Reflections on Chernobyl

HBO’s Chernobyl Miniseries starts up this Monday and I decided to get familiar with the topic again prior to the first episode. I just finished re-reading Andrew Leatherbarrow’s Chernobyl 01:23:40 and James Mahaffey’s Atomic Accidents, both fantastic books. I love abandoned places, enjoy science and technology, and have a small obsession with disasters; Chernobyl fits right in

A Nuclear Primer

All commercial nuclear reactors in operation today are of the fission type. Fission occurs when atoms split apart. It creates a large amount of energy, and when properly directed, can cause other atoms to split and create more energy that continues in a chain reaction. This energy is typically used for one of two things: to cause catastrophic damage in a nuclear bomb or to boil water into steam to generate electricity in a power plant.

While the materials and principles used in each scenario are similar, they are harnessed in opposite ways. In a bomb, the goal is to cause an uncontrolled fission chain reaction that quickly consumes all of the fuel and results in massive damage and devastation. In a power plant the goal is to create a very controlled fission reaction that maintains a constant temperature to boil water into steam.

For nuclear power, controlling the reaction and cooling the reactor are very important. If the reaction spins out of control or cooling is insufficient the reactor will become so hot that its contents – fuel rods, control rods, casings, and even the structure itself – will literally melt into a radioactive lava, hence the word “meltdown”. A meltdown can cause explosions that destroy everything around the core and disperse vast quantities of radioactive material into the atmosphere to spread far and wide. Acoolant meltdown is bad but an uncontrolled reactor meltdown is far worse.

Nuclear power plants are designed to balance on a razor’s edge: enable a reaction that generates enough heat to make large amounts of electricity while keeping that reaction controlled so that it doesn’t destroy all life around it. To do this, reactors are designed with numerous redundant safety systems, sensors, and procedures to keep them safe. The Chernobyl disaster occurred due to a combination of flaws in all of these systems.

The RBMK Reactor

Construction on Chernobyl started in 1970 and the first of six planned reactors was online in 1977 generating 3200 Megawatts (MW) thermal output (1000 MW electricity). Three more reactors were built, with reactor 4 coming online in 1983. Two more were under construction at the time of the accident in 1986, and if completed, would have made Chernobyl the highest capacity non-hydro power plant in the world. All reactors were based on the proprietary Russian RBMK design, which was considered a classified state secret.

The RBMK is an early second-generation reactor design, considered outdated by the West at the time of its introduction. While the RBMK is a Boiling Water Reactor (BWR) like its contemporaries, it has major differences, the largest being that it is moderated by graphite instead of water.

A moderator is a substance that slows the speed of neutrons to increase the likelihood of a fission reaction. Modern reactors, and US/UK/Western reactors at this time, use water as both a coolant and a moderator. This design reduces the risk of an uncontrolled reaction meltdown due to loss of coolant — if the coolant water is lost, the reaction immediately stops and the reactor shuts down. There is no risk of increased fission and a runaway chain reaction. The RBMK uses graphite to enable the reaction and water to inhibit it; if coolant is lost the reaction will actually speed up, causing a catastrophic meltdown followed by a possible nuclear explosion.

To make matters more complicated, the RBMK runs hotter than typical reactors of the time (500 C vs 275 C), making cooling even more important. It supports online refueling which allows spent fuel rods to be replaced with fresh fuel while the reactor is running. This provides a practical advantage by avoiding multi-month shut downs for refueling every few years and lends itself to retrieving weapons-grade plutonium byproducts from the core before they start to degrade. The unfortunate effect of this design is that the uneven mix of fuel in the reactor causes hot spots of up to 700 C in various parts of the core, making it more difficult to manage.

Control rods, like their names imply, are used to control the reaction. They are what prevent a reactor from becoming a bomb. Control rods are made of a neutron absorbing material such as boron which will slow the reaction down. Each reactor has hundreds of control rods that are inserted and removed to varying degrees to control the reaction. If all control rods are inserted, the reaction ceases.

RBMK control rods had a couple major design flaws. First, they were extremely slow to insert, requiring up to 21 seconds to descend all the way into the core. That’s not good when an out of control reaction can double in thermal power every second. The second issue is that they were tipped with graphite, the same material used in the reactor to increase reactivity. The graphite was intended to displace water so that the boron could work more effectively, but the Soviets discovered that inserting many control rods at once could cause a power surge before the reaction began to slow down. This flaw would prove devastating to Chernobyl.

