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Why Chernobyl Exploded - The Real Physics Behind The Reactor

Why Chernobyl Exploded - The Real Physics Behind The Reactor
Hello it's Scott Manley here And today I want to do an extra episode of going nuclear where we talk about why the

reactor

number four at

Chernobyl

exploded

in 1986 now obviously the catalyst for this is the TV show

chernobyl

on HBO and sky and I'm gonna say right away I love the show. It's fantastic I watched it all the way through and I think the some of the details it goes into is great It's not perfectly accurate in every way but I do think that the courtroom scene in the final episode is an exceptionally good Explanation of what happens for the layperson? However, I do think that they glossed over a number of important mechanisms, which I would love to talk about So, I mean the TV show is still superior to practically any documentary. I've seen on the subject So, you know, there are some

real

ly terrible documentaries out there. I would watch this TV show before those. I'm also Not gonna talk about the people involved. I'm not gonna name names I'm not gonna talk about the aftermath which is also full of amazing stories of you know, Human heroism and tragedy because of course the TV show does this exceptionally well? So let's try to get down into the weeds on how nuclear

reactor

s operate, many of the details I'm gonna talk about here have already been discussed in my series "going nuclear" so that's a good place to be but if you haven't watched that it'll be suitable for everyone a nuclear

reactor

works as a...
why chernobyl exploded   the real physics behind the reactor
self-sustaining chain reaction where atoms of uranium Our plutonium are split into smaller lighter atoms and as they do that it releases energy This is called fission now as the atoms split They also spit out high-energy neutrons which bounce around inside the core and some of them end up hitting other Uranium or plutonium atoms and make them split now

reactor

s are balanced so that the average of one Neutron per reaction Goes on to trigger another reaction if the average is slightly higher Then the reaction rate will actually increase and if it's slightly lower the reaction rate will decrease over time Since each fishing produces two to three high-energy neutrons, they need to get rid of the excess neutrons somehow There are three things that can happen to these, firstly they can go on to trigger another reaction They could escape the core completely and be lost or they could be lost by being absorbed By another type of atom that doesn't undergo fission

reactor

designs include Materials inside them that are designed to absorb the actual extra neutrons These are our materials like boron or cadmium so that it can keep the reaction rate constant some of these absorbers are in the form of movable control rods that can be moved in and out of the core to dynamically change the neutron absorption rate and keep the amount of Excess neutrons constant The nuclear fuel used in the

reactor

at

Chernobyl

is uranium Naturally occurring uranium is made of two primary isotopes...
why chernobyl exploded   the real physics behind the reactor
There's uranium 238 which is the most common and instead of splitting when hit with neutrons. It tends to absorb them instead Uranium-235 is much rarer, but when it's hit by uranium That's the one that splits and releases the energy. So for naturally occurring uranium only about 0.7 percent is the fissile uranium-235 However, in the first nuclear bomb dropped the little boy its uranium was enriched to something like 80% uranium-235 because of complex nuclear quantum

physics

Which I'm going to completely gloss over, the chances of a neutron and being absorbed or triggering fission changes with the energy of the neutrons in uranium-235 the chance of a neutron causing fission is about 1,000 times larger for low energy neutrons as it is for high-energy neutrons The neutrons that come out of each fission event are very high-energy They are moving at a good fraction of the speed of light So

reactor

s are designed that they can slow these neutrons down closer to the speed of sound and they do this by having the neutrons bounce around off the nuclei of atoms and every time they Bounce they slow down a bit The best atoms for this job are the ones which rarely absorb neutrons and which are very light So that each bounce transfers as much energy as possible most commonly we will see

reactor

s using either carbon in the form of graphite hydrogen and oxygen in the form of water or These materials which slow neutrons down are called Moderators and they make it possible to run...
why chernobyl exploded   the real physics behind the reactor
a nuclear

