UK Electricity: Renewables and the problem with inflexible nuclear via

In recent years, the share of the UK’s electricity supplied by renewable energy (RE) sources has increased substantially to the point that RE is now the second largest source after gas: It now supplies 20% to 25% of our electrical needs. This is greater than the amount supplied by nuclear – about 15% to 18%. Coal, hydroelectric, and mainly gas (~40%) constitute the other sources. See chart for Britain’s electrical power supplies in 2019.

Why are AGR reactors inflexible?

Before his untimely death in 2018, the nuclear engineer, John Large, explained that while advanced gas reactors (AGRs) were considered safe and reliable once they were up and running, they were difficult to control, ie less safe, when ramping up and down, especially in comparison to pressurised water reactors (PWRs). (PWRs were originally designed for flexible use in US nuclear submarines.)

For this reason, in the 1970s and 1980s, the former CEGB set the pattern of a nuclear “base load” in which its Magnox and AGR reactors operated flat out most of the time. Essentially this pattern is still adhered to today by the National Grid[1].

Difficult situation re nuclear and RE

The National Grid keeps supply and demand balanced in real time to prevent blackouts, such as occurred on August 9 2019, when a million UK homes were cut off. But when demand is low – as in recent months during the pandemic – it is difficult  to have all nuclear and all RE sources running at the same time as the Grid would end up with too much electricity. To avoid this, the Grid requests utilities to shut off their supplies and makes “constraint” payments to those who do so. A complicated reverse auction system exists for these payments in which operators bid as low as they feel able in order to secure such payments.

Because of the inflexibility of the AGRs, RE suppliers are shut off first. This is explained in a recent report by the newly-formed pressure group, 100percentrenewableuk, which explains that the inflexible nature of nuclear power is instrumental in forcing the National Grid to turn off large amounts of wind power (ie in the jargon to be ‘constrained’) in Scotland when there is too much electricity on the network.

This means nuclear reactors are also mainly responsible for the large constraint payments paid by the National Grid to wind farms to be turned off. These compensation payments are eventually paid for by all electricity consumers in the fixed element of their electricity bills.

The problem is that these constraint payments are now very large, For example, National Grid ESO, the UK system operator expects to spend an additional £500 million to balance the grid over the course of the 2020 summer, much of it in payments to wind farms to stop generating. In total, National Grid expects to spend £826 million to balance the grid in 2020.

This appears nonsensical as the Grid is turning off cheap renewables to preserve expensive nuclear, and then paying large compensation payments to them to do so. One wonders what OFGEM makes of this? As pointed out by the National Audit Office, this problem will get even worse if Hinkley C were ever to be allowed to finish construction and allowed to operate.

The situation has recently become so problematic that the Grid has been forced to request EDF Energy to shut half the generating capacity of its nuclear reactor at Sizewell in Suffolk. See “More than” £50 million is to be paid to EDF just to reduce the output from Sizewell to avert the risk of blackouts this summer.

One surmises that the reason the Grid and EDF chose the Sizewell reactor to restrict is its large capacity (1200 MW). It is by far the largest reactor in Britain (the remaining AGRs are about 460 MW), and this presents a problem. If it failed or quickly went off-line (eg it scrammed its control rods for safety reasons), the sudden loss of 1200 MW in supply would present severe problems for the grid. It could result in a drop in frequency triggering other plants to fail. This happened after the simultaneous failures of two (non-nuclear) power stations in August 2019. One shudders to think what would occur if Hinkley C (2 x 1600 MW) were ever to operate and it failed.

Can we manage the intermittency of renewables and attain 100% renewables?

Yes. In fact, many ways are possible, including

  • improved resource and weather forecasting
  • interconnecting the grid over larger UK regions
  • digitally-controlled smart grids giving better control of demand
  • power storage, in the form of pumped hydroelectric dams, dedicated batteries and electric car batteries
  • the increased use of the many existing interconnectors with Europe
  • the increased use of smart wind turbines, and
  • the use of heat pumps, heat batteries and hydrogen fuel cells.


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