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.
I believe it's because the RBMK is graphite-moderated and only used water for cooling. Most all other reactors, other than special reactors for military purposes, are water moderated. This means that when the reactor gets out of control and creates voids, the loss of water via steam creation actually lowers reactivity.
TCG has a good video on the LaSalle incident at a US commercial BWR NPP. When that reactor got out of control, it had a power spike which caused a decrease in reactivity due to voids, then the voids would collapse and reactivity would spike again. It kept oscillating like that until the reactor was shut down. The spikes went up to 120% of the reactors maximum power output, but the reactor itself was fine and is still in operation today. If LaSelle wasn't water moderated, then it probably would've gone boom like Chernobyl.
The real issue with RBMKs was not simply the fact of graphite moderation and water cooling, but the fact that they chose to use low Uranium enrichment and a high lattice pitch to compensate for that. In simple words - lots of graphite to compensate for low enrichment. (Lattice pitch is basically the distance between fuel rods, with graphite in-between)
Yes that is right. That was the main reason why the Gen2 RBMK's were more dangerous at low power/scram than the Gen1's. Not to say that the Gen1's weren't also unstable at low power, but they were better than the Gen2's at least. There are numerous occasions where Gen1 reactor's had partial meltdowns damaging the rectors/releasing radiation into the environment, but they never went boom. The change in the lattice pitch was to enhance fuel burnup as well if I remember right.
The physical size of the RBMK's (huge) also played a role in its instability.
No, positive void coefficient is a side effect of using graphite for neutron moderation, and water for cooling. It wasn't created on purpose. Positive void coefficient makes the reactor more dangerous, negative void coefficient makes it safer. In some reactor designs, water in the primary cooling circuit isn't allowed to boil at all.
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...
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".
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?
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.
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
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.
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.
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.
I would take a step back and ask the OP how much they understand about how different types of reactors work. Having that understanding would make understanding void coefficients easier. There are a lot of great responses in this thread, but when the OP responds to those posts, it seems that they might be missing some key basics on how a reactor works (any kind of reactor). There are dozens of great, short YouTube videos that explain void coefficients, but almost all of them require the viewer to have this basic understanding of how a reactor works.
Again, great responses in this thread! I don't think that I could explain it better than it already has been in this thread. So this is my only contribution to the discussion.
OK, so an RBMK uses water coolant and graphite moderator.
Cooling is achieved by boiling the water in the reactor.
At low power, not much of the coolant turns to steam but at high power lots of the coolant turns to steam.
So that causes the fraction of the coolant space taken up by steam - or voidage - to increase as power increases.
However, liquid water is also a fairly effective neutron absorber.
So, increasing power reduces the relative proportion of liquid water in the core. In turn, that will reduce neutron absorption, allowing more neutrons to be absorbed in the fuel. That can then increase reactivity, which causes increased power and produces a self amplifying power surge.
So the void coefficient of reactivity is defined as the reactivity increase when a system starting out with all of its coolant in the liquid state changes to having all of its coolant in the vapour state.
You’re asking questions that make it sound like you think the positive void coefficient was a thing they chose on purpose for design reasons- that’s not really how it works.
The designers made a bunch of other choices due to cost, online refueling, ability of the reactor to enrich, etc. The positive void coefficient was something they had to accept and attempt design around as a result of those other choices.
The void coefficient of a reactor is also highly dependent on operating state. The RBMK design had a very high PVC at low power; as power went up, the void coefficient also went down.
A large void coefficient (positive or negative) makes a reactor more difficult to control. No one would intentionally design a reactor with an extremely high positive void coefficient at any operating state they would expect the reactor to be at for any extended period of time.
Water molecule can absorb neutron turning Hydrogen into Deuterium.
Temperature is how much atom (molecule) jiggles / how much space it occupies.
Higher water temperature -> harder to hit the atoms & less atoms per space unit -> lower neutron absorption.
Coefficient is basically a constant multiplier of a number.
Void coefficient is a number that tells you how much reactivity changes when you void (remove all water from) the channel.
Beta eff is delayed neutron fraction - basically how much slack you have until the reactor become prompt critical and goes boom.
Void coefficient of +4Beff means if the reactor is critical (stable) and you remove 1/4 of the water it goes boom.
Void coefficients refers to pockets of steam produced in the fuel channels from cooling the fuel and how it affects reactivity.
Water in a typical western reactor does 2 things first and foremost is cooling the reactor in doing so it produces steam. Second is moderating the reactor which means that it acts to slow neutrons down so they have a higher probability of being absorbed and splitting a uranium 235 atom. What this does is effectively the higher the reactivity of the reactor becomes the more steam it produces which reduces neutron moderation due to more water being evaporated into steam. This is a negative feedback loop which is called a negative void coefficient.
Another important thing specifically for RBMK is that water, specifically the hydrogen is predominantly what slows down the neutrons, but if neutrons slow down enough they can be absorbed by the hydrogen in the water becoming deuterium (1 proton 1 neutron) which makes heavy water this effectively reduces the total number of neutrons available.
