News 19.9.2023
19min read

Inside the room 4: Akimov and Toptunov – Chernobyl Nuclear Power Plant Engineers

In the annals of history, there are moments that stand as stark reminders of the profound impact of human choices and the consequences that ripple through time. 

The Chernobyl Nuclear Power Plant disaster of April 26, 1986, is one such moment. At the heart of this catastrophic event were two young engineers, Leonid Toptunov and Aleksandr Akimov, who found themselves in the control room of Reactor No. 4 on that fateful night.

This blog will take you deep into the heart of the Chernobyl disaster, specifically into “Room 4,” where Toptunov and Akimov worked tirelessly to avert the unimaginable. It’s a tale of heroism, sacrifice, and the indomitable human spirit in the face of the most consequential events of the 20th century.

Join us as we step back in time to understand the roles of these two engineers, their decisions, the chaos that ensued, and the tragic consequences they faced. This is the story of Room 4, where the destiny of a reactor, a city, and the lives of countless individuals hung in the balance.

The Chernobyl Nuclear Power Plant: A Vital Energy Source

ImageThe Chernobyl NPP, 1983-1984.
Photo: Unknown

The Chernobyl Nuclear Power Plant, often simply referred to as Chernobyl, held a pivotal position within the Soviet Union’s energy landscape. It emerged as a testament to Soviet engineering prowess and a symbol of the country’s ambitious foray into nuclear energy.

Constructed in the town of Pripyat, Ukraine, the Chernobyl Nuclear Power Plant was part of the Soviet Union’s broader nuclear energy program. Its construction began in the late 1970s and was emblematic of the USSR’s drive for technological advancement.

The Chernobyl NPP was not just a facility; it was a symbol of progress and power. With its four RBMK-1000 reactors, each capable of producing immense amounts of electricity, it became a significant contributor to the Soviet Union’s energy grid.

ImageBuilding of the atomic reactor of Chornobyl NPP, October 1977.
Photo: Unknown

According to Soviet scientists, the RBMK-1000 reactor design was known for its versatility and efficiency. It was used not only for electricity generation but also for producing heat, which was vital for both industrial processes and district heating systems.

Among the four reactors at Chernobyl, Reactor No. 4 held a position of paramount importance. It was one of the workhorses of the plant, generating electricity to meet the ever-growing energy needs of the Soviet Union.

The operation of Reactor No. 4 was central to the plant’s power generation capacity. Its electricity output was channeled to homes, industries, and infrastructure across Ukraine and beyond.

The Fateful Night: April 26, 1986

ImageBuilding of the Control Room 4 (1983).
Photo: Unknown

As the evening of April 25, 1986, transitioned into the early hours of April 26, the Chernobyl Nuclear Power Plant, like any other night, operated under the assumption of routine. Reactor No. 4 was ready for a safety test—an experiment that was meant to gauge the reactor’s ability to function during a power outage. However, this seemingly ordinary night would soon descend into chaos, forever altering the course of history.

The safety test planned for that night was not a routine operation; it was a complex and risky endeavor. It aimed to simulate a power outage and assess whether the reactor’s cooling systems could provide sufficient power until backup generators came online.

The Chernobyl engineers believed that this test could help improve reactor safety, even though it would temporarily lower the power output of Reactor No. 4.

Errors and Flaws Unveiled:

ImageDestroyed reactor 4 at the Chernobyl Nuclear Power Plant, April 30, 1986.
Photo: Unknown

In the hours leading up to the test, a series of critical errors and design flaws emerged. These would ultimately set the stage for the disaster.

The reactor’s power output dropped much lower than anticipated during the early stages of the test. To compensate, operators began removing control rods to increase power, a decision that proved to be ill-fated.

A combination of unstable reactor conditions and a sudden surge in power triggered a catastrophic explosion within Reactor No. 4, resulting in a massive release of radioactive materials into the atmosphere.

Timeline according to the World Nuclear Association

The timeline which follows has been compiled following a review of a large number of reports and it represents what is considered the most likely sequence of events, but there remain some uncertainties.

April 25
01:06 The scheduled shutdown of the reactor started. Gradual lowering of the power level began.
03:47 Lowering of reactor power halted at 1600 MW (thermal).
14:00 The emergency core cooling system (ECCS) was isolated (part of the test procedure) to prevent it from interrupting the test later. The fact that the ECCS was isolated did not contribute to the accident; however, had it been available it might have reduced the impact slightly.

The power was due to be lowered further; however, the controller of the electricity grid in Kiev requested the reactor operator to keep supplying electricity to enable demand to be met. Consequently, the reactor power level was maintained at 1600 MWt and the experiment was delayed. Without this delay, the test would have been conducted during the day shift.

