Nuclear fuel comes in the form of enriched uranium, which naturally produces heat as uranium atoms split (a similar process is used in Nuclear Bombs).
This heat is used to boil water, which then produces steam that drives a turbine, which then spins a generator to create electricity; the steam is then cooled and fed back through the system in a cycle. The amount of heat generated by the reactor is so vast that it needs a constant flow of water in order to ensure the fuel rods (made of uranium pellets) don’t overheat and therefore melt down which would be catastrophic.
The temperature is also controlled by aptly named control rods, which control how much heat is generated by the fuel. When fully inserted they have essentially shut down the fuel rods, when pulled all the way out the fuel produces maximum heat. Now when the earthquake hit the control rods automatically inserted fully into the fuel rods and the plant went into shut down. Done! Well not quite….
Uranium fuel doesn’t have an off switch and even in shut down continues to produce heat for quite some time and therefore needs the cooling system to continue operation to prevent meltdown. Obviously this was already considered and the plant has back up systems in case the electricity to the pumps supplying water was ever cut.
1: Their main electricity source
2: The main grid can then supply the power
3: If that fails they have back up diesel generators
4: If they fail they have battery-powered pumps that can last several hours until electricity is restored.
When the earthquake hit the worst-case scenario unfolded. The plant went into shut down and therefore their electricity supply went too. Next the main power grid became unstable and therefore also shut down. Then the tsunami hit and destroyed the diesel generators, which were not high enough to withstand the force or quantity of water. The batteries were therefore the last source of electricity but only lasted a few hours. More generators were sourced but weren’t hooked up in time and without power to the pumps the fuel rods began to overheat. This cracked the metal tubes holding the uranium, exposing the fuel to water where it began producing hydrogen gas. This is called Thermolysis! As you are probably aware, hydrogen is highly explosive and as the pressure built up we saw a series of explosions. These were hydrogen explosions not nuclear explosions.
In a last ditched attempt to contain the situation they flooded the reactors with boron infused seawater. The boron similar to the control rods acts as a coolant to the fuel. However flooding the reactor has now permanently damaged the plant but this was a better alternative to a complete meltdown, which would have been catastrophic.
The uncertainty now comes in Japan’s future supply of electricity. About half of Japan’s power came from these plants damaged by the tsunami and they are now potentially permanently shut down. Building a new plant may take a decade.
To put it in perspective, Three Mile Island disaster was a level 5 incident… this is level 6, Chernobyl was a 7.
So there it is: the not so simple answer to why you can’t just turn a power station off.
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