How Does an RBMK Reactor Explode
09-03-2019, 05:15 PM (This post was last modified: 09-06-2019, 08:05 PM by Dartz.)
09-03-2019, 05:15 PM (This post was last modified: 09-06-2019, 08:05 PM by Dartz.)
Or the weird things that you come across when doing your research for a fic exactly thirty people have read.
----Pt 1---------
Everybody knows about Chernobyl. Everybody knows either one of two things. The accident was caused by gross incompetence. Or the accident was caused by a gross design error. Or the accident was caused by both. Yes, that's three things - but this isn't the Spanish Inquisition.
Anyway, to figure out why an RBMK reactor can explode, first we have to understand how a nuclear fission reactor works in the first place. It's not actually that complicated. A uranium atom splits - small particles called neutrons fly out from the atomic shrapnel and they each find another uranium atom, collide with it and break it apart too, letting more neutrons find more Uranium atoms in a chain reaction. And out of each split we get energy - once split, the fragments snap apart as the energy holding them together is released - like cutting an elastic band. These fragments explode away, run into the atoms next to them at high speed and all that speed is turned into heat. Heat is used to boil water. This steam turns a turbine. This turbine turns an electric generator and, eventually you get electric power out of it.
Now, it's not quite that simple. Because big, heavy atoms like Uranium come in multiple different versions - called Isotopes - sort of like different models of the same car. They're all Uranium, but they're all slightly different at the same time. The most well known, are Uranium 235, and Uranium 238. U238 is by far and way the most common - on the order of 99% or more of Uranium on Earth is U238. It's a big, heavy fat atom that doesn't really like to split - it takes more energy to break it apart than it gives up when it breaks. On the other hand, U235 is much happier to break apart - unfortunately, it's about as rare as common sense. So, out of a large block of Uranium, only a small amount can actually split. Only the U-235 is actually fissile.
Out of the ground, less than 1% of your Uranium fuel is actually fuel.
More than that, in order to have a chain reaction, the neutrons released by one atom splitting have to be able to cause further atoms to split. Otherwise everything just runs down. It turns out, that the neutrons released by fission are extremely fast - too fast in fact to actually find their way to the next atom and split it. For a fast neutron, the probability that it will cause another fission is really low. Either you need a lot more U-235 around it to get the probability up, or you need to find someway of slowing it down. If you slow it down - the probability of fission goes way up. This is called a Moderator.
For most reactors, this moderator is water. Ordinary - albeit extremely pure - water. Water is a good moderator, but it has one slight drawback - it absorbs neutrons. Absorbed neutrons do not get to go and make another fission happen - they just turn the water radioactive. Using water to moderate a reactor absorbs so many neutrons, that the quantity of U235 in the reactor fuel has to be increased. This process is called enrichment. It's expensive and energy intensive - and it turns out if you enrich Uranium to about 80% U235 it can be used to make an atomic bomb.
Which, naturally, is why Isreal got so pissy about Iran having the ability to enrich Uranium. The difference between safe reactor fuel and weapons-grade bomb fuel, is time in the centrifugal oven to bake.
Now, doesn't it seem eminently sensible to find a way to build a reactor that will be happy on regular, non-enriched Uranium?
Canada did it with the CANDU reactor. Instead of regular water, a CANDU reactor uses 'heavy' water. 'Heavy' water is like ordinary 'Light' water, except the Hydrogen atom that's the H in H2O is a little different. It has one neutron and one proton, rather than just a single proton. It absorbs less neutrons, which means more neutrons are free to cause more fission. In fact, Heavy Water is so effective that CANDU reactor doesn't need enriched fuel. The Nazis appearred to have tried a similar appoach, and the destruction of their Norwegian Heavy Water factories stalled their weapons program. On the other hand, heavy water - while common - is still fairly expensive.
What if you could build a reactor that ran on natural uranium, that didn't need heavy water?
