Chernobyl: How did the RBMK nuclear reactor explode and could it happen again?

Chernobyl, the dark and brutal mini-series Telling the story of the worst nuclear disaster in the world will probably be one of the best television shows of this year and perhaps even of all time.

Written by Craig Mazin and directed by Johan Renck, Chernobyl stoically adhered to the epoch and crisis he described as an irradiation attached to abandoned firefighter uniforms. It may have taken artistic liberties for history, but refused to sweep the truth about the disaster under the carpet. He has made historical truths and innumerable lies cruel.

At every step, Chernobyl evoked the inefficiency of Russian governance, the uncompromising courage of the liquidators responsible for cleaning up the site, the weight that weighed on the shoulders of all the scientists investigating the disaster and the harsh reality of atomic power. .

But the greatest asset of Chernobyl is to have inspired an immense scientific curiosity to its spectators through horror. We know that Chernobyl really took place – and the uncompromising and honest approach to the catastrophic disaster only increased curiosity. Google Trends shows a huge spike in the search for terms related to the science of the show: "RBMK reactor", "nuclear reactor" and "radiation sickness" have all experienced huge leaps since the Chernobyl TV debut.

During its five episodes, Chernobyl has constantly sought to answer a question – "How?" – and we wanted to go ahead and find the answers ourselves. The last episode, broadcast on June 3, finally revealed the truth on this morning of April 1986.

Valery Legasov, the head of the commission to investigate the disaster, is participating in the trial of three power plant officials responsible for the blast and its immediate consequences. With the politician Boris Shcherbina and the physicist Ulana Khomyuk, the trio details the main causes of the disaster and clearly highlights the shortcomings of those responsible, including chief engineer Anatoly Dyatlov, who was responsible for the explosion of the factory.

But we are talking here about nuclear physics. Things are messy and confusing. The term "positive vacuum coefficient" is used and this is not a term you hear every day. Even the Chernobyl engineers could not fully understand the consequences of their actions. So we rummaged through the radioactive quagmire to present the science behind the Chernobyl RBMK reactor explosion – and the reasons why it is unlikely to happen again.

What is a RBMK reactor?

The Russian nuclear program developed the technology for RBMK reactors in the 1950s, before the start of construction of the first RBMK-1000 reactor at Chernobyl in 1970. RBMK is an acronym for Reaktor Bolshoy Moshchnosti Kanalniy, which translates as "reactor high power channel type ". "

In simple terms, the reactor is a giant tank full of atoms, the cornerstone of everything we see. They are themselves composed of three particles: protons, neutrons and electrons. In a reactor, neutrons collide with atoms, separating them and generating heat in a process called nuclear fission. This heat helps to generate steam and the steam is used to spin a turbine which, in turn, forces a generator to create electricity in the same way as hot coal.

The RBMK reactor that exploded at Chernobyl, No. 4, was 7 meters high and almost 12 meters wide. The most important segment of the reactor is the heart, a huge piece of graphite, sandwiched between two "organic shields" like meat in a hamburger. You can see this design below.

The nucleus is where the fission reaction occurs. It has thousands of channels containing "fuel rods", composed of uranium that contains "easy" atoms to split. The core also includes control rod channels, made of boron and with graphite tips, designed to neutralize the reaction. The water flows through the fuel rod channels and the entire structure is covered with steel and sand.

Water is essential to understand what happened at Chernobyl. In a RBMK reactor, water has two jobs: keep things cool and slow down the reaction. This design is implemented in the same way in no other nuclear reactor in the world.

Fuel rods are the core of the nucleus and are composed of uranium atoms. The uranium atoms cast a net into the nucleus and pass through the solid graphite around them as sneaky neutrons mix. Graphite "slows down" these neutrons, much like water, which makes them more likely to be captured by the network of uranium atoms. Collision with this network can release more neutrons. If the process repeats in a chain reaction, it generates a lot of heat. Thus, the water in the channel boils, turns into steam and serves to create energy.

If left unchecked, this reaction will run away and cause a meltdown, but the control bars are used to balance the reaction. Simplistically, if the reactor generates too much power, the control rods are placed in the core, thus preventing neutron collision as regularly and slowing down the reaction.

In a perfect world, the systems and the people who control them ensure that the balance does not tilt too much in one direction or in the other. The control rods enter and exit the reactor, water is pumped continuously so that everything remains cold and the plant produces energy.

But if the plant itself loses its energy, what happens? This is one of the weaknesses of the RBMK reactor. No electricity means that the water is no longer pumped to cool the reactor – and this can quickly lead to a disaster. In the early hours of April 26, 1986, the reactor was subjected to a safety test to solve this problem.

The security test

The safety test is the starting point for a series of errors that led to the explosion of reactor 4.

