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
Start the reactor up? With what power? Power from the outside grid was lost, and the backup generators were damaged by the flood and non-functional.
Essentially, the plant would need power before it could start making power.
https://en.wikipedia.org/wiki/Fukushima_nuclear_accident
Some of the failed reactors were restarted, but due to damage to the facility had been without power for extended periods. Even when restarted, lots of the supporting systems to manage the reactor were damaged, too – this meant the reactors didn’t work as designed and/or it was too dangerous to run the plant as normal.
At one point the workers vented a reactor by opening its safety relief valve using electricity from strung-together scavenged car batteries. The plant was absolutely wrecked.
Imagine that you’re sick and driving to the hospital but then get into a car accident and the front of the vehicle is crushed. Your life may depend on restarting the vehicle. But can you? It’s possible that trying to do that could be a danger to yourself and other people. What if it’s leaking gasoline or oil and goes up in flames while you’re sitting there? And if you can get it started, who’s to say the car won’t fail again due to the damage? All the options to deal with the problem suck.
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.
While it might seem logical to attempt to restart the reactor after it had automatically shut down, several significant challenges made this option unfeasible:
1. Loss of External Power: The primary reason for the cooling system failure was the loss of external power caused by the tsunami. Without external power, the plant’s cooling systems, including pumps and generators, could not function properly. Restarting the reactor would not solve the problem of the lack of power to operate the cooling systems.
2. Damage to Infrastructure: The tsunami caused extensive damage to the infrastructure of the Fukushima Daiichi plant, including flooding and destruction of critical components. Restarting the reactor would have been impossible without first restoring the damaged infrastructure.
3. Safety Concerns: Even if power could have been restored, there were concerns about the safety of attempting to restart the reactor amidst the chaos and uncertainty caused by the natural disaster (the extent of damage to the reactor and surrounding infrastructure was not fully known at the time)
4. Core Meltdown: More importantly, the inability to cool the reactor cores led to a meltdown of fuel rods in several of the reactors. Once a core meltdown occurs, restarting the reactor becomes impossible, as the damage to the reactor is severe and irreversible.
The issue is what’s known as a “black start”. Power stations need power to turn on. Some smaller power stations have generators, or can be started mechanically but typically nuclear plants can’t do this.
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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