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
So some people have mentioned other good reasons I’m going to mention a more nerdy one, xenon poisoning.
In nuclear reactors, the power output is more or less proportional to the amount of neutrons going around. The goal of a reactor is to balance the amount of neutrons, not enough and you just don’t get much power, too much and you end up making so much energy so fast it has a meltdown.
Neutron absorbers can be a part of the equation, for example control rods are made of the stuff, we want to have a way to slow or shut down the reactor, rods all the way in will do that.
Xenon-135 is one of the best neutron absorbers we know of. And its unfortunately produced slowly from byproducts in a reactor core.
Under normal reactor operations, it gets “burned off” pretty quickly, its creation rate is nowhere near the neutron creation rate and since it can only take one neutron per atom. But when the reactor is shut down there’s a different story.
In that case, Xenon can build up in the reactor core cause it keeps being produced after shutdown and you can’t really do anything about it but wait it out. Xenon does decay with a 9 hour half life so given two days it will mostly be gone.
So with Xenon poisoning taking hold it becomes very hard to restart the reactor for a bit, you usually need neutrons to make more neutrons, the chain reaction is part of why it works, and if you don’t have enough neutrons to deal with both the xenon and keeping the reactor going it just wont work.
You can try to force your way through it of course if it hasn’t been too long, but the longer you wait the riskier it gets. You can try to get your reactor to produce maximum neutrons, there are a number of levers you can use to do that. The problem is that you risk issues the moment after the Xenon is out and your reactor is at full throttle… that’s like a car on a treadmill, sure you can floor the gas pedal enough to get off the treadmill and get your wheels spinning fast but the moment your rear wheels are off you are going to accelerate at max speed into the nearest wall in front of you.
This is exactly what happened at Chernobyl. They shut down the reactor for a bit then tried to bring it back up cause the power guys in Kyiv called them up and asked them for more power. And we know how that ended.
The Fukushima guys had an hour probably to determine if the reactor should be restarted before Xenon poisoning would’ve made things too risky, at least that’s my armchair nuclear engineer logic. One hour to determine that their systems worked and that the earthquake didn’t cause any other issues.
Well that’s not a risk we are willing to take. And this is why most protocols involve just shutting it down.
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