Nuclear Reactor Fail-safe
In this video, we explore two different fail-safe designs used with nuclear reactors.
Out of the Skies, Under the Earth by Chris Zabriskie is licensed under a Creative Commons Attribution license (https://creativecommons.org/licenses/…)
When you think of a system being “fail-safe” you may imagine lawnmowers with a hand-closed switch that has to be held down at all times, and if let go of, the blades stop rotating. Or you may think of the way that automatic doors at shopping marts can still be pushed open by hand in the case of power loss.
But can something as powerful and potentially dangerous as a nuclear reactor be designed in a way to fail-safe?
Nuclear reactors work by fission. Special atoms, such as Uranium-235, are bombarded with neutrons. When a uranium atom absorbs a neutron, it becomes unstable and breaks apart into lighter atoms plus free neutrons. These free neutrons bombard other nearby uranium atoms, and we have a fission chain-reaction on our hands.
Both nuclear weapons and nuclear reactors rely on this chain reaction, but with reactors, the process is much slower and controlled. One of the most important parts of a nuclear reactor is the control rod assembly. These rods are used to essentially “soak up” surplus neutrons, preventing excess fissions.
The rods are made of materials like boron and cadmium, which do a great job of absorbing free neutrons. The rods can be raised out of the reactor to increase the rate of fission, and can be lowered into the reactor to slow or even stop fission.
What’s amazing is that these rods can be made to fail-safe. They do this by being held up by electromagnets. So, if, for some reason, the power fails, the electromagnets holding up the control rods no longer work. And although the power has failed, gravity never will. The rods will automatically fall down into the reactor, shutting it down by stopping the fission chain reaction.
I love the simplicity, yet elegance of this safety feature. But there’s still another way that nuclear reactors can be made to fail-safe.
We’ve already discussed how free neutrons are released during the fission process, and we need these neutrons to create a chain reaction. But what I didn’t mention is that these free neutrons actually move too fast to cause further fissions.
So, in a reactor, they have to be slowed down by a substance known as a moderator. The moderator’s job isn’t to absorb neutrons, just to slow them down to the correct speed for the chain reaction to take place.
And this chain reaction creates a lot of heat. So, reactors rely on coolants to take thermal energy away from the nuclear fuel and transfer it to a steam generator where a turbine can be driven to generate electricity.
But if coolant leaks from the reactor, the nuclear fuel can heat up to the point that a meltdown occurs.
A way to combat this scenario is to make the moderator and coolant the same substance. For example, water can be used as both a moderator and a coolant. In this case, if coolant leaks from the reactor, it will fail-safe because it is also a moderator leak. And once the moderator leaves the system, the free neutrons will be moving too fast for fission to take pace, shutting down the chain reaction.
Because of these fail-safe designs and designs like them, nuclear reactors are one of the safest ways to generate electricity. There is an unbelievable emphasis put on safety in nuclear power stations, and the precautions stretch well beyond the two fail-safe designs covered in this video.