On April 26, 1986, a significant incident occurred in Ukraine that would profoundly impact human history. It unfolded in the heartland of the country, near the town of Pripyat, at the Chernobyl Nuclear Power Plant. During the early hours of that morning, reactor number four experienced a catastrophic explosion, resulting in the release of an unprecedented amount of radioactive material into the atmosphere.

This event stands as one of the most severe nuclear disasters in history and has significantly influenced our understanding of the power and potential risks associated with nuclear energy.

Chernobyl’s Design

Chernobyl was an RBMK-1000 nuclear reactor, which differed from the commonly used water-moderated and water-cooled pressurized water reactors (PWRs) or boiling water reactors (BWRs) found in the West.

The RBMK design featured graphite moderation and water cooling.

However, the RBMK reactors did have safety systems in place to prevent meltdowns. While the RBMK design had inherent safety flaws, such as a positive void coefficient that could lead to power increases with increased steam formation, the safety features were intended to mitigate risks and were not deliberately deactivated.

The RBMK design, developed in the 1950s, was a second-generation reactor that originated in the Soviet Union. It was designed with a focus on rapid production and cost efficiency rather than redundancy and extensive safety measures. The reactor had certain design features that became highly unstable when operated beyond their intended specifications.

Unlike heavy water reactors, which required substantial maintenance and expensive volumes of heavy water for startup, the RBMK design utilized a graphite core and natural uranium fuel. This enabled the reactor to generate massive amounts of power at a significantly lower cost, making it an attractive option for power generation purposes. However, the design’s deficiencies and lack of safety redundancies ultimately led to significant safety concerns.

How does a Nuclear Reactor Function?

A nuclear reactor uses uranium or plutonium fuel to produce heat through a process called nuclear fission. When the fuel atoms are struck by neutrons, they split apart, releasing a large amount of energy. This energy is used to generate steam, which drives a turbine connected to a generator, producing electricity. Control rods are used to manage the reaction and prevent overheating. Overall, nuclear reactors produce electricity by harnessing the heat generated from splitting atoms.

Mechanism of Reactor Stability

1. Graphite Moderation: The RBMK-1000 reactor used graphite as a moderator (moderating the speed of the neutrons). A moderator is a material that slows down the fast-moving neutrons released during the nuclear fission process. By slowing down the neutrons (thermalization), the graphite allows them to efficiently cause further fission reactions in the reactor’s fuel.
2. Water Cooling: The RBMK-1000 reactor used water as a coolant. The primary purpose of the coolant is to absorb the heat generated by the nuclear fission reactions. In the RBMK-1000 design, the water served two main functions: cooling the fuel and generating steam for electricity production.
3. Boiling Water Reactor: The RBMK-1000 reactor is classified as a boiling water reactor (BWR). In a BWR, the coolant (water) itself boils directly in the reactor core, and the resulting steam is used to drive the turbine and generate electricity. The steam is then condensed back into water and returned to the reactor core.

Design Basis Accident

In the event of a total power loss at the Chernobyl station, each reactor was equipped with three backup diesel generators. However, these generators took approximately 60-75 seconds to reach full load and generate the required 5.5-megawatt output to run one main pump. During this interim period, special counterweights on each pump were designed to allow them to provide coolant through inertia, bridging the gap until the generators were fully operational.

Nevertheless, there was a potential safety risk if a station blackout occurred simultaneously with the rupture of a 600-millimeter coolant pipe, which was known as the Design Basis Accident. In such a scenario, the emergency core cooling system (ECCS) was required to pump additional water into the core to replace the coolant lost through evaporation. This was crucial to prevent overheating and potential damage to the reactor core.

The Test, Part I

In order to generate electrical power for the Emergency Core Cooling System (ECCS) at the Chernobyl nuclear power plant, it was proposed to utilize the slowly reducing rotational momentum of the reactor’s steam turbine. The idea was that the turbine’s energy could be used to power the coolant pumps for 45 seconds, bridging the gap of 60 seconds before the backup generators could power the pumps or the ECCS.

However, before implementing this concept, it was necessary to confirm its feasibility through experimental testing. Previous attempts at conducting such tests had been unsuccessful. An initial test in 1982 revealed that the excitation voltage of the turbine generator was insufficient to maintain the required magnetic field after the turbine was tripped. Modifications were made to the electrical system, and a second test was conducted in 1984, but it also proved unsuccessful. A third test in 1985 faced issues with the recording equipment and yielded no results.

Despite the unsuccessful tests, another test was scheduled to be carried out in 1986 during a controlled power-down of reactor No. 4, which was part of a planned maintenance outage. The test procedure had been written, but the authors were not aware of the peculiar behaviour of the RBMK-1000 reactor under the planned operating conditions.

