Eli5: What causes a nuclear core to go super critical?

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Eli5: What causes a nuclear core to go super critical?

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Every nuclear reactor is built differently but its basically like this.

There is a reaction going on that cannot easily be stopped. So, there is also some other medium that is made to absorb that heat or energy produced and the radiation and that medium (lets say water) is used to not only cool the reactor but used to keep the nuclear reaction running at a predictable level. Well, in the most basic example possible, if the water begins traveling too slow OR too fast it could change the rate of absorption and the nuclear reaction could speed up to levels where not even the water can contain it

[https://phys.org/news/2011-03-nuclear-meltdown-video.html](https://phys.org/news/2011-03-nuclear-meltdown-video.html)

Remember every reactor is built differently.

[https://www.youtube.com/watch?v=hJfziSPWB7o](https://www.youtube.com/watch?v=hJfziSPWB7o)

Free classes from schools like MIT or Harvard will break down different power reactors and different elements they use and most of the time its human error that causes the meltdown but they will walk you through how every reactor in history has melted down specifically.

“Critical” is when the reaction is balanced, so on average 100 fissioning atoms lead to 100 more fissioning atoms.

When you want more power from the reactor, you want it to be “supercritical” for a while. So pull out the control rods, and watch the activity in the core increase. This is fine if it’s under control, and doesn’t exceed the safe maximum level.

The demon core is slightly different than the reactor failures.

The Demon Core was just a chunk of metal. Very refined metal though.

There is a critical mass of nuclear material, an amount where if enough of it is together, it will react with itself and give off energy in the form of radiation.
Kind of like how if you cram enough people moving around together in a crowd, eventually someone will get hurt, which may lead to more people getting hurt. Spread them out and it’s not a problem.
Radioactive material gives off energy, when a particle hits another radioactive particle it can cause that one to decay as well.
Nuclear bombs and reactors work on this principle to either control the release slowly like in a reactor, or to make it release a crap ton of energy all at once, like in a bomb. Both take radioactive material and place it close together in certain conditions, the material gives off high energy particles which then collide with other radioactive material that splits off and gives off more particles, which collide with other radioactive material.. this just repeats until either it is out of energy, or physically is too far away/the conditions are no longer right for this to happen.

This is because radioactive materials are unstable. They have extra particles and energy.
Think of them at the top of a staircase. Highly radioactive things are at the top, and stable things like lead are at the bottom.
[Each time it goes from step to step, it gives off energy, and converts to a different elemen](https://geoinfo.nmt.edu/resources/uranium/images/decaychain.jpg)t. This is for uranium instead of plutonium like the demon core but the idea is still the same.

Reactor failures can be from the same principle, but also can be more complex with mechanical failures, safety feature failures, design issues, operator error, cooling system failures, steam and hydrogen explosions etc. These all stem though from the energy from the radioactive materials.

Side note, channel I’d recommend if you’re looking at nuclear accidents, both criticality and releases of radioactive material, would be [Plainly Difficult.](https://www.youtube.com/watch?v=j5wZoswSNwc&list=PLeJkgZkJSc0T0PbDphJi5KIMCL-6uPHsd) They have some on the Demon Core, but also other criticality incidents like when it happens when mixing the products that make the core.

There’s 3 states, sub critical, critical, and super critical.

When you hit uranium 235 with a neutron hard enough, it undergoes fission and some of the products include 3 neutrons that can go off and start fission in other atoms.

There’s also control rods made of materials that can absorb neutrons into their nuclei while remaining stable.

How many control rods you add controls the speed at which the reaction takes place.

Sub critical means not every fission causes another atom to undergo fission. The reaction is slowing down and will eventually stop.

Critical means every fission causes exactly one atom to undergo fission. The reaction is stable and staying at exactly the same speed.

Super critical means that every fission causes more than one atom to undergo fission. The reaction is speeding up.

A radioactive atom emits radiation by, basically, carving off a chunk of itself and shooting it off in a random direction. If that chunk (often a neutron) hits another radioactive atom it might cause it to also split off a chunk and emit it.

Left to its own devices, any given amount of radioactive material does this slowly enough that we can’t get any useful energy out of it. In order to build a nuclear reactor we need some way to ensure the process is *self-sustaining*: every neutron fired off by a radioactive atom causes at least one other atom to also fire off a neutron. We do this by inserting control rods that bounce the neutrons back into the radioactive material until they finally collide with something.

When this is all working properly the radioactive material produces a predictable amount of heat and we can use that to boil water and turn turbines to produce electricity. But if it starts happening too quickly then it can produce more heat than the system can handle. Things start overheating, steam pressure builds up, things start cracking and exploding.

Usually this is because something went wrong with the control rods: they were inserted too far or left in place too long or some such. There are generally failsafes for this built into the design and procedures of nuclear reactors so it’s pretty rare. When it does happen it’s usually because multiple things have gone wrong at the same time.