The first Hanford Reactors, built in the United States for the Manhattan project used Graphite - pencil 'lead' - as a moderator. They also used regular, light water as a coolant to keep the reactor from melting down with its own heat. This hot water was dumped merrily into the local river. Of course, somebody had worked out that if the reactor lost cooling water, it would very quickly begin to run out of control so the Hanford reactors were built miles from anywhere inhabited. They never generated a watt of electricity- but they did create the Plutonium for your nuclear weapons.
The British Government, aware of this risk, built the Windscale Piles to be Air cooled - with giant fans blowing air over hot graphite and metal. These then went and caught fire. In the end, the solution was to use graphite and an inert gas, like carbon dioxide, to cool the core.
This was still extremely expensive.
So, the Soviet Union looked at this and though; We can build a graphite moderated reactor, cool it with regular light water and so long as we don't fuck up, we'll have a shitton of free energy.
We know the result already. But that's being a little bit churlish. These people weren't fools.
This why an RBMK reactor is different to every other reactor built anywhere else in the world. The majority of modern nuclear reactors are basically big pressurised kettles, filled mostly with pressurised water. This water either boils in the kettle- or it is under such a high pressure that it remains liquid and is used to boil water in a second circuit. In an RBMK reactor, the fuel is contained in more than 1600 vertical channels cut through the graphite moderator. Inside these channels, light water flows as coolant. It enters a pair of drums high above the reactor where any steam bubbles in the water seperate and are drawn off to the turbine to generate power - while the liquid water is recirculated, being mixed with cooler water coming back from the turbine. Interspersed within these channels are more than 200 others - each contaning a control rod. These control rods are also cooled by liquid water - but at a much lower temperature.
The main steam circuit on an RBMK reactor operates at somewhere around 270 degrees and 60-odd Bar of pressure. The control rod circuit operated at 70 degrees.
The light water in the channels still absorbs neutrons sure - but because there's so much less of it, the reactor will still run on natural unenriched Uranium. It also means that, since each fuel assembly has its own individual channel, it can be removed, moved and replaced without shutting down the reactor. This is a feature few other reactors have - most reguire a shutdown to open the reactor vessels to refuel. This is goof for fuel economy, good for efficiency, and good for creating weapons grade plutonium on the sly if you were that way inclined.
The people who designed the Chernobyl reactor weren't fools. There were compelling reasons for making the decisions they did. It made a big, powerful reactor cheap and easy to build, while improving the reactor's fuel economy and general uptime. And you could potentially fuel a weapons program with it.
The one clear drawback with this should be obvious. When the cooling water boils, its replaced by a bubble of steam. This steam absorbs far fewer neutrons than liquid water - meaning more neutrons availably for fission, which means more fission, more heat, more steam, more neutrons, more fission.
This is called a Positive Void Coeeficient. It is an example of positive feedback - an action creates a stronger action in the same direction - like setting a ball rolling at the top of a hill, it's only going to start rolling faster as it gets further down. Engineers love positive feedback. It usually results in entertaining explosions.
This is potentially problem for an RBMK reactor specifically because the water does not act as a moderator - more correctly, it provides little to no moderation. On a conventional reactor, the water also provides moderation - so if it is boiled away by heat, the moderation in the reactor reduces, neutrons get faster, the probability of fissions gets lower, less fission happens and the problem self-corrects. On an RBMK reactor - even if all the water in the core somehow is removed, the moderator is still present in the form of the graphite to keep the reactor going.
It would be dangerous to have a reactor which behaved like this. The engineers who designed Chernobyl were, of course, aware of this. But real physics is not that simple. As the fuel heats up and gets more energetic, it responds to neutrons differently. The hotter it gets, the harder it is for a neutron to cause a fission. Hotter fuel is less likely to fission - so an increase in power will actually reduce the ability of the fuel to fission and create power - in effect an automatic brake provided free by simple physics. This is called Negative feedback, and is basically the same as you feeling a tug in the steering wheel of your car, and steering the other direction to compensate.
Positive feedback acts to destabilise. Negative feedback acts to stabilise.
But - if the negative feedback from the fuel heating is stronger than the positive feedback from the steam boiling, the reactor's power level will self stabilise and everything will be fine.