It’s worth noting that the test procedure was considered to be purely an electrical test of the generator and not a comprehensive unit test, even though it involved critical unit systems. According to the regulations in place at the time, such a test did not require approval from the chief design authority for the reactor (NIKIET) or the Soviet nuclear safety regulator.

The test program involved disabling the emergency core cooling system, which is a system designed to cool the reactor core in the event of a loss-of-coolant accident. If the reactor started to rapidly heat up, there would be no way to save it.

The Test, Part II

Based on the information available from 1986, the test procedure was intended to run as follows:

1. Test Preparation:

• The test would be conducted before a scheduled reactor shutdown.
• The thermal power of the reactor would be reduced to a range of 700 MW to 1,000 MW. This reduction was necessary to ensure adequate cooling because the turbine would be spun at its operating speed while disconnected from the power grid.
• The steam turbine generator would operate at its normal operating speed.
• Four out of the eight main circulating pumps would be powered by off-site power, while the remaining four would be powered by the turbine.

2. Electrical Test:

• Once the correct conditions were achieved, the steam supply to the turbine generator would be closed off, and the reactor would be shut down.
• The voltage provided by the coasting turbine would be measured, along with the voltage and RPMs (Revolutions Per Minute) of the four main circulating pumps that were powered by the turbine.
• When the emergency generators supplied full electrical power, the turbine generator would be allowed to continue free-wheeling down.

The Test, Part III

The test was scheduled to take place on April 25, 1986, during the day shift as part of a routine reactor shutdown. The day shift crew had received instructions in advance regarding the operating conditions required for the test. Additionally, a team of electrical engineers was present to conduct a one-minute test of the new voltage-regulating system once the appropriate conditions were met. The power unit’s output had been gradually reduced to 50% of its nominal 3,200 MW thermal level by 01:06 on April 25.

During the day shift, various unrelated maintenance tasks were carried out, and the test was planned to be performed at 14:15. Preparations for the test, including the disabling of the emergency core cooling system, were carried out. However, there was an unexpected outage at another regional power station at 14:00. As a result, the Kiev electrical grid controller requested a postponement of the further reduction of Chernobyl’s output, as the power was needed to meet the peak evening demand. Consequently, the test was postponed.

After the delay, the day shift was replaced by the evening shift. Unfortunately, the emergency core cooling system remained disabled. Although the system would not have impacted the subsequent events, leaving the reactor operating without emergency protection for 11 hours outside of the test indicated a general lack of safety culture.

At 23:04, the Kiev grid controller permitted the reactor shutdown to resume. However, this delay had significant consequences. The day shift had already finished their work, the evening shift was preparing to leave, and the night shift would not take over until midnight, which was well into their shift. According to the original plan, the test should have been completed during the day shift, and the night shift would have only been responsible for maintaining the cooling systems to manage the decay heat in a shut-down plant.

Inexperience of the Night Shift

The night shift had very limited time to prepare for and carry out the experiment. Anatoly Dyatlov, deputy chief engineer of Chernobyl, was present to supervise and direct the test. He was one of the test’s chief authors and was the highest-ranking individual present in the Reactor 4 control room. Unit Shift Supervisor Aleksandr Akimov was in charge of the Unit 4 night shift, and Leonid Toptunov was the Senior Reactor Control Engineer responsible for the reactor’s operational regimen, including the movement of the control rods of the core itself. Mr. Toptunov, who was working as the Senior Reactor Control Engineer, had 3 months of experience.

Unexpected Power Drop

The Test had called for a gradual drop of the power to 700-1000 MW. A power of 720 MW was reached at 00:05 in the morning.

However, due to the reactor’s continuing production of Xenon-135 (see Xenon-135), in normal circumstances, the xenon is “burned off” by the neutrons, creating highly stable xenon-136, which doesn’t absorb neutrons easily. However, as the power was kept at half power, the xenon did not “burn off” as there were fewer neutrons being given off. As such, the power continued to drop below 700 MW.

When the power dropped to ~500 MW, the reactor power control was shifted from the Local Automatic Regulator (LAR) to the Automatic Regulator (AR) to manually retain power. AR-1 then activated, removing all four of its control rods automatically, but AR-2 failed to activate due to an imbalance in its ionization chambers. In response, Toptunov reduced power to stabilize the ionization sensors. The result was a sudden power drop to an unintended shutdown state, with a power output of 30 MW thermal or less.

The reactor was now operating at 5% of the required power level for the test. In response, the operators of 211 control rods in the core left only a few remaining fully in the reactor, still high enough for safety regulations to dictate.

As several minutes went by, the reactor power slowly climbed to 200 MW thermal, which was not high enough for the test, but high enough by manually managing the water flow, and they produced enough steam to drive the turbines up to speed.