The facts are so:

• In case of power failure or power failure, the RBMK reactor will stop pumping water through the heart.
• A backup group of diesel-powered generators comes into play after 60 seconds in such a case – but this delay may put the reactor at risk.
• Thus, the test hoped to show how a RBMK reactor could fill the 60 second delay and continue to pump cold water into the system using available energy generated by the slowing down of the plant turbines.
• The test was initially scheduled for April 25 but was delayed by 10 hours by the power grid officials in Kiev.
• Because of this delay, a team of night teams had to perform the test, for which she had not been trained.
• To carry out the test, the reactor had to be put into a dangerous state of low power.

The state of low power consumption in the RBMK reactor is not like you put your computer in sleep mode. He can not quickly return to his usual state of nutrition. However, the Chernobyl control room team tried to do this and ignored the security protocols in place.

In an attempt to restore the current to an acceptable level, the workers removed the kernel control rods, hoping to restart the reaction and restore it. But they could not do it. During the 10 hour delay, the low power state of the nucleus caused a build up of xenon, another type of atom that essentially blocks the nuclear fission process. The core temperature also dropped so much that she stopped boiling the water and producing steam.

The usual procedure with such a low power would be to bring the power level of the heart to over 24 hours. The head of the plant, Dyatlov, did not want to wait and therefore continued his safety test.

"Any commissioning test involving changes to protection systems must be very carefully planned and controlled," says Tony Irwin, who advised the Russians on the safe operating practices of RBMK reactors following Chernobyl.

"In this accident, they acted out of their rules and bypassed the protection that was intended to keep the reactor safe."

A disregard for the rules – and science – exposes them to the great danger of the RBMK: The positive vacuum coefficient.

Positive vacuum coefficient

We hear about "positive void coefficient" in the latest episode of Chernobyl, written by Legasov of Jared Harris, who plays a key role in the explosion – but this is not explained precisely.

Remember how water at once cools the core and "slows down" the reaction down. However, when the water turns to steam, it does not have the ability to effectively do both of these things because it bubbles and turns into bubbles or "voids". The water / steam ratio is called the "vacuum coefficient". In other nuclear reactors, the vacuum coefficient is negative – more steam, less reactivity.

In the RBMK reactor, it is the opposite: more steam leads to increased reactivity. This positive vacuum coefficient is specific to Russian RBMK reactors.

Once the workers at the plant shut down the reactor at 1:23:04 am, the water is no longer pumped into the heart. The catastrophic Chernobyl waterfall is launched.

The safety test stops the reactor and the remaining water is evaporated. So, no more steam.

Steam makes nuclear fission more efficient and faster. So, more heat.

More heat boils the water faster. More steam.

More steam … you get the point.

If we stop the image here, the scenario is dark. The core quickly generates steam and heat during a runaway reaction. All 211-plus control rods, with the exception of six, have been removed from the core and the water no longer produces any cooling effect. The nucleus is now a giant children's bullet pit in an earthquake, with neutrons bouncing around the chamber and constantly colliding with each other.

The only thing the factory workers could do was press the emergency stop button.

The Chernobyl explosion

At 1:23:40, the night shift leader, Alexander Akimov, pressed the emergency stop button. This forces all the control bars in the kernel.

Command bars must decrease the reaction, but because they're finished with graphite, they make sure that the power increases even more. Over the next five seconds, the power increases dramatically and reaches levels that the reactor can not withstand. The caps on the top of the reactor core, weighing more than 750 pounds, begin to bounce in the reactor hall.

Then, at 1:23:45, the explosion occurs. This is not a nuclear explosion, but a steam explosion, caused by the huge buildup of pressure in the nucleus. This eliminates the biological shield from the top of the core, breaks the fuel channels and detonates the graphite. This results in another chemical reaction: air enters the reactor hall and ignites, causing a second explosion that puts an end to nuclear reactions in the core and leaves a huge hole in the reactor building. Chernobyl.

Could this happen again?

It's a bit crazy to think that humans can control the power of the atom. The Fukushima disaster, which hit a Japanese nuclear plant in 2011, shows that disasters still exist in reactors around the world and that we are not always prepared for these disasters.

After Chernobyl, a number of modifications were made to RBMK reactors in Russia. Today, there are still 10 such reactors across the country – the only place they currently operate.

These sites have been equipped with safety devices to prevent the creation of a second Chernobyl. Command bars have been made more numerous and can be inserted more quickly into the kernel. Fuel rods contain slightly more enriched uranium, which allows better control of nuclear reactions. And the positive void coefficient, although it still exists in the design, has been drastically reduced to avoid the possibility of a new low-power fusion.

Of course, the only thing that has not changed, is us. Chernobyl was a failure on the human scale long before the failure of the atom. Trying to control nuclear fission reactions will always involve risks and these risks can only be mitigated – not reduced to zero.

So, can this kind of nuclear disaster happen again? Yes. As long as we try to exploit the power of the atom, the odds will be more favorable to the disaster.