The last of four regular power lines supplying the Russian-held nuclear power plant at Zaporizhzhia in Ukraine was briefly cut on Thursday, meaning the plant had to resort to backup power for the first time, thinning its defences against a meltdown.
In all nuclear power plants, fission of nuclear fuel gives off heat to then produce steam that turns turbines to generate electricity. That process must be controlled to prevent a runaway chain reaction that would overheat and in the worst case cause a meltdown like those at Chornobyl or Fukushima.