For a large part of the reactor's life this was true. But this changed as the reactor got older, and more and more fuel was used up. the negative reactivity from the fuel heating up, no longer counterbalanced the positive reactivity from the steam boiling.
This balance changed with old fuel. Where a reactor had been running for several years- the fuel gets more and more depleted. In addition, more and more poisons are added, each of which absorbs neutrons differently or introduces additional hazards into the reactor. The reactor in Chernobyl had been running for about three years. After this time, changes in the fuel meant that the negative feedback from the fuel heating was no longer enough to counterbalance the effect of the void coefficient. The reactor operated in a positive feedback loop.
An increase in power, left unchecked would create a further increase in power. Only the reactors control rods kept the reactor under control, The majority of these control rods inserted from the top of the reactor. Some inserted from the bottom. They served to absorb any excess neutrons in the core and act as the final brake on the reactor to keep it in control, to keep the reactor critical.
Fission reactors are at their happiest when they're critical. A critical reactor is a reactor running in a balanced steady state at a constant power. It's the desired state of being. Every fission is creating one further fission and that's it. A reactor that is supercritical, is a reactor that's accelerating - each fission creates more than one further fission. A reactor that is subcritical, is a reactor that's decelerating - each fission creates less than one further fission.
The reactor is moved from state to state by adding or removing reactivity. Reactivity is like the throttle and brake on the reactor. It's not really the current power level - it's sort of the potential change in power level. Positive reactivty means fission is more likely to happen than it is now - which will cause an increase in power. Negative reactivity, means making fission less likely to happen - which will cause a decrease in power.
In theory, there is no limit to the amount of negative reactivity you can add - all it does is stop the reactor faster. But there is a limit to the positive reactivity.
When an atom fissions, the vast majority of neutrons are released instantaneously - at the moment of fission. The neutrons fly away, get themselves moderated, and in the space of microseconds find more atoms to collide with and split. The scientists of the Manhattan project called this a 'Shake' and it is an extremely short interval of time - from nanoseconds to microseconds. These are called Prompt neutrons.
Fission with prompt neutrons happens so quickly, that there is little to no mechanical process capable of controlling and regulating it. If the universe had been created in such a way that there were only prompt neutrons - controllable fission power would likely be impossible.
But, luckily, a very small fraction of the neutrons released by a fission event are delayed - they happens seconds, to minutes later. It is this small fraction of delayed neutrons which enables every nuclear fission reactor to be controlled. It is possible therefore, to have a reactor which is critical on the combination of the Prompt, and the Delayed neutrons. In fact, this is how things normally are. Even a supercritical reactor, will take seconds to minutes, to change power output. There's time there for the process machinery of the reactor to respond to changes and stabilise.
But, if the reactor is pushed to the point that it is capable of achieving criticality on the Prompt neutrons alone - before any of delayed neutrons are emitted - then things get interesting. Instead of a power increase that happens on the order of seconds to minutes - now the only limiting factor the the reactor's power increase is the time it takes for a neutron to find an atom to fission, and however long the reactor manages to stay together in a critical assembly against the energies that are being very rapidly liberated. A reactor which has going prompt critical, has become, in effect, a really, really shit nuclear bomb. The big difference being that bombs take advantage of physics, inertia and a dozen other things to keep the reaction going that few nanoseconds longer it takes to go from 5 tons of TNT, to 15 Kilotons of TNT.
Scientists at the Manhattan project, for whatever reason, called this interval a 'Dollar' of reactivity. Once you get a reactor past that point - unless it's a type specifically designed to go there and self recover - the reactor will be destroyed. Importantly, this does not have to happen within the entire reactor - it can be limited to a very small part of the core where conditions align like the stars.
At the Chernobyl reactor, Reactivity was added by fresh fuel, by removing control rods and by boiling water to make steam. Reactivity was removed by increasing water flow, adding control rods, and by another factor.
The shrapnel left over from fission creates what's known as 'fission products'. Most of these are hideously radioactive. Many of these are effective at absorbing neutrons. Absorbed neutrons reduce reactivity, which has to be compensated for either by withdrawing control rods, or by removing the used fuel and replacing it with fresh fuel. One of the most effective neutron absorbers is an isotope of Xenon, called Xenon-135.
