The sixth and final operating reactor at Ukraine's Zaporizhzhia nuclear power plant was shut down on Sunday to lower the risk of a radiation disaster amid the continuing fighting between Russia and Ukraine.
The move became possible after the plant was partially reconnected to Ukraine's power grid, following weeks of disconnections and reconnections caused by heavy shelling in the area damaging power lines.
The facility had been isolated from the power grid and operating in "island mode" since September 5, meaning only one reactor remained operational, generating power to run the cooling systems and other crucial processes for the rest of the plant.
Fighting near the plant, one of the 10 biggest atomic power stations in the world, fuelled fears of a disaster like the one that took place at Chernobyl in northern Ukraine in 1986, when a reactor exploded and contaminated a vast area that remains unsafe to live in.
So, are we out of the woods yet?
While Zaporizhzhia's reactors are protected by a reinforced containment shelter that should withstand being hit by a shell or rocket, a disruption in the electrical supply to the plant risks knocking out its cooling systems, which are essential for the reactors' safety — even after they have been shut down.
Nuclear reactor cores continue to emit radioactivity and generate heat long after they have finished operating, meaning systems that circulate water through the cores to cool them are required to stay operational for extended periods of time.
The risk of a full-scale meltdown diminishes over time once reactors cease operating, meaning Zaporizhzhia's "cold shutdown" is a significant risk-reduction strategy — but the danger is far from over yet.
Further damage to the plant's electricity supply, a high likelihood given the continued fighting, would require the plant to begin running its safety systems using emergency diesel generators.
The chief executive of Energoatom, Ukraine's nuclear operator, on Thursday told the Associated Press the plant only had enough diesel fuel for 10 days.
Operating a nuclear power plant safely also requires cooling the plant's spent nuclear fuel, which is thermally hot and highly radioactive for years after it is removed from the plant's reactor.
Fuel rods are stored vertically in racks called assemblies, which are cooled in steel-lined concrete pools of water for several years before being transferred to dry storage facilities.
Zaporizhzhia has a single dry fuel storage facility on-site and spent fuel pools at each separate reactor site
While those pools are located inside the plant's containment area, a serious reactor mishap would likely affect the pools as well, as fresh water must continually be added to offset evaporation.
Failure to do so could result in a fire of irradiated uranium, potentially releasing radioactive isotopes into the atmosphere.
A 2017 submission by Ukraine to the International Atomic Energy Agency (IAEA), the UN nuclear watchdog which currently has two experts at the plant, listed 3,354 spent fuel assemblies at the dry spent fuel facility and 1,983 spent fuel assemblies in the pool.
That's a total of more than 2,200 tonnes of nuclear material on site, excluding the reactors and the material produced in the years since.
Who's in charge of preventing disaster?
The Zaporizhzhia power plant has been under the control of Russian forces since the early days of the war.
Ukrainian staff continue to operate it, but special Russian military units guard the facility and Russian nuclear specialists give advice.
The IAEA has warned that the staff are operating under extremely stressful conditions.
If there were a nuclear accident, it is unclear who would deal with it during a war, said Kate Brown, an environmental historian at MIT who wrote a book documenting the full scale of the Chernobyl disaster.
"In 1986 everything was running as well as it ran in the Soviet Union, so they could mobilise tens of thousands of people and equipment and emergency vehicles to the site.
“Who would be taking charge of that operation right now?”
IAEA director-general Rafael Grossi has called for the establishment of a safe zone around the plant to avert a disaster, but the fighting has continued, with each side blaming the other for continued shelling.