Then, the operators turned two cooling water pumps on, which reduced reactivity in the core. In response to this problem, the operators pulled even more control rods, and at this time, the control rod number in the core fell below the required number of 15. As the RBMK didn’t have a system for counting the number of control rods pulled, the operators couldn’t know. Of 211 control rods, only 6 remained.

The Test Begins

At 01:23:04, the test finally began; 4 of the 8 Main Circulating Pumps were to be powered by the coasting turbine, while the four other pumps were to be powered by outside power.

The steam to the turbines was shut off, beginning a run-down of the turbine generator. The diesel generators started and started to pick up loads; as the rotational momentum of the turbine generator decreased, so did the power it produced for the pumps. The water flow rate decreased, leading to increased formation of steam voids in the coolant flowing up through the fuel pressure tubes (Positive Void Coefficient).

As the voids increased, the power started to jump rapidly.

1:23:40, AZ-5

At 01:23:40, as recorded by the SKALA central control system, Akimov signals to Toptunov to engage AZ-5, the emergency SCRAM, in effect like brakes on a car. However, unbeknownst to the operators, the control rods had a graphite displacer rod on the ends of them, and when AZ-5 activated, it displaced the water at the bottom portion of the core, accelerating the reaction.

At 01:23:43, the water remaining in the core vaporizes into steam, rupturing the fuel channels; the control rods in those channels are now stuck at 1/3 insertion, endlessly accelerating the reaction.

The caps on the fuel channels, weighing 300 kilograms each, start jumping up and down. An operator sees this and runs to stop AZ-5, but it is too late.

1:23:45

At 1:23:45, Chernobyl Nuclear Power Plant, designed to operate at 3200 MW thermal, went beyond 33000 MW, simulations indicate that it may have exceeded 300000 MW thermal.

In an instant, the Upper Biological Shield of the Reactor gets blown away. At this point, the core is blown apart, the water inside the core gets so hot that it dissociates into hydrogen and oxygen, coupled with superheated graphite, ignites and explodes, sending the core, or lack thereof, into the open air. A fire instantly ignites.

An Accident has occurred; Send the Fire Brigade.

The Fire Brigade arrived at Chernobyl, scrambling to put out the fires before it could damage the core. In reality, there was no core left.

The immediate priority was to extinguish fires on the roof of the station and the area around the building containing Reactor No. 4 to protect No. 3 and keep its core cooling systems intact.

Many firefighters climbed up the reactor and tried to put out the fires from the top. A firefighter tried to hook the hoses to the pumps, but only air came. He called, “Get me some pressure!” He didn’t know that the pipes had all burst.

A firefighter, Vasily Ignatenko, climbed up the ladder to the roof, and when he started to come down, every firefighter on the roof vomited while descending. He didn’t know that he had seen the eyes of death.

We will live forever

Anatoli Zakharov, a firefighter who had been helping at the construction of Chernobyl when it was first being built, saw smooth black chunks of rock on the ground. He remembered when he was helping to construct it, he saw it in the core, and he felt disturbed. His fellow firefighters saw the look on his face; they asked, “Anatoli, what is it?” Anatoli replied, “Lads, it’s the guts of the reactor.” He then said, “If we survive until morning, we will live forever.”

Containment

As the fire burned at several hundred degrees Celsius, regular water could not extinguish it. They could only use sand and boron (a neutron-absorbing material to contain radiation). However, as the sand insulated the heat, the sand and boron melted, creating radioactive lava (corium). Fearing a supercriticality because of full water tanks inside the reactor, they sent three men to drain them, in effect, sending three men to die. However, as water absorbs radiation, the three men were relatively unharmed; two now live today, and one died from an unrelated heart attack.

Effects

The Chernobyl disaster, which occurred in 1986, had profound effects on both humanity and the environment. The immediate impact resulted in the loss of life and acute radiation sickness among workers and nearby residents. Long-term health effects, particularly an increased incidence of thyroid cancer, have been observed. The accident led to the permanent displacement of communities and the creation of an exclusion zone due to high levels of contamination. The environment suffered from widespread contamination of soil, water, and vegetation, which affected the local ecosystem and wildlife. The economic burden was significant, with substantial costs for cleanup, healthcare, and ongoing management. Despite mitigation efforts, the effects of Chernobyl continue to be felt to this day. Chernobyl will be safe 24,000 years from today.