It starts to appearr about 6 hours after the fission events that effectively 'created' it. The amount of it that's created, is in direct proportion to the quantity of fission that happened six hours ago. So if a reactor is run at full power for a long time, and then throttled down, Xenon will continue to appearr according to that fuel burned six hours previously. It's a bit like the exhaust from your cars engine magically taking longer to form after the combustion in the cylinders. Normally, with the reactor in a steady-state, Xenon is created as quickly as it is consumed - the physics balances out. It can make it very difficult to increase or reduce power - if power is reduced too quickly, and the Xenon continues to build, the reactor might even be stalled by it.
It can also mean that, if the fuel in the reactor has been burned for a long time - there may not be sufficient reactivity in the remaining fuel to overcome this Xenon pit - the reactor is stalled and effectively impossible to start.
This is important. Because after a few hours more, the Xenon goes away. More than that, Xenon which absorbs a neutron also 'goes away' - it's no longer Xenon-135 and it's massive ability ot hoover up neutrons is suddenly gone.
Keeping all of these positive and negative reactivities in balance is the job of the Senior Reactor Engineer, who manipulates the reactor core's systems and control rods to achieve the required stable power output. The Engineer has only so much control as the rods will give them.
Finally, there is the concept of the Reactivity Margin. And that's basically the count of control rods left inside the reactor, which are required to maintain criticality. The higher the reactivity margin, the more control rods remain in the core and the more reactivity can be added to the core. A reactor with fresh fuel will have a very high reactivity margin. A reactor with old fuel, or with xenon poisoning, will have a low reactivity margin. Other factors like Xenon can push the reactivity margin down. It may seem that a low reactivity margin might be 'safer', because now there's less reactivity that can be added by the control rods (which are already out of the reactor at low margin). At a high reactivity margin, the control rods are inside the reactor. More of them can be withdrawn, to push the reactor further into the supercritical - more positive reactivity can be added to the reactor.
the Chernobyl reactor was happy around about 30 Rods of reactivity. At this point, fresh fuel was being added frequenctly enough to keep the reactor stable, but not so frequently as to be uneconomical. The official limit, was somewhere around 15 Rods of reactivity. This wasn't thought to be a safety limit - it was simply economic. The reactor was far more likely to stall at low reactivity margins, resulting in downtime and lost energy production - or surprise blackouts.
But, below 30 rods of reactivity, another insidious effect began to occur.
The Control rods of an RBMK reactor are manufactured from boron. Boron absorbes neutrons, which reduces power. The deeper they go into the reactor - the more neutrons are absorbed - the slower the reactor goes. They can also be moved independently of each other - which changes where and how power is produced throughout the reactor, to compensate and balance for old and fresh fuels and how they're distributed through the reactor.
But, at the tip of the control rod on a telescoping extension, is a single slug of graphite and the end of a telescopic rod. The graphite tip of the control rod acted as a displacer. Its purpose was to push water out of the control rod channel, to remove it and its neutron absorbtion effect after the rod was withdrawn. In effect, instead of giving the control rod an action of -1,0 - they are something like -1,+1. They graphite displacer gives the control rod a stronger control action. It makes it more powerful by adding reactivity after the rod is withdrawn.
This led to an interesting effect.
If a number of rods in the same area of the reactor were inserted at the same time, and were in the same vertical position as they moved, a small amount of postivie reactivity could be momentarily added to the bottom of the reactor. This would cause an uptick in power for a few seconds before the boron control rod travelled the entire height of the core and finally quenched the reaction.
This was not thought to be much of a concern - power changes in the reactor after all, take longer than it takes for the rod to travel. It was just something the RBMK reactor did. Methods to mitigate it had been known and discussed for a decade prior Chernobyl Disaster, but were not seen as too much of a big deal. An RBMK reactor cannot explode, after all.
It would also reflect badly on the director of the Kurchatov institude if the reactor he had overseen were found to have a potentially fatal flaw. It was quietly buried in the documentation.
