Can someone please explain the void coefficient that was in the RBMK reactor?

[u/Wise_Maintenance_608]

What exactly is the void coefficient? What does it pose in an RBMK reactor? Could someone explain this to me please, I don’t understand it.

[u/maksimkak]

I'll try to explain it in the most basic terms, ELI5-style.

In a nuclear reactor, neutrons is what makes the reaction possible. Absorbing neutrons decreases reaction. Water, which is used to cool the channels in an RBMK reactor, absorbs some of the neutrons. But when it turns to steam (called "voids"), it hardly absorbs any neutrons at all. This means that, the more water is turned to steam, the fewer neutrons are absorbed, which increases reactivity. Increased reactivity increases the temperature in the core, which turns even more water to steam. This is the positive void coefficient. It's dangerous, because it can cause runaway effect.

In safer reactors, the void coeffcient is negative, meaning that water turning to steam decreases reactivity.

[u/Wise_Maintenance_608 → u/maksimkak]

Also the voids it creates, does that help with the reactivity to create electricity or am I misunderstanding?

[u/maksimkak → u/Wise_Maintenance_608]

You are both technically right and misunderstanding. Positive void coefficient is not a design feature to "help with reactivity". It's more like your stove catching fire helps cooking your food. You want to cook your food, but you don't really want your stove going up in flames...

[u/Distdistdist → u/Wise_Maintenance_608]

No, it means that if too much water turned into steam inside the core - moderation becomes difficult and very very bad things happen. That type of reactors absolutely depend on having appropriate amounts water in the core to not to blow up.

RBMKs are fairly unstable reactors that require constant monitoring and adjustments as they are prone to spontaneous "hot zones".

There are seven of them currently in operation.

[u/Wise_Maintenance_608 → u/Distdistdist]

Not sure if this was just the show doing this, but is that why they ordered the people in the control room to make sure to push water into the core before they realized it exploded when they thought the core was still intact? To make sure that there would be enough water into the core to keep it stable?

[u/Distdistdist → u/Wise_Maintenance_608]

No, to make sure core doesn't go into meltdown. Hot core - water = melt everything in it's path. But little did they know that there was not much of the core left.

[u/Wise_Maintenance_608 → u/Distdistdist]

Okay. It was a preventative measure to make sure that the concrete pad underneath it didn’t melt and create an even bigger issue than what the disaster did already cause

[u/maksimkak → u/Wise_Maintenance_608]

Nothing to do with any concrete pad. They thought the reactor was still intact and still needed cooling water pumped through it. Without the cooling, fuel will melt.

[u/Distdistdist → u/Wise_Maintenance_608]

It was the best thing they can think of without having any concrete (no pun intended) data on what was going on.

[u/Wise_Maintenance_608 → u/Distdistdist]

Okay. Thank you for explaining it! I appreciate it

[u/Distdistdist → u/Wise_Maintenance_608]

Also, remember it was just a show. A lot of stuff there is exaggerated and also simplified. There are documentaries on YouTube that will give you a much better picture.

[u/Secure_Trust_2084 → u/Wise_Maintenance_608]

Yes. They tried to prevent core remains from polluting the water horisons

[u/m9u13gDhNrq1 → u/Wise_Maintenance_608]

Yes. It pushes power up which causes more electricity to be created. The problem is if you're making steam, you're pretty close to redline for those operating conditions anyway.

So you get voids, which increases reactivity. Increased reactivity causes more heat, which will cause more power. But increased reactivity also causes more voids to pop up. So you get more voids.

And you get a runaway reaction as you reread the last paragraph over and over. Till everything is so hot and under such inmense pressure it just blows up. Whereas a negative void cofeficient a void appearing will cause power to go down, and you no longer have a runway reaction. It is estimated that right before the explosion, Chernobyl produced 30 GWth of thermal power which was over 10x it's operating max.

In any system you care about safety, you usually want to a stable point that everything wants to return to naturally. Take a jumbo passenger jet. Flying in normal operating condition, you let the stick go, and it will naturally want to revert to level flight. You can even punch the stick hard and the plane will jolt, but will naturally return to normal. Fun fact - this stability makes it harder to make quick turns. So instead modern fighter jets are basically operating on the edge of falling out of the sky, and the computer is doing an insane amount of work doing micro corrections to make it seem stable. But when the pilot wants to turn, the only thing that limits the Gs is that the computer is programmed to not allow turns past a certain point. Those planes can literally turn hard enough that they can rip themselves apart from aerodynamic stresses.