While the RBMK reactors at Chernobyl had backup coolant, backup generators, sensors, warnings, and procedures like their Western counterparts, they were missing a critical safety protection: a containment building. A containment building is a sealed structure designed to sustain high pressure and keep the radioactive contents of a reactor from escaping due to a meltdown. The RBMK was rather large and required lots of cranes above it for refueling, making a containment structure cost prohibitive. Instead, its sides were lined with reinforced concrete and it was topped with a thick 450 ton metal plate called the Upper Biological Shield which was intended to contain a blast. The building around the reactor was rather frail and the roofs of both Reactor 4 and its next door neighbor Reactor 3 were built with flammable materials because fire-proof material wasn’t available at the time.

The RBMK was based on a simple design that made it cheaper to build and easier to maintain with the ability to harvest fuel for atomic weapons to boot; unfortunately that simplicity was traded for safety.

A Recipe for Disaster

The top of an RBMK reactor showing the fuel channels above the Upper Biological Shield

So we’ve got a nuclear reactor, technology that is constantly on the brink of a meltdown, combined with a reactor design that runs hot, requires lots of coolant, will go out of control in the event of a coolant loss, increases power when control rods are inserted to stop the reaction, and doesn’t have a complete sealed containment structure. Combine that with the testing that was done on the night of April 26th 1986 and we have a disaster.

The RBMK reactors at Chernobyl required an uninterrupted flow of coolant to prevent a meltdown. The coolant was pumped in using power generated by the reactor itself. When the reactor was shut down, the pumps were powered by the grid. If the grid was down due to a blackout, backup diesel generators would pick up the slack. Unfortunately the generators needed 50 seconds to start up, leaving an unacceptable gap in coolant to the reactor. The RBMK was designed so that the power generated as its turbine spun down could be used to power the pumps while the generators spun up, however this feature was not enabled during construction and had to be retrofitted and tested.

To ensure that the turbine could power the pumps, the team would drop the output of the reactor core down to around 700 MW thermal to simulate blackout conditions, disable the emergency generators, and measure the voltage of the turbine to see if it could power the pumps. This test had been conducted on reactor 3 in 1982, 1984, and 1985 and failed each time. This fourth test was supposed to be conducted by the day shift crew who had been trained for it but the test was postponed until after the peak power demand period had ended. This left it up to the untrained third shift to execute the test using a set of notes with numerous hand-written corrections on it.

With major safety systems disabled, the third shift team started the test under the guidance of Deputy-Chief Engineer Anatoly Dylatov. They inserted control rods into the reactor to reduce its output, but output fell all the way down to 30 MW thermal, which is essentially a full shut down. At this point they should have stopped the test and done it again at a later time but Dylatov insisted that they bring the reactor back up to the correct values and continue the test. The staff protested but eventually relented and pulled control rods out to bring the output back to desired levels. After a few minutes output had not increased as expected so the crew continued removing control rods until output rose. By the time there was enough output for the test there were barely any control rods inserted. The RBMK was difficult to control at low power levels and the core had become critically unstable without the operators’ knowledge.

The test continued when the turbine was disconnected from the reactor for measurement. This slowed coolant flow to the reactor, which created more steam, which reduced coolant, and started increasing the speed of the reaction. The automatic safety systems would have prevented the test from progressing this far had they not been disabled.

The crew noticed the sudden and rapidly increasing thermal measurement and, fearing a meltdown, took the only logical action – hit the emergency shutdown button and kill the reaction. Almost all of the control rods started their 20 second descent into the core but halted just seconds later. The graphite tips of the control rods caused an instant rise in heat so powerful that their tubes distorted and the rods became stuck with their graphite tips in the middle of the reactor.

The result was devastating. All of the water in the reactor instantly turned into steam. The pressure was so high that the 450 ton Upper Biological Shield blasted completely off and landed back down at an angle, blowing the roof off of the reactor building. A few seconds later a second explosion occurred and spewed 50 tons of vaporized nuclear fuel into the air and 700 tons of radioactive material out onto the ground and reactor roofs. The core ignited in a graphite fire that burned for days as the roofs of Reactors 3 and 4 caught fire.


