Nuclear power had become routine and a bright atomic future lay ahead.
The Soviet Union had five power stations of this type but there is only one of these we all know about ... Chernobyl.
On this evening there was no hint of the catastrophe about to unfold that would have a profound impact on the USSR and nuclear power around the world.
At 1:23 am during a safety test the AZ-5 button was pushed. About 8 seconds later reactor 4 exploded.
Why did the emergency shutdown blow up the reactor? This is the central Chernobyl question.
At the time of the disaster Ukraine was one of the 15 soviet republics. Chernobyl was in northern Ukraine near the Belarus border.
The centerpiece of these power stations was the RBMK reactor.
The RBMK was a graphite reactor which had been powering Soviet growth since 1970.
Chernobyl's reactor no. 4 was only 2.5 years old.
**THE SYSTEMS**
These are the main systems. The nuclear reactor produces large quantities of heat.
Pumps circulate water vertically up through the reactor to collect the heat.
Steam collects in the separator tanks . The steam spins the turbine generators which power the electrical grid.
The steam is cooled back into water from a large lake and fed back into the main circuit.
It runs like this at full power for months on end.
The buildings required to accommodate the systems were extensive.
The components were located inside individual concrete compartments.
The concrete vault around the reactor was 2.5m thick.
The reactor was housed inside a steel drum.
** GRAPHITE MODERATOR **
A central component of this story is graphite.
The core contained huge amounts of graphite which was essential for the nuclear reaction.
When an atom splits it releases neutrons at near the speed of light.
These neutrons go too fast to split the surrounding uranium. Neutrons slow down as they bounce around inside the graphite until they travel slowly enough to split more atoms.
Graphite is called the moderator because it moderates the neutron speed.
Each graphite block has a hole down the center which contains the pipes for the fuel channels and control rods.
Water flows inside these pipes at high pressure.
The uranium fuel is formed into these small pellets.
Each pellet is extremely fuel dense and contains the heat energy equal to a truckload of coal.
** CONTROL RODS **
A reactor must have an on off switch and a way to set the power level. This is done with control rods.
These are made of boron which is a very powerful neutron absorber.
When the rods are in the core they kill the reaction.
With about 80% of the rods withdrawn from the core the reactor runs at full speed.
When the control rods were extracted they left a space behind.
The lower half of the control rods were made of graphite which filled this void.
Exchanging boron with graphite was a sound design principle.
The reactor was more efficient and it made the control rods twice as effective.
** DESIGN FLAW **
The explosion was the result of a design error called the "end effect".
This caused a momentary power spike precisely when the control rods were supposed to kill reactivity.
The graphite displacers did not extend all the way to the floor of the reactor.
The lowest 1.25 meters were cut short because of lack of space below.
The floor of the reactor was where things went badly wrong, but we will come back to this.
** THE TEST **
The objective of the test was to simulate loss of grid power and verify if the flywheel effect of the turbines
could power the water pumps until the diesel generators powered up.
3 previous attempts at this test had failed.
The test had to be performed during a reactor shutdown. On this day the shutdown was paused at 50 % power for most of the day.
This resulted in two crucial contributors to the disaster.
1. the reactor became xenon poisoned and
2. the test was performed by the less experienced night shift who had not been properly briefed.
At 1am the shutdown began but was paused because the electrical grid needed additional power and Chernobyl unit 4 had to make up the shortfall.
After a 19hr pause the shutdown was permitted to continue.
The new shift took over at midnight and put the reactor onto auto control to keep it steady at 22% power.
Except auto mode was not designed for a poisoned core power plummeted to 1% before the test had been done.
** XENON POISONING **
While the reactor was running at half power it became poisoned with xenon gas.
Xenon is a powerful neutron poison. A basic reactor rule is not to restart a reactor which has been poisoned because it is unstable.
Xenon forms 6 hours later and interferes with the fine equilibrium of the nuclear reaction.
The engineer in charge Anatoly Dyatlov was under pressure to do the test.
He made the fateful decision to extract almost every control rod to bring the reactor back up to enough power for the test.
