Sizewell C nuclear plant isn't perfect. But it's a necessary step if we're to avoid 'dunkelflaute'

Imagine its 2035. Every other car you see on the street is electric, millions of homes have heat pumps providing heat, large data centres the size of ten football pitches are dotted across the country – this has caused the electricity demand to surge by more than 50% over the last decade.

The good news is that a lot of this is powered by renewables, after an aggressive strategy to build huge offshore wind farms and covering large tracts of brownfield land with solar panels over the past decade.

Now also imagine a cold, windless Christmas evening that year, millions of ovens are on, heat pumps are whirring and energy demand is outpacing renewable power supply. The Germans call it dunkelflaute, a long lull of dark windless days. 

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Ask anyone of the very smart people managing our electricity grid today and they will tell you that this is what concerns them the most. How do we prepare for the dunkelflaute and how do we do so without paying through our noses and without burning more fossil fuels? This is where nuclear comes into the mix. As a steady, relatively reliable source of energy, nuclear is a useful insurance policy.   

The UK government’s recent approval of Sizewell C with a massive downpayment of £14.2 billion is a preparation for the predicament I described above. For an insurance policy it is rather expensive and in fact, the real costs are still unknown and considering the nuclear industry’s history of cost overruns, one can safely assume the numbers will be higher.

But these plants will be around for several decades and are projected to provide 15-20% of our electricity over that period. Unlike gas power stations, they also operate without the volatility of fossil fuel markets or the carbon emissions that worsen the climate crisis. In this sense, nuclear can help balance and stabilise a zero-carbon grid.

The opposition to Sizewell C goes beyond the usual suspects. Some Conservative backbenchers oppose it on cost and local disruption grounds, particularly the impact on Suffolk’s protected natural areas and water supply. The Green Party, meanwhile, opposes it on principle – arguing that nuclear’s long build time, radioactive waste legacy, and poor cost competitiveness make it a distraction from renewables. These are not fringe objections.

They reflect legitimate concerns about nuclear’s place in the energy mix when solar and wind are already so cheap. The reality however is that our energy system must be built for the worst case scenario, not the best case. Some have argued that batteries can provide the backup needed when dunkelflaute hits but long duration energy storage is still several years away and we not yet in a position to deploy such storage solutions to avoid nuclear altogether. 

Sizewell C won’t be operational until mid 2030s, meaning it won’t help meet our 2030 climate targets. Its financing model places long-term costs on consumers via a regulated asset base (RAB) model, where customers begin paying before any electricity is generated. This departs from how renewable projects like offshore wind are financed, and certainly raises questions about fairness and affordability. But in the world of energy policy, cost is only one priority and ensuring energy security and decarbonisation are equally important. So on balance, the investment in Sizewell C is defensible. 

Alongside Hinkley point C and a supposed wave of new smaller nuclear reactors, there is no doubt that the UK is locking itself into a decades-long commitment to a technology that is expensive and contentious – but it is also carbon-free, dependable, and increasingly necessary as our carbon budgets tighten. The task now is to ensure that Sizewell C is not a diversion from faster, cheaper climate action, but a complement to it. That means accelerating renewables, reforming the grid, investing in energy efficiency, and developing the skills and supply chains for a broader green industrial strategy.

Chaitanya Kumar is head of economy and environment at the New Economics Foundation.

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