The destroyed and fully exposed core of Reactor 4. Reactor 3 is the building on the other side of the chimney and continued to operate until the year 2000.

The meltdown was something nobody had prepared for. It took firefighters all night to put out the fires on top of reactors 3 and 4, exposing them to incredible amounts of radiation that made them sick and killed many within weeks. The graphite fire burned for days inside the reactor, spewing radioactive smoke as all the material in the core melted through the Lower Biological Shield into the concrete basement below.

During this entire ordeal, the citizens of Pripryat and nearby villages were told nothing. They went about their normal lives for nearly an entire day before they were evacuated by the government. All residents within 10 km were evacuated on April 27th; six days later the evacuation radius was expanded to 30 km. Nobody has been allowed to live in this radius since.

Two days went by before the rest of the world found out about the disaster. The Soviet government didn’t admit to it until radiation alarms went off in a power plant in Sweden. At first they denied that anything happened. Once they made the world aware they kept many of the details secret.

Scientists were concerned that the melted reactor core would make its way into a nearby containment pool or into the ground water and explode, igniting the remaining three reactors and ultimately making Northern Ukraine and Belarus uninhabitable for hundreds of years. Brave plant workers risked their lives and health to drain the pool and miners dug for days to freeze the ground and prevent catastrophe. The reactor started cooling and crisis was averted.

Thousands of people spent the rest of the year containing the accident. They bulldozed and buried villages, killed all wild and domestic animals, cleared radioactive trees, sprayed down buildings, and dumped huge amounts of polymer liquid from planes to weigh down radioactive dust. They used robots, and later humans, to clear radioactive graphite off of the reactor roofs. Thousands of men spent 1 – 3 minutes on roofs throwing radioactive materials away with rubber gloves and lead panels strapped to their bodies.

After the roofs were clean, a huge containment building, termed the Sarcophagus, was built around reactor 4 to contain all of the radioactive material. It was built quickly, had lots of open air gaps, and was not intended to last more than 20 years. In 2010, construction on a new, fully sealed containment unit was begun. From November 14-29, 2016 the New Safe Containment structure was slowly slid over the existing Sarcophagus to seal it off and allow the reactor to finally be cleaned up. The outside walls of the new structure were completed last month, finally sealing it from the elements. Cranes inside the structure will now start the job of dismantling the Sarcophagus and the reactor. The New Safe Containment structure is rated for 100 years; the cleanup might take that long.


The Chernobyl disaster is the greatest nuclear disaster in history. It was caused by a combination of bad design, miscommunication, disregard of protocol, and lack of safety. These characteristics permeated the culture of the Soviet Union at the time. The RBMK was considered a state secret so its design flaws were never communicated to the operators running it. Safety features that should have been configured and tested prior to operation were skipped to meet deadlines and collect bonuses. The Soviet Union fell in 1991. Many believe that the impact of Chernobyl and the cost of its cleanup played a part.

If it were not for the extremely brave men and women who risked their lives to contain the fires, cool the reactor, and clean up the radioactive waste, Chernobyl could have been a tragedy affecting hundreds of thousands if not millions of people. While secrecy permeated the Soviet government, a sense of duty permeated the Soviet culture. The crews that cleaned up Chernobyl did it because they believed it was the right thing to do for the good of their country; even if it killed them.

After the accident, work on reactors 5 and 6 was canceled, but reactors 1, 2, and 3 continued to operate through the nineties, with reactor 3 finally closing at the turn of the century. The Soviet Union altered the design of the RBMK by redesigning the control rods to reduce the likelihood of increased reaction during an emergency shutdown and removed the ability for steam to increase reactivity in the core. Of the 16 full-scale RBMK in operation over its life span, 11 are still operating, with one licensed to operate through 2034.

Chernobyl changed the nuclear industry and the world. I’ve been extremely interested in its story for the past several years and I’m really excited for the HBO miniseries. If the initial reviews are any indication, the five part series will be amazing. It starts Monday.

Additional Resources

If you want to learn more about Chernobyl, read the two books I mentioned at the beginning of this post or take a peek at these:

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