We do not yet know how an RBMK reactor explodes. But we know what we need to know
----Pt 1---------
Everybody knows about Chernobyl. Everybody knows either one of two things. The accident was caused by gross incompetence. Or the accident was caused by a gross design error. Or the accident was caused by both. Yes, that's three things - but this isn't the Spanish Inquisition.
Anyway, to figure out why an RBMK reactor can explode, first we have to understand how a nuclear fission reactor works in the first place. It's not actually that complicated. A uranium atom splits - small particles called neutrons fly out from the atomic shrapnel and they each find another uranium atom, collide with it and break it apart too, letting more neutrons find more Uranium atoms in a chain reaction. And out of each split we get energy - once split, the fragments snap apart as the energy holding them together is released - like cutting an elastic band. These fragments explode away, run into the atoms next to them at high speed and all that speed is turned into heat. Heat is used to boil water. This steam turns a turbine. This turbine turns an electric generator and, eventually you get electric power out of it.
Now, it's not quite that simple. Because big, heavy atoms like Uranium come in multiple different versions - called Isotopes - sort of like different models of the same car. They're all Uranium, but they're all slightly different at the same time. The most well known, are Uranium 235, and Uranium 238. U238 is by far and way the most common - on the order of 99% or more of Uranium on Earth is U238. It's a big, heavy fat atom that doesn't really like to split - it takes more energy to break it apart than it gives up when it breaks. On the other hand, U235 is much happier to break apart - unfortunately, it's about as rare as common sense. So, out of a large block of Uranium, only a small amount can actually split. Only the U-235 is actually fissile.
Out of the ground, less than 1% of your Uranium fuel is actually fuel.
More than that, in order to have a chain reaction, the neutrons released by one atom splitting have to be able to cause further atoms to split. Otherwise everything just runs down. It turns out, that the neutrons released by fission are extremely fast - too fast in fact to actually find their way to the next atom and split it. For a fast neutron, the probability that it will cause another fission is really low. Either you need a lot more U-235 around it to get the probability up, or you need to find someway of slowing it down. If you slow it down - the probability of fission goes way up. This is called a Moderator.
For most reactors, this moderator is water. Ordinary - albeit extremely pure - water. Water is a good moderator, but it has one slight drawback - it absorbs neutrons. Absorbed neutrons do not get to go and make another fission happen - they just turn the water radioactive. Using water to moderate a reactor absorbs so many neutrons, that the quantity of U235 in the reactor fuel has to be increased. This process is called enrichment. It's expensive and energy intensive - and it turns out if you enrich Uranium to about 80% U235 it can be used to make an atomic bomb.
Which, naturally, is why Isreal got so pissy about Iran having the ability to enrich Uranium. The difference between safe reactor fuel and weapons-grade bomb fuel, is time in the centrifugal oven to bake.
Now, doesn't it seem eminently sensible to find a way to build a reactor that will be happy on regular, non-enriched Uranium?
Canada did it with the CANDU reactor. Instead of regular water, a CANDU reactor uses 'heavy' water. 'Heavy' water is like ordinary 'Light' water, except the Hydrogen atom that's the H in H2O is a little different. It has one neutron and one proton, rather than just a single proton. It absorbs less neutrons, which means more neutrons are free to cause more fission. In fact, Heavy Water is so effective that CANDU reactor doesn't need enriched fuel. The Nazis appearred to have tried a similar appoach, and the destruction of their Norwegian Heavy Water factories stalled their weapons program. On the other hand, heavy water - while common - is still fairly expensive.
What if you could build a reactor that ran on natural uranium, that didn't need heavy water?
The first Hanford Reactors, built in the United States for the Manhattan project used Graphite - pencil 'lead' - as a moderator. They also used regular, light water as a coolant to keep the reactor from melting down with its own heat. This hot water was dumped merrily into the local river. Of course, somebody had worked out that if the reactor lost cooling water, it would very quickly begin to run out of control so the Hanford reactors were built miles from anywhere inhabited. They never generated a watt of electricity- but they did create the Plutonium for your nuclear weapons.
