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Sky Atlantic and HBO's Emmy-winning Chernobyl (you can also watch it on NOW TV) is currently horrifying viewers up and down the country with its unflinching exploration of the nuclear accident that nearly destroyed the planet in 1986 and the cover-ups, before and after, that unquestionably made it worse.
Its grim authenticity – with its horde of naked, radiation-sick coalminers – is absolutely faultless, even down to the fabrics worn by '80s Soviet communist party officials.
But while its relentless exposure of the failings of the Soviet bureaucracy to prevent the explosion, to handle the danger, to deal with the consequences or to safeguard its own citizens makes for compelling TV, you may have a couple of questions.
Like… what actually happened?
The show does a great job of explaining the jeopardy at any given time, but the physics is, like, really complicated. So Digital Spy is here to help.
Here's what happened.
First, a super-sketchy guide to how nuclear reactors work
Inside a nuclear reactor core, the atoms of highly unstable metals (specifically uranium) are bombarded with neutrons escaping from other atoms, creating a chain reaction that just keeps going: neutrons split uranium atoms, creating more rogue neutrons, which split more uranium atoms and so on. Splitting atoms creates an enormous release of energy in the form of heat.
That heat is contained and used to turn water into steam, which spins turbines that convert movement into electricity. The nuclear chain reaction, meanwhile, is managed by control rods that absorb neutrons and therefore reduce the extent of the reaction at any given time.
Okay, so what went wrong?
On 26 April 1986, the night staff at VI Lenin Nuclear Power Station near Pripyat, Ukraine (popularly known as the Chernobyl Power Station) had turned off the emergency safety and power-regulating systems in order to conduct a test that simulated a power outage. This test had been delayed from earlier in the day by the Ukrainian power grid authorities, meaning the night shift staff were unprepared.
The idea was to see whether, in the event of an emergency shutdown, they could use residual power from the plant's steam turbines to run the water pumps that would cool down the reactor core until the back-up diesel generators kicked in.
First, for safety's sake, they had to run down the plant's power output to 700 megawatts (but no lower).
Due to human error, the control rods (which slowed down the chain reaction, remember, and therefore the power output) were inserted too far, slowing the output to 30 megawatts. Due to complex reasons of physics – chiefly xenon 'poisoning' – that we won't explain here because we don't understand them, they were only able to restore power to around 200 megawatts by manually adjusting the rods.
They went ahead with the test anyway, which they totally shouldn't have done.
More mistakes
They pumped in more water, which reduced the reactor power. It was meant to be no lower than 700 megawatts, but was running at 200, so they removed more control rods to increase the reaction and generate more steam to maintain the overall power output.
Remember the automatic emergency shutdown facilities had all been switched off at this point.
at 1.23.04 am, they began the test: they turned off the turbine generator to see if its residual power would keep the water pumps ticking over till the diesel generators kicked in in the next 45 seconds or so.
As the turbine coasted down, so did the water flow into the reactor, meaning there was less water cooling the reaction. This created pockets of superheated steam – and here's where it gets tricky.
In the "RBMK" type of reactor, like Chernobyl's, those steam pockets, or "voids", reduce the absorption of neutrons, which, you'll remember, means there are more neutrons around to set off nuclear fission, which means the reactor gets hotter. A hotter reactor creates more steam voids, which create a hotter reactor which creates more steam voids which, etc, etc… It's a feedback loop.
The Soviet authorities knew about this flaw in RBMK reactors but decided not to tell the engineers who actually worked in them, because that would have been an admission that the USSR wasn't perfect.
Even now, the automatic control system would have inserted the neutron-absorbing control rods at this point and cooled the reactor – but it had been overridden, because as far as the engineers knew, there was no danger.
The reactor was now out of control.
At 1.23.40 am, they pressed the emergency button that would override the manual controls and re-insert all the control rods, bringing the reaction to heel and reducing the power to manageable levels. To their knowledge, that should have fixed everything.
From bad to worse
Unfortunately, the control rods in an RBMK-type reactor have graphite tips – and in the time it took to reinsert the rods, the graphite tips, being considerably less accommodating to rogue neutrons that the water they were displacing, caused an even greater power spike.
The massively overheated core caused the fuel rods to fracture, which blocked the control-rod mechanism and meant they could no longer cool down the nuclear reaction.
The fuel rods ruptured into the coolant, the power output jumped by a factor of ten (to 33,000 megawatts), all the water nearby turned instantly to steam and the reactor exploded.
…And still worse
With no water to cool the reactor, it got even hotter – the "meltdown" you've heard of. Two seconds after the first explosion, there was a bigger one, which blew a 1000-ton protective steel slab up through the ceiling and dispersed the reactor core around the vicinity, starting a fire on the roof of the next-door reactor.
Yes, there were three more live nuclear reactors in the immediate vicinity still chuntering away at this point.
… And still worse
Nobody knew what had happened. Nobody wanted to believe the worst could have happened, so they treated it like a regular fire. The horrific consequences of that can be seen in Chernobyl.
Meanwhile, the molten reactor core, graphite and concrete slag were blazing at 1200 degrees celsius and about to burn through the floor into the water reservoir below – which would have created another explosion. So three volunteers – knowing it was probably a suicide mission – waded into the pitch-black chamber through extreme radiation exposure and opened the valves to allow the water to be pumped out. (Miraculously, they all survived – one into his 60s, when he died of heart failure, and the others are reportedly still alive today.)
Hey, guess what?
So the immediate danger of another explosion was averted, but there was still the problem of the molten core. If it burned down through the ground and reached the water table, it would have rendered large parts of Europe uninhabitable.
So with the help of some of the most resilient coalminers in the world, they dug underneath the power station and pumped the Soviet Union's entire supply of liquid nitrogen into a bodged-up heat exchanger to freeze the ground.
On the roof, they had to clear the radioactive debris from the explosion back into the ruptured station so that it could be sealed off. They discovered minesweeper robots would fail almost immediately under the bombardment of electromagnetism – the Soviet authorities had lied to the West German suppliers about how much radiation the robots would need to withstand – so they sent conscripted citizens up to do the job instead. The radiation levels were so high these men were only permitted to work in shifts lasting a maximum of 90 seconds. Ever. 90 seconds on that roof was long enough to absorb an entire lifetime's maximum allowable dosage of radiation.
All that remained was sealing off the station, evacuating the region, decontaminating thousands of square miles and exterminating every living creature within range (that means shooting cats and dogs). The wider consequences to people and the environment have so far proven incalculable.
So that's what happened.
Chernobyl is on Sky Atlantic at 9pm on Tuesdays
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