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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 really 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 reactors 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
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 reactors 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
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 reactors 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 reactors 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 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 reactors 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 reactors use water as a moderator in the RBMK reactors
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 realize 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 really 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 reactors
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 reactors 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 really 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 really good example of this fuel temperature
Coefficient is the triger reactors
Which are research reactors 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 reactors are designed to operate in regimes for all these coefficients all these factors
result in a self stabilizing reaction and then the RBMK reactors
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 realize 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
Reactors 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 reactors operating around the world
Even though though Chernobyl's reactors 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 reality 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 reactors 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.
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