from their “lack of flexibility.” Which leads to your point about coal: Modern pressurized water reactors are still slightly slower and less flexible than open gas turbines or multi-mode CCGT, but they are one hell of a lot faster than old, baseload-optimized lignite or hard coal fired units.

      • Good point. I did not want to make a biased comparison, and I was genuinely surprised about the results, as it’s well known that open and modern CCGTs are very fast to load follow.

        By the way, some modern nuclear plants are effectively as quick or even quicker than gas turbines, at least over certain power ranges. I seem to recall the EPR being promised something close to 5% per minute ramp-up/down rates (can’t recall the power range though); with 1600 MW(e) rated power, this translates to rather significant ramping speeds.

        Thanks again for your comments, this has been a valuable discussion for me.

  2. Pingback: 100% renewables and 100% nuclear are both practically impossible | The unpublished notebooks of J. M. Korhonen

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