Chernobyl: How did the RBMK nuclear reactor explode and will it occur once more? – CNET

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Chernobyl, the bleak and brutal miniseries telling the story of the world’s worst nuclear catastrophe, is prone to go down as top-of-the-line TV reveals this 12 months and perhaps even all-time.

Written by Craig Mazin and directed by Johan Renck, Chernobyl stoically adheres to the period and disaster it portrayed like radiation clinging to discarded fireman uniforms. It might have taken some inventive liberties for the sake of story, however refused to comb the reality of the disaster underneath the rug. It rendered historic truths, and the numerous lies, in a harrowing mild.

At each step, Chernobyl touched on the ineptitude of Russian governance, the uncompromising braveness of the liquidators tasked with cleansing up the positioning, the load that hung over the shoulders of each scientist investigating the catastrophe and the stark actuality of atomic energy.

However Chernobyl’s crowning achievement is the way it impressed an immense scientific curiosity in its viewers by means of the horror. We all know Chernobyl actually occurred — and the hard-nosed, trustworthy method to the disastrous meltdown solely served to intensify that curiosity. Google Developments reveals an enormous spike in searches for phrases associated to the science of the present: “RBMK reactor”, “nuclear reactor” and “radiation illness” have all seen big leaps since Chernobyl’s TV debut.

Over its 5 episodes, Chernobyl consistently moved towards answering one query — “How?” — and we have needed to skip forward and discover the solutions out for ourselves. The ultimate episode, which aired on June three, lastly revealed the reality of that April morning in 1986.

Moments after the reactor explosion, Chernobyl burns.


Valery Legasov, the chief of the fee tasked with investigating the catastrophe, takes half within the trial of three energy plant officers chargeable for the explosion and its speedy aftermath. Together with politician Boris Shcherbina and physicist Ulana Khomyuk, the trio element the important thing causes behind the catastrophe and squarely level to the failings of these officers, together with chief engineer Anatoly Dyatlov, because the trigger for the plant’s explosion.

However we’re speaking about nuclear physics right here. Issues are messy and complicated. The time period “constructive void coefficient” will get thrown round and that is not a time period you hear day by day. Even Chernobyl’s engineers could not absolutely grasp the implications of their actions. So we have dug by means of the radioactive quagmire to deliver you the science behind Chernobyl’s RBMK reactor explosion — and the explanations we’re not prone to see it occur once more.

What’s an RBMK reactor?

The Russian nuclear program developed the expertise for RBMK reactors all through the ’50s, earlier than the primary RBMK-1000 reactor started development at Chernobyl in 1970. RBMK is an acronym for Reaktor Bolshoy Moshchnosti Kanalniy, which interprets to “excessive energy channel-type reactor.”

Within the easiest phrases, the reactor is a huge tank filled with atoms, the constructing block that makes up the whole lot we see. They’re themselves composed of three particles: protons, neutrons and electrons. In a reactor, the neutrons collide with atoms one other, splitting them aside and producing warmth in a course of referred to as nuclear fission. That warmth helps generate steam and the steam is used to spin a turbine which, in flip, drives a generator to create electrical energy in a lot the identical method burning coal would possibly.

The RBMK reactor that exploded at Chernobyl, No. four, was an enormous 23 toes (7 meters) tall and nearly 40 toes (12 meters) vast. Crucial phase of the reactor is the core, an enormous chunk of graphite, sandwiched between two “organic shields” just like the meat in a burger. You’ll be able to see this design beneath.


A schematic of the plant utilized in HBO’s Chernobyl displaying the graphite core and the organic shields.

HBO/Annotated by CNET

The core is the place the fission response takes place. It has hundreds of channels which comprise “gas rods”, composed of uranium which has atoms “simple” to separate. The core additionally has channels for management rods, composed of boron and tipped with graphite, designed to neutralize the response. Water flows by means of the gas rod channels and your entire construction is encased in metal and sand.

The water is crucial to understanding what occurred at Chernobyl. In an RBMK reactor, water has two jobs: Maintain issues cool and sluggish the response down. This design will not be carried out in the identical method in another nuclear reactors on the earth.

The gas rods are the powerhouse of the core and are composed of uranium atoms. The uranium atoms forged a internet within the core and as rogue neutrons ping round inside they go by means of the stable graphite that surrounds them. The graphite “slows” these neutrons down, very like the water does, which makes them extra prone to be captured by the uranium atoms internet. Colliding with this internet can knock extra neutrons free. If the method happens time and again in a sequence response, it creates plenty of warmth. Thus, the water within the channel boils, turns to steam and is used to create energy.

Unchecked, this response would runaway and trigger a meltdown however the management rods are used to stability the response. Simplistically, if the reactor is producing an excessive amount of energy, the management rods are positioned into the core, stopping the neutrons from colliding as commonly and slowing the response.

In an ideal world the methods, and males controlling the methods, be sure that the scales by no means tip too far by some means. Management rods transfer out and in of the reactor, water is continually pumped by means of to maintain the entire thing cool and the facility plant produces vitality.

But when the plant itself loses energy, then what occurs? That is one of many RBMK reactor’s shortcomings. No energy means water is not being pumped to chill down the reactor — and that may rapidly result in catastrophe. Within the early hours of April 26, 1986, the reactor was present process a security check which aimed to repair this situation.

The protection check


Valery Legasov testifies earlier than the fee, in entrance of the three energy plant officers chargeable for the catastrophe. 


The protection check is the start line for a sequence of errors which in the end resulted in reactor four’s explosion.