The British Government, aware of this risk, built the Windscale Piles to be Air cooled - with giant fans blowing air over hot graphite and metal. These then went and caught fire. In the end, the solution was to use graphite and an inert gas, like carbon dioxide, to cool the core.
This was still extremely expensive.
So, the Soviet Union looked at this and though; We can build a graphite moderated reactor, cool it with regular light water and so long as we don't fuck up, we'll have a shitton of free energy.
We know the result already. But that's being a little bit churlish. These people weren't fools.
This why an RBMK reactor is different to every other reactor built anywhere else in the world. The majority of modern nuclear reactors are basically big pressurised kettles, filled mostly with pressurised water. This water either boils in the kettle- or it is under such a high pressure that it remains liquid and is used to boil water in a second circuit. In an RBMK reactor, the fuel is contained in more than 1600 vertical channels cut through the graphite moderator. Inside these channels, light water flows as coolant. It enters a pair of drums high above the reactor where any steam bubbles in the water seperate and are drawn off to the turbine to generate power - while the liquid water is recirculated, being mixed with cooler water coming back from the turbine. Interspersed within these channels are more than 200 others - each contaning a control rod. These control rods are also cooled by liquid water - but at a much lower temperature.
The main steam circuit on an RBMK reactor operates at somewhere around 270 degrees and 60-odd Bar of pressure. The control rod circuit operated at 70 degrees.
The light water in the channels still absorbs neutrons sure - but because there's so much less of it, the reactor will still run on natural unenriched Uranium. It also means that, since each fuel assembly has its own individual channel, it can be removed, moved and replaced without shutting down the reactor. This is a feature few other reactors have - most reguire a shutdown to open the reactor vessels to refuel. This is goof for fuel economy, good for efficiency, and good for creating weapons grade plutonium on the sly if you were that way inclined.
The people who designed the Chernobyl reactor weren't fools. There were compelling reasons for making the decisions they did. It made a big, powerful reactor cheap and easy to build, while improving the reactor's fuel economy and general uptime. And you could potentially fuel a weapons program with it.
The one clear drawback with this should be obvious. When the cooling water boils, its replaced by a bubble of steam. This steam absorbs far fewer neutrons than liquid water - meaning more neutrons availably for fission, which means more fission, more heat, more steam, more neutrons, more fission.
This is called a Positive Void Coeeficient. It is an example of positive feedback - an action creates a stronger action in the same direction - like setting a ball rolling at the top of a hill, it's only going to start rolling faster as it gets further down. Engineers love positive feedback. It usually results in entertaining explosions.
This is potentially problem for an RBMK reactor specifically because the water does not act as a moderator - more correctly, it provides little to no moderation. On a conventional reactor, the water also provides moderation - so if it is boiled away by heat, the moderation in the reactor reduces, neutrons get faster, the probability of fissions gets lower, less fission happens and the problem self-corrects. On an RBMK reactor - even if all the water in the core somehow is removed, the moderator is still present in the form of the graphite to keep the reactor going.
It would be dangerous to have a reactor which behaved like this. The engineers who designed Chernobyl were, of course, aware of this. But real physics is not that simple. As the fuel heats up and gets more energetic, it responds to neutrons differently. The hotter it gets, the harder it is for a neutron to cause a fission. Hotter fuel is less likely to fission - so an increase in power will actually reduce the ability of the fuel to fission and create power - in effect an automatic brake provided free by simple physics. This is called Negative feedback, and is basically the same as you feeling a tug in the steering wheel of your car, and steering the other direction to compensate.
Positive feedback acts to destabilise. Negative feedback acts to stabilise.
But - if the negative feedback from the fuel heating is stronger than the positive feedback from the steam boiling, the reactor's power level will self stabilise and everything will be fine.
For a large part of the reactor's life this was true. But this changed as the reactor got older, and more and more fuel was used up. the negative reactivity from the fuel heating up, no longer counterbalanced the positive reactivity from the steam boiling.