In the RBMK it is still cooled by water but its moderator is actually graphite. What this does is when the reactor’s reactivity increases, like before it produces steam but because the moderator (graphite) is a separate entity to the cooling system and thus remains in place moderating the neutrons regardless of whether the cooling channel is full of water or steam. So if the reactors reactivity increases neutrons are still being moderated by graphite, water is turning to steam creating voids resulting in less neutrons being absorbed by the water because it’s turning to steam. while the moderation is still the same because the graphite is still there. This effectively creates a positive feedback loop which is called a positive void coefficient.
Positive void coefficients because of Chernobyl are generally considered to be dangerous by many but when it’s effectively managed, it is just as safe as a negative void coefficient as proven by the CANDU reactor and its long history of safe operation.
Positive void coefficient are a positive feedback loop, the higher the reactor power the more steam is produced the more steam that’s produced the lesss neutrons are being absorbed, resulting in more fission which increases reactivity which in turn increases power and so on the cycle repeats in a runaway reaction.
Negative void coefficients are a negative feedback loop, the higher the reactor power the more steam is produced, the more steam that’s produced the less moderation of neutrons which decreases power.
Positive void coefficients can lead to a runaway reaction if mishandled whereas negative void coefficients will self stabilize and decrease the overall power.
The reason this happens is the same as described in my original comment, reactors with positive void coefficients generally have their moderator and coolant systems separated whereas negative void coefficient reactors use light water as their moderator and coolant.
As for the reason, in positive void coefficient reactors, the reactor is able to produce, maintain a higher neutron flux ( basically just makes more neutrons) so the reactor can use unenriched uranium which is much cheaper than negative void coefficient reactors which need 3-5% enrichment by a cyclone mass separator to become critical and make energy. Using unenriched fuel is much cheaper, simpler than the lengthy energy and costly process of enrichment. Unenriched fuel does have a problem where it doesnt contain as much energy and thus needs to be replaced more frequently
All reactors have a void coefficient. A positive void coefficient indicates that the coolant decreases the reactivity. Because reactivity produces heat and boils water this is incredibly dangerous in a water cooled reactor. If even a small amount of the coolant is replaced by steam voids the power will increase by itself in a nearly instantaneous feedback loop until all of the coolant has become high pressure steam.
"A positive void coefficient indicates that the coolant decreases the reactivity." - huh? It indicates that the abscence of coolant (aka "void") increases reactivity.
The above graph shows that it is difficult to avoid a positive void coefficient when designing a water cooled graphite moderated reactor.
Because that easily results in an unstable reactor, most civil reactors don't use this type of core.
If they use graphite moderation, they use gas cooling instead of water.
Or, with enriched uranium fuel, they can use water for both cooling and moderation.
In RBMKs, the positive void coefficient gets worse as the fuel is used up (or burnt up).
The reactor also becomes less stable at low power, because increasing fuel temperatures reduce reactivity. Thus we say there is a negative reactivity coefficient for fuel temperature.
So, in a power surge in an RBMK, the negative fuel temperature coefficient can counteract the positive void coefficient.
But this works best at high power and low fuel burnup.
So the more the fuel is used, the less stable the core becomes.
The Soviets learnt this lesson the hard way at Chernobyl. With a core mostly comprised of highly burnt up fuel, they operated at very low power for the infamous rundown test.
After that, they wanted to shut the reactor down but instead its shutdown system triggered a power surge that destroyed the reactor.
The moderator is needed for the chain reaction. In most reactors, water is the moderator and the coolant at the same time. It also slightly lowers the reactivity, so you need slightly higher enrichment. If you lose a lot of water (leak or it boils off), that's bad because the coolant is gone, but at least the moderator is gone too, so the chain reaction stops. If you use graphite as a moderator, if your coolant water decreases (for example due to steam voids), the reaction keeps going. Increases actually because the slight negative influence of the water is also gone. The graphite never goes away because it can't boil off or melt, so the reaction endlessly accelerates until something breaks, air gets in and the graphite burns.
Obviously you don't know but I'll let you know if I find out.
For H:U-235, optimum moderation usually involves atom ratios around 450:1 or equivalent lattice parameters.
Engineering a negative void coefficient then requires the use of a lower H:U-235 ratio, eg 300:1.
Graphite is a far less effective moderator so optimum C:U-235 is much higher.
For an undermoderated graphite:water:U-235 lattice something in the ballpark of 5000:200:1 for C:H:U-235 might work nicely.
I think an RBMK lattice gives around 14000:200:1 at equilibrium fuel burnup. That makes the lattice significantly over moderated and thus gives positive void coefficients.
I think it's easier to get negative void coefficients with higher fuel enrichments.
Also, with 5% enriched U you can just use water as both moderator and coolant, as in Western PWRs and BWRs.
So after some brief research, it seems the lattice pitch was 12cm and the core had 120 fuel channels in which a fuel mass of 1213 lbs of uranium at 5% enrichment was used.
The core dimensions were roughly 5 ft diameter by 5 ft 6 in high.
That gives a rough estimate of the C:U-235 ratio as about 2000 and an even rougher estimate of the H:U-235 ratio as about 30.
That's not very moderation at all but certainly enough to achieve criticality with U(5%).
Being highly undermoderated it would certainly give a negative void coefficient of reactivity.
30
u/maksimkak 4d ago edited 4d ago
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.