Spain's grid operator restarts nuclear plants after blackout

Grid operator restarts nuclear plants and 75% of Spain's power is now renewable

The blackout left an awkward picture and, two days later, a paradox: the nuclear plants the government wants to shut down have had to be started up again to keep the grid stable. Of the 7 GW of installed nuclear capacity, barely 1.4 GW was running at the most critical moment. The rest remained offline. Meanwhile, renewables were generating 75% of all national electricity.

The sequence is the key. The nuclear plants shut down under safety protocol — any wobble in their operation, the argument goes, can lead to an emergency with prolonged consequences — and they do not come back to full load overnight. Start-up is slow and measured, with ramps counted in days. That technical detail, which takes up two lines in an operating manual, is what has blown up the week's political narrative.

What peine to nuclear power during the blackout

On the day of the incident, nuclear generation was contributing 3.4 GWh of the 25 GWh total, with only 0.9 GWh of combined-cycle gas in the mix. In the snapshot after the blackout, the split was different: 4 GWh of combined cycle and 1.4 GWh of nuclear out of some 18 GWh of total output. The conclusion drawn from those numbers is uncomfortable for everyone: gas is filling the gap left by nuclear, not the one left by the sun.

Some argue that the underlying problem is that renewables cannot be the majority system without firm backup. Against that weighs the opposite argument: on the day of the blackout renewables were covering most of demand and it was precisely the gas and nuclear plants that went down or could not hold frequency. The debate, in reality, is not about ideology but about rotational inertia, a concept almost nobody uses on television and which explains almost everything.

Why a nuclear plant takes days to restart

A nuclear plant is not a light switch. After a shutdown, the reactor enters a process known as xenon poisoning: accumulated fission products absorb neutrons and block the chain reaction until they decay. That phenomenon means waiting days, sometimes more than a week, before power can be ramped up. It is not a political decision or sabotage: it is physics.

That timetable explains why a week after the blackout only 1.5 GW of the 7 GW installed was operational, according to the figures being handled. It also explains why a reactor at Almaraz that had been shut down — because the electricity price did not make it worthwhile — had to be asked to start up again. The paradox is rich: they are turning to the plants whose closure is planned for two years' time.

The 75% renewable figure nobody wants to repeat

The figures from the day after the blackout are the ones circulating most. Renewables generating ten times more energy than nuclear, with solar output peaks of 10 GW and combined cycle contributing 4 GW, with a small uptick of 5 GW. Nuclear, meanwhile, at a token 1.4 GW. The picture is sharp and each side reads it its own way.

The most optimistic calculation holds that the system held up precisely because renewables were there. The pessimistic scenario starts from another base: that renewables are being deliberately curtailed and a minimum of rotating generation — combined cycle — is being kept to avoid running out of inertia again. Both things can be true at once, and probably are. The full breakdown of the generation charts, hour by hour, leaves differences that surprise anyone looking at them for the first time.

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What is clear is that the Spanish electricity system has been through a week of forced learning. Nuclear plants are coming back gradually, gas is holding frequency and renewables keep setting records. Nobody has yet explained, with names and documents, what exactly triggered the blackout. And until it is explained, every chart will be a battleground.