This balance changed with old fuel. Where a reactor had been running for several years- the fuel gets more and more depleted. In addition, more and more poisons are added, each of which absorbs neutrons differently or introduces additional hazards into the reactor. The reactor in Chernobyl had been running for about three years. After this time, changes in the fuel meant that the negative feedback from the fuel heating was no longer enough to counterbalance the effect of the void coefficient. The reactor operated in a positive feedback loop.
An increase in power, left unchecked would create a further increase in power. Only the reactors control rods kept the reactor under control, The majority of these control rods inserted from the top of the reactor. Some inserted from the bottom. They served to absorb any excess neutrons in the core and act as the final brake on the reactor to keep it in control, to keep the reactor critical.
Fission reactors are at their happiest when they're critical. A critical reactor is a reactor running in a balanced steady state at a constant power. It's the desired state of being. Every fission is creating one further fission and that's it. A reactor that is supercritical, is a reactor that's accelerating - each fission creates more than one further fission. A reactor that is subcritical, is a reactor that's decelerating - each fission creates less than one further fission.
The reactor is moved from state to state by adding or removing reactivity. Reactivity is like the throttle and brake on the reactor. It's not really the current power level - it's sort of the potential change in power level. Positive reactivty means fission is more likely to happen than it is now - which will cause an increase in power. Negative reactivity, means making fission less likely to happen - which will cause a decrease in power.
In theory, there is no limit to the amount of negative reactivity you can add - all it does is stop the reactor faster. But there is a limit to the positive reactivity.
When an atom fissions, the vast majority of neutrons are released instantaneously - at the moment of fission. The neutrons fly away, get themselves moderated, and in the space of microseconds find more atoms to collide with and split. The scientists of the Manhattan project called this a 'Shake' and it is an extremely short interval of time - from nanoseconds to microseconds. These are called Prompt neutrons.
Fission with prompt neutrons happens so quickly, that there is little to no mechanical process capable of controlling and regulating it. If the universe had been created in such a way that there were only prompt neutrons - controllable fission power would likely be impossible.
But, luckily, a very small fraction of the neutrons released by a fission event are delayed - they happens seconds, to minutes later. It is this small fraction of delayed neutrons which enables every nuclear fission reactor to be controlled. It is possible therefore, to have a reactor which is critical on the combination of the Prompt, and the Delayed neutrons. In fact, this is how things normally are. Even a supercritical reactor, will take seconds to minutes, to change power output. There's time there for the process machinery of the reactor to respond to changes and stabilise.
But, if the reactor is pushed to the point that it is capable of achieving criticality on the Prompt neutrons alone - before any of delayed neutrons are emitted - then things get interesting. Instead of a power increase that happens on the order of seconds to minutes - now the only limiting factor the the reactor's power increase is the time it takes for a neutron to find an atom to fission, and however long the reactor manages to stay together in a critical assembly against the energies that are being very rapidly liberated. A reactor which has going prompt critical, has become, in effect, a really, really shit nuclear bomb. The big difference being that bombs take advantage of physics, inertia and a dozen other things to keep the reaction going that few nanoseconds longer it takes to go from 5 tons of TNT, to 15 Kilotons of TNT.
Scientists at the Manhattan project, for whatever reason, called this interval a 'Dollar' of reactivity. Once you get a reactor past that point - unless it's a type specifically designed to go there and self recover - the reactor will be destroyed. Importantly, this does not have to happen within the entire reactor - it can be limited to a very small part of the core where conditions align like the stars.
At the Chernobyl reactor, Reactivity was added by fresh fuel, by removing control rods and by boiling water to make steam. Reactivity was removed by increasing water flow, adding control rods, and by another factor.
The shrapnel left over from fission creates what's known as 'fission products'. Most of these are hideously radioactive. Many of these are effective at absorbing neutrons. Absorbed neutrons reduce reactivity, which has to be compensated for either by withdrawing control rods, or by removing the used fuel and replacing it with fresh fuel. One of the most effective neutron absorbers is an isotope of Xenon, called Xenon-135.