reactor

without spending a lot of time and effort enriching your fuel to weapons-grade levels It's worth pointing out that regular hydrogen still has a reasonable chance of absorbing neutrons Which does offset its utility as a moderator, but it does work if the fuel enrichment is high enough deuterium has a much lower chance of doing this which is why heavy water is used in some

reactor

designs at

Chernobyl

the

reactor

s were the RBMK design Which is a Russian acronym for reaktor Bolshoy Moshchnosti Kanalny Which I think is roughly translated as high power channel type

reactor

The channels are a series of pipes that run vertically through the core of the

reactor

Which carry cooling water and they contain things like fuel rods control rods Neutron sources instrumentation depending upon the configuration of the

reactor

while many water-cooled

reactor

s use water as a moderator in the RBMK

reactor

s The neutron moderator is primarily graphite blocks that are placed around the channels This was a design decision which made it possible for the

reactor

to sustain a reaction using unenriched natural uranium as a fuel without resorting to heavy water as a coolant and this made it a very cost-effective design a set of electrical pumps would drive the water through the core the pressurized water entered from the bottom of the core Absorbs heat from the reaction and after leaving the core the steam is then separated and used to drive the turbines And so we come to the test...
that was a series of procedures being performed On the night of the accident in the event of an emergency The

reactor

would be shut down but the cooling process Had to continue the

reactor

continues to produce heat after the reaction stops because the split atoms are radioactive and the And they're slowly decaying and releasing energy and that meant you couldn't immediately turn off the cooling water to the

reactor

So those pumps had to keep operating and to make sure this happened there were diesel Generators that could startup and provide power to the pumps in an emergency However, those generators would take about a minute or so to come up to speed So another energy source was needed for that first critical minute and that energy was supposed to come from the already spinning turbines and generators as the steam generation stopped the turbines would begin to spin down But they would still have kinetic energy and so they could continue to generate electrical power as they converted their kinetic energy into electricity and that would keep the pumps running for long enough that the generators could kick in this had never been successfully demonstrated for a

reactor

at

Chernobyl

the voltage Regulators had always let the power drop off too quickly So this test was supposed to be the one where they finally showed that this safety system could work The

reactor

was being brought offline for maintenance anyway, so it was scheduled for the time when they were shutting it...
down However, that day the power grid had needed more energy than expected that meant that

reactor

4 wasn't allowed to be taken offline when planned and instead the

reactor

continued operating at a relatively high power level until late into the evening now remember I explained that the balance of neutrons being absorbed is important and how neutron absorbing isotopes are used to control this one of the most critical fission products that builds up in the core is xenon 135 and it is exceptionally good at absorbing neutrons that would otherwise sustain their reaction but xenon-135 doesn't appear instantly About 95% of it comes from iodine 135 which has a half-life of about six and a half hours So if a

reactor

has been operating the xenon from the reactions only appears about six and a half hours later As a

reactor

operates the amount of xenon 135 in the core grows until it reaches an equilibrium And the extra Neutron absorption from this means that less Neutron absorption is needed from control rods in nuclear engineering Xenon-135 is referred to as a neutron poison because of its ability to kill the reaction by robbing off the neutrons that it needs Now this is expected and under normal operation the

reactor

control system will adjust the control rods to keep their

reactor

rate constant in an active

reactor

the Xenon-135 gets removed by being burned when it absorbs a neutron It becomes xenon 136 which is much less likely to absorb a neutron and is very stable or if...
the

reactor

is Idle then. It's not generating any neutrons So this will decay into cesium 135 with a half-life of something like nine and a half hours but it's important to

real

ize that if the

reactor

power is reduced the xenon production will continue at the rate from six and a half hours ago, but the burn rate now occurs at the lower power levels So bringing a

reactor

powered down puts it in a situation where the neutron poison builds up and slows things down even more Normally the

reactor

was designed to operate at about 3,200 megawatts, but for most of the day preceding the accident it had been running at about 1,600 megawatts for the test The power was supposed to be brought down to about 700 megawatts and to be clear when I talk about