In Remembrance of Those Who Saved the World

The 30 who died for the world:

• Akimov, Aleksandr Fyodorovich (CoD: ARS)
• Baranov, Anatoly Ivanovich (CoD: ARS)
• Brazhnik, Vyacheslav Stepanovych (CoD: ARS)
• Degtyarenko, Viktor Mykhaylovych (CoD: ARS)
• Ignatenko, Vasily Ivanovych (CoD: ARS)
• Ivanenko, Yekaterina Alexandrovna (CoD: ARS)
• Khodemchuk, Valery Ilyich (CoD: Crushed by debris, permanently entombed inside)
• Kibenok, Viktor Mykolayovych (CoD: ARS)
• Konoval, Yuriy Ivanovych (CoD: ARS)
• Kudryavtsev, Aleksandr Gennadiyevych (CoD: ARS)
• Kurguz, Anatoly Kharlampiyovych (CoD: ARS)
• Lelechenko, Aleksandr Grigoryevich (CoD: ARS)
• Lopatyuk, Viktor Ivanovich (CoD: ARS)
• Luzganova, Klavdia Ivanovna (CoD: ARS)
• Novyk, Aleksandr Vasylyovych (CoD: ARS)
• Orlov, Ivan Lukych (CoD: ARS)
• Perchuk, Kostyantyn Grigorovich (CoD: ARS)
• Perevozchenko, Valery Ivanovich (Looked into the core; CoD: ARS)
• Popov, Georgi Illiaronovich (CoD: ARS)
• Pravyk, Volodymyr Pavlovych (CoD: ARS)
• Proskuryakov, Viktor Vasilyevich (CoD: ARS)
• Savenkov, Vladimir Ivanovych (CoD: ARS)
• Shapovalov, Anatoliy Ivanovych (CoD: ARS)
• Shashenok, Vladimir Nikolaevich (CoD: blunt force trauma)
• Sitnikov, Anatoly Andreyevich (CoD: ARS)
• Tishura, Vladimir Ivanovych (CoD: ARS)
• Titenok, Nikolai Ivanovych (CoD: ARS)
• Toptunov, Leonid Fedorovych (CoD: ARS)
• Vershynin, Yuriy Anatoliyovych (CoD: ARS)
• Vashchuk, Nikolai Vasilievich (CoD: ARS)

The following saved the world and died for it:

• Valery Legasov (Cause of Death: Suicide)
• Boris Scherbina (Cause of Death: Cancer)

Coda: Вічная Пам’ять, Memory Eternal

As the years passed, Chernobyl became a testament to the resilience of the human spirit and the healing power of nature. The aftermath of the disaster remained etched in the memories of those who lived through it, but slowly, hope began to replace the anguish that had gripped the region.

In the immediate aftermath of the Chernobyl disaster, a massive sarcophagus was constructed to contain the damaged reactor and prevent further release of radiation. It stood as a stark reminder of the tragedy that had unfolded. But with time, scientists, engineers, and dedicated individuals from around the world came together to devise a long-term solution.

Years of planning and collaboration led to the construction of the New Safe Confinement, an enormous structure designed to enclose the damaged reactor and ensure the containment of radioactive materials for centuries to come. Completed with unprecedented engineering precision, it provided a sense of closure and security to the affected area.

Nature, too, began to reclaim the land. Wildlife, once decimated by the disaster, gradually returned to the evacuated zone. Wolves, bears, and lynxes roamed freely, and birds filled the air with their songs. The absence of human activity allowed ecosystems to flourish, creating a unique haven for biodiversity.

The people of Ukraine, though forever marked by the tragedy, found solace and purpose in rebuilding their lives. Many chose to return to their abandoned homes, determined to go back to their old lives.

Today, Chernobyl stands as a living testament to the consequences of a catastrophic event and the indomitable human spirit. It serves as a constant reminder of the importance of safety, accountability, and the pursuit of knowledge. The lessons learned from Chernobyl have shaped global policies on nuclear safety, ensuring that such a disaster will never be repeated.

Visitors from around the world come to witness the haunting beauty of the Chernobyl Exclusion Zone. They walk through abandoned towns, their curiosity mixed with a deep sense of respect for those whose lives were forever altered. It is a place of remembrance, a place where the past is preserved to guide future generations.

The story of Chernobyl is one of tragedy, but also of resilience, hope, and the power of human endeavor. It is a story that reminds us that we are not the masters of the world, for we are only pawns in the fragile balance of nature. And as the sun sets over the abandoned city of Pripyat, casting its golden light on the overgrown ruins, it serves as a poignant reminder that the true mistress of the world is Mother Nature.

The Story of Chernobyl is also a testament to the power of lies. When the truth offends, humanity will lie and lie until we no longer remember that the truth is even there, but it is still there. Every lie humanity tells incurs a debt to the truth. The truth will wait for all eternity to be repaid; it cares neither about whether we are Christian, Muslim, Jewish, Buddhist, or Atheist. It doesn’t care whether we are American, German, Irish, Russian, Ukrainian, or Kazakh. It will be paid. It always will be paid.

Now I ask, “What is the cost of lies?”
The answer is the happiness of all mankind.

In remembrance of those who saved the world.

Submitted on April 26, 2024. Thirty-eight Years after Chernobyl.