It starts to appearr about 6 hours after the fission events that effectively 'created' it. The amount of it that's created, is in direct proportion to the quantity of fission that happened six hours ago. So if a reactor is run at full power for a long time, and then throttled down, Xenon will continue to appearr according to that fuel burned six hours previously. It's a bit like the exhaust from your cars engine magically taking longer to form after the combustion in the cylinders. Normally, with the reactor in a steady-state, Xenon is created as quickly as it is consumed - the physics balances out. It can make it very difficult to increase or reduce power - if power is reduced too quickly, and the Xenon continues to build, the reactor might even be stalled by it.
It can also mean that, if the fuel in the reactor has been burned for a long time - there may not be sufficient reactivity in the remaining fuel to overcome this Xenon pit - the reactor is stalled and effectively impossible to start.
This is important. Because after a few hours more, the Xenon goes away. More than that, Xenon which absorbs a neutron also 'goes away' - it's no longer Xenon-135 and it's massive ability ot hoover up neutrons is suddenly gone.
Keeping all of these positive and negative reactivities in balance is the job of the Senior Reactor Engineer, who manipulates the reactor core's systems and control rods to achieve the required stable power output. The Engineer has only so much control as the rods will give them.
Finally, there is the concept of the Reactivity Margin. And that's basically the count of control rods left inside the reactor, which are required to maintain criticality. The higher the reactivity margin, the more control rods remain in the core and the more reactivity can be added to the core. A reactor with fresh fuel will have a very high reactivity margin. A reactor with old fuel, or with xenon poisoning, will have a low reactivity margin. Other factors like Xenon can push the reactivity margin down. It may seem that a low reactivity margin might be 'safer', because now there's less reactivity that can be added by the control rods (which are already out of the reactor at low margin). At a high reactivity margin, the control rods are inside the reactor. More of them can be withdrawn, to push the reactor further into the supercritical - more positive reactivity can be added to the reactor.
the Chernobyl reactor was happy around about 30 Rods of reactivity. At this point, fresh fuel was being added frequenctly enough to keep the reactor stable, but not so frequently as to be uneconomical. The official limit, was somewhere around 15 Rods of reactivity. This wasn't thought to be a safety limit - it was simply economic. The reactor was far more likely to stall at low reactivity margins, resulting in downtime and lost energy production - or surprise blackouts.
But, below 30 rods of reactivity, another insidious effect began to occur.
The Control rods of an RBMK reactor are manufactured from boron. Boron absorbes neutrons, which reduces power. The deeper they go into the reactor - the more neutrons are absorbed - the slower the reactor goes. They can also be moved independently of each other - which changes where and how power is produced throughout the reactor, to compensate and balance for old and fresh fuels and how they're distributed through the reactor.
But, at the tip of the control rod on a telescoping extension, is a single slug of graphite and the end of a telescopic rod. The graphite tip of the control rod acted as a displacer. Its purpose was to push water out of the control rod channel, to remove it and its neutron absorbtion effect after the rod was withdrawn. In effect, instead of giving the control rod an action of -1,0 - they are something like -1,+1. They graphite displacer gives the control rod a stronger control action. It makes it more powerful by adding reactivity after the rod is withdrawn.
This led to an interesting effect.
If a number of rods in the same area of the reactor were inserted at the same time, and were in the same vertical position as they moved, a small amount of postivie reactivity could be momentarily added to the bottom of the reactor. This would cause an uptick in power for a few seconds before the boron control rod travelled the entire height of the core and finally quenched the reaction.
This was not thought to be much of a concern - power changes in the reactor after all, take longer than it takes for the rod to travel. It was just something the RBMK reactor did. Methods to mitigate it had been known and discussed for a decade prior Chernobyl Disaster, but were not seen as too much of a big deal. An RBMK reactor cannot explode, after all.
It would also reflect badly on the director of the Kurchatov institude if the reactor he had overseen were found to have a potentially fatal flaw. It was quietly buried in the documentation.
We do not yet know how an RBMK reactor explodes. But we know what we need to know
I love the smell of rotaries in the morning. You know one time, I got to work early, before the rush hour. I walked through the empty carpark, I didn't see one bloody Prius or Golf. And that smell, you know that gasoline smell, the whole carpark, smelled like.... ....speed.
One day they're going to ban them.