reactor

powers here This is the thermal power being generated inside the

reactor

. This is not the electrical power that is coming out of it so the power reduction from 1600 megawatts to 700 began at a bit ten minutes past eleven and Just over an hour later the operators who just changed shifts Managed to stall the

reactor

and the power levels plummeted down to about 30 megawatts too low to run the test At this point the

reactor

was in a state where it was gonna be

real

ly hard to bring it back up to power that xenon 135 was still building up and it wasn't being burned off because there weren't enough neutrons flying around any neutrons that were being generated were being used to burn up the xenon This is colloquially...
referred to as being stuck in the xenon pit But that wasn't the only effect of choking the

reactor

of its neutrons Remember that water is a weak neutron absorber now during normal operation the cooling water is being boiled and that creates low-density voids in the water and this reduces the effect of density of the water and therefore the amount of Neutron absorption so when the

reactor

power crashed all the way down the washer wasn't being boiled and And that means it was absorbing even more neutrons than normal So under pressure from the lead engineer the controllers attempted to restore the

reactor

to a power level Where the test could occur which meant they had to reduce the Neutron absorption And of course they did this by pulling control rods further and further out of the core the control rods in the RBMK

reactor

s Used boron carbide as their neutron absorber But if they were simply pulled out of the core Then the space left behind would contain water and that is also a neutron absorber so to enhance the effectiveness of the rod they would instead pull in a piece of graphite that would act as a moderator and Therefore enhance the reactivity of the system therefore this in theory made the control rods much more powerful controls of the

reactor

Normally, there were more than two hundred rods used to control the core but with all the xenon stealing their neutrons the operators pulled almost all of them out there were Equivalent of less than eight rods left...
actively controlling the core and yes, that does sound dangerous but I imagine that the operators were no doubt comforted and encouraged to do this to push the limits because They knew that if things got out of hand there was always the emergency power shut down switch They could reinsert all of the control rods as quickly as possible By manually pulling so many rods out of the core They were able to get the

reactor

back up to power for about 200 megawatts well below the power that they should have had according to the test protocol but high enough that by Manually managing the water flow through the

reactor

. They were generating enough steam to spin the turbines up to their operating speed and so the test began, the turbines were isolated and they began to spin down and It was at this point things went wrong in a big way with so many of the control rods removed the water had become a significant contributor to the neutron absorption and with the power levels so low and the cooling system running at a Correspondingly low speed for the power. The

reactor

becomes very very Sensitive to the boiling of the water as the water begins to boil and generate void Fewer neutrons get absorbed which in turn means that the reaction accelerates and heats the water even more This positive feedback mechanism is summed up in the phrase positive void coefficient and this term comes up regularly when describing the accident at

Chernobyl

in

reactor

s where water is a coolant and a moderator, then...
an increase in voids reduces the moderating effect and therefore slows the

reactor

This is what's called a negative void coefficient and it tends to make the

reactor

more stable But the void coefficient is just one of several Reactivity coefficients that describe how the

reactor

core responds us to changes in

reactor

conditions Another

real

ly good example is the fuel temperature coefficient which tells us how the

reactor

reactivity changes as the fuel heats up This is usually negative too. So that as the reaction gets going it acts to slow things down a

real

ly good example of this fuel temperature Coefficient is the triger

reactor

s Which are research

reactor

s that supposedly could be operated by high school students These have extremely negative fuel temperature coefficients They generate large Short-lived pulses of power that are very quickly arrested as the fuel heats up the mechanism behind this is actually pretty complicated But roughly speaking as the fuel heats up the atoms in the solid Vibrate more and this motion has to be added to the neutrons speed Changing the effective velocity of your neutrons flying through the core which in turn affects the absorption and scattering Parameters and so the fission cross-section effectively drops off and it's sometimes called the Doppler Coefficient because it's a result of Doppler shift of the neutrons encountering the atom But look the point here that I'm making is that

reactor

s are designed to operate in regimes...
for all these coefficients all these factors result in a self stabilizing reaction and then the RBMK