The info are so:

Within the occasion of a blackout or lack of energy to the plant, the RBMK reactor will cease pumping water by means of the core.A backup set of diesel-fuelled turbines kick in after 60 seconds in such an occasion — however this timeframe dangers placing the reactor at risk.
Thus, the check hoped to indicate how an RBMK reactor may bridge the 60 seconds and maintain pumping cool water into the system through the use of spare energy generated because the plant’s generators slowed down.
The check was initially scheduled for April 25 however was delayed for 10 hours by energy grid officers in Kiev.
The delay meant a crew of nightshift workers must run the check — one thing they’d not been educated to do.
To carry out the check, the reactor needed to be put right into a harmful low-power state.

The low-power state within the RBMK reactor will not be like placing your pc in sleep mode. It can’t be returned to its common energy state rapidly. Nevertheless, the crew within the management room at Chernobyl tried to do exactly that and disregarded the security protocols in place.

To aim to get the facility again as much as an appropriate degree, the employees eliminated the management rods within the core, hoping to kickstart the response once more and transfer the facility again up. However they could not do it. Throughout the 10 hour delay, the core’s low-power state prompted a build-up of xenon, one other kind of atom that in essence blocks the nuclear fission course of. The core temperature additionally dropped a lot it stopped boiling water away and producing steam. 

The same old plan of action with such low-power could be to deliver the core’s energy degree again up over 24 hours. The facility plant chief, Dyatlov, didn’t wish to wait and so solid forward with the security check.

“Any commissioning check involving modifications to safety methods needs to be very fastidiously deliberate and managed,” explains Tony Irwin, who suggested the Russians on secure working practices of RBMK reactors within the wake of Chernobyl.

“On this accident they had been working exterior their guidelines and defeating safety which was designed to maintain the reactor secure.”

A disregard for the principles — and the science — uncovered them to the RBMK’s nice hazard: The constructive void coefficient.

The constructive void coefficient

We hear the time period “constructive void coefficient” bellowed by Jared Harris’ Legasov in Chernobyl’s ultimate episode and it’s key to the explosion — but it surely’s not precisely defined.

Recall how the water each cools the core and “slows the response down. Nevertheless, when water turns to steam it lacks the power to successfully do each of these issues, as a result of it boils away and turns into bubbles or “voids.” The ratio of water to steam is named the “void coefficient.” In different nuclear reactors, the void coefficient is unfavorable — extra steam, much less reactivity.

Within the RBMK reactor, it is the alternative: Extra steam leads to larger reactivity. This constructive void coefficient is exclusive to the Russian RBMK reactors.


Emily Watson is riveting as a nuclear physicist who represents the entire actual life scientists that labored to unravel how Chernobyl exploded.


As soon as the plant employees shut down the reactor at 1:23:04 a.m., water is not pumped into the core. The catastrophic cascade at Chernobyl is about in movement.

The protection check shuts down the reactor and the remaining water boils away. Thus, extra steam.

The steam makes the nuclear fission extra environment friendly, rushing it up. Thus, extra warmth.

Extra warmth boils the water away quicker. Extra steam.

Extra steam… you get the purpose.

If we freeze-frame proper right here, the situation is grim. The core is rapidly producing steam and warmth in a runaway response. All however six of the 211-plus management rods have been faraway from the core and the water is not offering any cooling results. The core is now an enormous child’s ball pit in an earthquake, with neutrons bouncing across the chamber and consistently colliding with each other.

The one factor the plant employees may do was hit the emergency cease button.

The Chernobyl Explosion

At 1:23:40 a.m., the emergency cease button was pressed by chief of the evening shift, Alexander Akimov. This forces the entire management rods again into the core.

The management rods ought to lower the response however as a result of they’re tipped with graphite, they really trigger the facility to spike much more. Over the subsequent 5 seconds, the facility will increase dramatically to ranges the reactor can not face up to. The caps on the highest of the reactor core, weighing greater than 750 kilos, start to actually bounce within the reactor corridor.


The 700-plus pound metal blocks resting on prime of the reactor core began rumbling round and being lifted into the air within the moments earlier than the explosion.


Then, at 1:23:45 a.m., the explosion happens. It isn’t a nuclear explosion, however a steam explosion, attributable to the large buildup of stress throughout the core. That blows the organic protect off the highest of the core, ruptures the gas channels and causes graphite to be blown into the air. Consequently, one other chemical response takes place: air slips into the reactor corridor and ignites inflicting a second explosion that terminates the nuclear reactions within the core and leaves a mighty gap within the Chernobyl reactor constructing.

Might it occur once more?

It is sort of insane to suppose that people can management the facility of the atom. The Fukushima catastrophe that affected a Japanese nuclear plant in 2011 demonstrates that catastrophes nonetheless lurk inside reactors all over the world and we’re not all the time ready for them.

After Chernobyl, a variety of modifications had been carried out within the RBMK reactors throughout Russia. Right this moment, 10 such reactors nonetheless exist in operation throughout the nation — the one place the place they’re at the moment working.

These websites had been retrofitted with security options which goal to forestall a second Chernobyl. The management rods had been made extra plentiful and will be inserted into the core quicker. The gas rods characteristic barely extra enriched uranium which helps management the nuclear reactions a bit higher. And the constructive void coefficient, although it nonetheless exists within the design, has been dramatically diminished to forestall the opportunity of a repeat low-power meltdown.

In fact, the one factor that hasn’t modified is us. Chernobyl was a failure on the human scale, lengthy earlier than it was a failure on the atomic one. There’ll all the time be dangers in making an attempt to manage nuclear fission reactions and people dangers can solely be mitigated — not diminished to zero.

So can this sort of nuclear disaster occur once more? Sure. So long as we attempt to harness the facility of the atom, the chances will fall in favor of catastrophe.


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