If I understand it correctly fukushima disaster happens because reactor lost all sources of power and was not able to cool down. But why they didn’t just restart the reactor? Reactor had shut down automatically as a safety measure in case of earthquake, but it was still functional. Just restart it and use energy from reactor.
In: Physics
Most grid scale power, with mostly the exception of photovoltaic solar and wind, is generated by heating up water, creating steam and using that to turn a turbine which generates electricity. Starting a power plant, be it oil, coal, gas or nuclear, requires power. There are computers to monitor and control systems, pumps for moving the water that actually turns the turbines, motors to feed the fuel into the generator, safety equipment, lights and tons of other stuff. To start a power plant from nothing without external power is called a black start and that requires onsite generators* that can run at least most of those systems at some level while the power generators come on line and synchronize with the rest of the grid (if there is anything, there wouldn’t be in a black start).
In the case of Fukushima, the external power and generators were both knocked offline. Without power, the pools of water that house and cool the fuel rods were not being circulated and replenished by pumps, so they got hot and the water boiled off. Without water to cool the rods they got really hot and melted. When the water is boiling off and the rods are partially exposed hydrogen can be formed from the water and that can build up lead to explosions which breach the containment vessels that are designed to keep the radioactive stuff in the power plant.
* I’m not certain the generator capacity at Fukushima, even if it was working, was large enough to perform a black start. I believe in a true black start scenario smaller and easier to start gas and oil power plants are brought online first to give the larger plants enough power to start up. The generators at Fukushima may have only been sized for emergency off grid maintenance of the facility not a restart.
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