reactor

s The destabilizing positive voico efficient of the water was normally offset by this and other stabilizing mechanisms With the

reactor

in this low power low flow state some changes in the pressure flow or temperature began, a power feedback loop and the

reactor

power began to rise rapidly and as the power began to rise the washer began boiling at lower and lower Points in the channels allowing the reaction to effectively move down in the course He normally at higher water flow rates If this happened the pressure of the washer would push the voids back up and it would self stabilize. But with the water flow being low because of the low power that contributed to

reactor

instability around this time the

reactor

scrammed control was triggered This was the az5 switch or the EPS emergency protection system five control This is actually a cluster of six buttons with plastic covers and wax seals So you couldn't casually trigger them each of buttons do initiate a different safety procedures For example, I think one would bring it down to 50% Slowly and then with one that would bring it down to 50% quickly but a Zed 5 Quenched the power to zero as quickly as possible And of course the way it quenched the power to zero was by inserting those control rods as quickly as possible unfortunately, this wasn't a particularly fast process because the rods had to push down these...
channels and push the water out of the way and it would only move at about 40 centimeters per second. The core was about 7 meters long So we take 18 seconds to completely push the rod down through the core the control rods also had to push the graphite rods that had taken their place out of the way now those reaction enhancing rods were about four and a half meters long and there were positions that they were central to the core that meant they You had about 1.25 meters of water above and below them This had the effect that when they were being pushed through the core the lower couple of metres Initially had water displaced and replaced with by graphite and that meant the reaction at the very bottom of the core was enhanced as soon as you started shutting the

reactor

down so in an effort to get power for the test The controllers had pushed the

reactor

into a configuration where they had started getting the power But it was no longer stable And when that power started to come in, they tried to shut it down but the process of shutting it down actually Temporarily enhanced the reaction for a few seconds and in those few seconds the power skyrocketed it shot up way beyond the design limit and that's what happened seconds after the scram started. There was an explosion followed, a few seconds later by an even larger explosion which blew the top off the

reactor

and left it sitting at an angle the final power reading that was recorded in the control system was 33 gigawatts About...
10 times higher than the design power of the station, but most models of this excursion suggest that it may have peaked at over 300 gigawatts The actual mechanics of the explosion are still somewhat open to debate most people believe that the increase in power flash boiled the water to steam in the channels and then caused the pipes to rupture and the second larger explosion may have been due to water being Disassociated into hydrogen oxygen by the heat and then subsequently collecting and combusting elsewhere But there are those that argue that this was literally a small nuclear explosion where the reaction went prompt critical and ran away so quickly that it only stopped when the fuel was vaporized and

exploded

beyond the point of criticality In one scenario the vaporized material literally shoots out of the channel like a jet of plasma directed skywards regardless, once the power ran away there were many mechanisms that could have led to the destruction of the

reactor

and its containment it was Important to

real

ize that the time most experts in the world thought the idea for nuclear

reactor

having a runaway excursion and explosion like this was impossible and of course the reason this was so far-fetched was because

Reactor

s are designed to prevent these circumstances and even the shortcomings of the RBMK

reactor

Were remedied in later years There are still a number of RBMK

reactor

s operating around the world Even though though

Chernobyl

's

reactor

s one through three...
were only shut down a few years ago but that's my cold analytical

physics

side of things The events that happen after the explosion are where the

real

meat of the drama lives and the TV show does do a fabulous job Of putting the viewers into some of these situations and yes there are some horrific scenes of victims suffering from extreme radiation poisoning And that's the

real

ity some of the first responders had to experience but having seen that it's important that you keep in mind that scientific study of energy generation and the human cost of energy generation shows that nuclear is actually one of the safest options out there So I don't want you to come away from this video thinking that all nuclear

reactor

s are a potential

Chernobyl

waiting to happen That's simply not the case But I do hope you come away from this with a bit more Understanding of the

physics

and perhaps a bit more respect for power in general. I'm Scott Manley. Fly safe.