This was like in a car applying more than full throttle to counter a jammed handbrake.
1 Hr after the reactor stalled the controllers had raised the power back up to 6%.
** AZ-5 BUTTON **
The test was performed at 1:23 am and lasted 40 seconds.
The control staff had agreed beforehand upon completion of the test the AZ-5 button was to be pushed to complete the shutdown.
The control staff reported all was calm at this time.
You may be familiar with the safety cover and rotating switch but this was the retrofit after the disaster.
The historic button was most likely this one.
The control rods were in this configuration when the AZ-5 was pushed.
The core should have had less than the equivalent of 15 rods active.
But on this evening it had almost none because of the misguided attempt to counterbalance the xenon poison.
The reactor had been pushed too far. The AZ-5 inserted all the top control rods simultaneously.
unfortunately the lower control rods were not coupled with the button. If they had moved the disaster would have been avoided.
Reactor 4 was teetering on the brink and the end effect would be the final straw.
Water is a weak neutron poison. As the graphite displaced the water a temporary power surge formed across the floor of the reactor.
The boron would arrive too late.
The rapid jump in power caused steam bubbles. Less water meant less neutron poison - more heat and more reaction.
This was the feedback loop to destruction. There was now nothing stopping a reactivity runaway.
The base of the reactor was overwhelmed and fuel channels vaporized and smaller explosion heard in the control room.
Reactor pressure was lost and coolant water flashed to steam increasing reactivity across the core.
Reactor 4 in that moment was producing the energy equivalent of much of the soviet electric grid.
Reactor power was an estimated 20 times the maximum. We will never know because there was nothing left to do the measuring.
The reactor drum and the building structure could not hold all that sudden energy release.
The explosion blew the 1 000 ton reactor lid through the roof and it landed back in the reactor pit on its side.
The north face of the building fell away in the explosion and exposed the steam tanks and main pumps.
Firemen spoke of warming their hands over the graphite lying all around on the ground.
They were unaware they were exposing themselves to lethal doses of radiation.
This was a heat and gas explosion. It was not an atomic bomb and nuclear scientists get frustrated when Chernobyl is called one.
This is the control room about 25 years later.
It's equipment had been reused elsewhere or removed as souvenirs.
** ACCOUNTABILITY **
Soviet nuclear scientists were aware of the "end effect" problem.
There had been previous channel ruptures from the AZ-5 "tip effect" in Leningrad 1975 and Ignalina 1983.
Design changes had been recommended but the RBMK was considered very robust and it was widely believed it could not explode.
No changes are made and crucially the lurking danger was kept secret from controllers.
Had they known they could have compensated for it in their decision-making.
The controllers took comfort in the AZ-5 emergency shutdown button always being there if they needed to shut down the reactor in a hurry.
But it turned out to be the hidden detonator in the wrong configuration.
The IAEA commission found that at below 50% power the control instruments provided incomplete feedback.
The low power dangers were not well described in the operating documentation and the procedures were sometimes contradictory.
The operators would have been somewhat blind to what was going on inside the reactor and had to rely more on experience and intuition for control.
** SAFER REACTORS **
To build inherently safer reactors later designs including those in Russia utilized water as the moderator.
Water is a far superior moderator. It added a vital additional automatic reactivity control and the core could be way smaller.
As the water boils into steam, the moderator reduces and there is an instantaneous drop in reactivity.
With higher grade uranium water can be used as the moderator.
Graphite was a favored moderator in earlier reactors around the world for low-grade uranium.
The switch to water happened when uranium enrichment technology had advanced so large quantities could be produced.
With perfect hindsight it is so easy to be critical of a time when the science was in earlier stages and there was minimal computer modelling.
Man can dream up the most amazing things but humans are responsible for the things going wrong.
We will never really know what happened in the control room that night and tragically most who were there paid with their lives
as they heroically scrambled to deal with multiple other dangers
that could have put the other reactors at risk of meltdown and a wider disaster.
It's natural to focus on the recklessness of a few but this is as much a story about the epic bravery and personal sacrifice of so many.
Occasionally humans fail but mostly we succeed.
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