23:10 Power reduction recommenced.
24:00 Shift change.
April 26
00:05 Power level had been decreased to 720 MWt and continued to be reduced. Although INSAG-1 stated that operation below 700 MWt was forbidden, sustained operation of the reactor below this level was not proscribed.
00:28 With the power level at about 500 MWt, control was transferred from the local to the automatic regulating system. The operator might have failed to give the ‘hold power at required level’ signal or the regulating system failed to respond to this signal. This led to an unexpected fall in power, which rapidly dropped to 30 MWt.
00:43:27 Turbogenerator trip signal blocked in accordance with operational and test procedures. INSAG-1 incorrectly reported this event occurring at 01:23:04 and stated: “This trip would have saved the reactor.” However, it is more likely that disabling this trip only delayed the onset of the accident by 39 seconds.
01:00 The reactor power had risen to 200 MWt and stabilized. Although the operators may not have known it, the required operating reactivity margin (ORM) of 15 rods had been violated. The decision was made to carry out the turbogenerator rundown tests at a power level of about 200 MWt.
01:03 A standby main circulation pump was switched into the left hand cooling circuit in order to increase the water flow to the core (part of the test procedure).
01:07 An additional cooling pump was switched into the right hand cooling circuit (part of the test procedure). Operation of additional pumps removed heat from the core more quickly leading to decreased reactivity, necessitating further absorber rod removal to prevent power levels falling. The pumps delivered excessive flow to the point where they exceeded their allowed limits. Increased core flow led to problems with the level in the steam drum.
01:19 (approx.) The steam drum level was still near the emergency level. To compensate, the operator increased feedwater flow. This raised the drum level, but further reduced reactivity to the system. The automatic control rods went up to the upper tie plate to compensate but further withdrawal of manual rods was required to maintain the reactivity balance. System pressure began to fall and, to stabilize pressure, the steam turbine bypass valve was shut off. Since the operators were having trouble with the pressure and level control, they deactivated the automatic trip systems to the steam drum around this time.
01:22:30 Calculations performed after the accident found that the ORM at this point proved to be equal to eight control rods. Operating policy required that a minimum ORM of 15 control rods be inserted in the reactor at all times.
01:23 (approx.) Reactor parameters stabilized. The unit shift supervisors considered that preparations for the tests had been completed and, having switched on the oscilloscope, gave the order to close the emergency stop valves.
April 26: the test
01:23:04 Turbine feed valves closed to start turbine coasting. This was the beginning of the actual test. According to Annex I of INSAG-7, for the following approximately 30 seconds of rundown of the four coolant pumps, “the parameters of the unit were controlled, remained within the limits expected for the operating conditions concerned, and did not require any intervention on the part of the personnel.”
01:23:40 The emergency button (AZ-5) was pressed by the operator. Control rods started to enter the core, increasing the reactivity at the bottom of the core.
01:23:43 Power excursion rate emergency protection system signals on; power exceeded 530 MWt.
01:23:46 Disconnection of the first pair of main circulating pumps (MCPs) being ‘run down’, followed immediately by disconnection of the second pair.
01:23:47 Sharp reduction in the flow rates of the MCPs not involved in the rundown test and unreliable readings in the MCPs involved in the test; sharp increase of pressure in the steam separator drums; sharp increase in the water level in the steam separator drums.
01:23:48 Restoration of flow rates of MCPs not involved in the rundown test to values close to the initial ones; restoration of flow rates to 15% below the initial rate for the MCPs on the left side which were being run down; restoration of flow rates to 10% below the initial rate for one of the other MCPs involved in the test and unreliable readings for the other one; further increase of pressure in the steam separator drums and of water level in the steam separator drums; triggering of fast acting systems for dumping of steam to condensers.
01:23:49 Emergency protection signal ‘Pressure increase in reactor space (rupture of a fuel channel)’; ‘No voltage – 48 V’ signal (no power supply to the servo drive mechanisms of the EPS); ‘Failure of the actuators of automatic power controllers Nos 1 and 2’ signals.
01:24 From a note in the chief reactor control engineer’s operating log: “01:24: Severe shocks; the RCPS rods stopped moving before they reached the lower limit stop switches; power switch of clutch mechanisms is off.”

Press the AZ-5 button: Aleksandr Akimov

ImageAleksandr Akimov
Photo credits: Historical photography

The Chernobyl Nuclear Power Plant disaster of April 26, 1986, stands as one of the most catastrophic nuclear accidents in history. At its epicenter was Reactor Unit 4, a place where a series of events unfolded, leading to unimaginable consequences. Among the brave souls who grappled with the unfolding crisis was Aleksandr Akimov, a man whose dedication and heroism are forever etched into the annals of Chernobyl’s tragic history.

Aleksandr Akimov was born on May 6, 1953, in Novosibirsk, Russian SFSR, which was part of the Soviet Union. His early life was marked by the backdrop of a changing world, with the Cold War and the nuclear arms race casting shadows over international relations. Little did he know that he would become a central figure in a nuclear tragedy that would shape the course of history.

In 1976, Akimov embarked on a journey of education, graduating from the Moscow Power Engineering Institute. His specialization in engineering and automation of heat and power processes equipped him with the knowledge required for a career in the complex and highly specialized world of nuclear energy.

ImageSam Troughton (on the left) as Aleksandr Akimov, shift supervisor of the night crew, in Chernobyl HBO TV series.
Photo: HBO

Akimov’s career path led him to the Chernobyl Nuclear Power Plant in September 1979. His initial roles at the plant included serving as a senior turbine management engineer and later as a shift supervisor in the turbine hall. These positions provided him with a solid foundation in the operations of the plant and the complexities of nuclear energy.

However, it was on July 10, 1984, that his life took a fateful turn when he was appointed as the shift supervisor of Reactor Unit 4. This reactor, along with its control room, would become the stage for a harrowing ordeal that would test Akimov’s mettle and courage in ways he could never have imagined.

The Night That Changed Everything:

Alexander Akimov. He celebrated his 33rd birthday in a hospital bed with an acute radiation sickness, in 5 days he will disappear. Little fragment from movie “I love these places”:

On the night of April 26, 1986, Akimov found himself in the control room of Reactor Unit 4. The reactor was in the midst of preparations for a safety test, a complex and risky endeavor meant to assess its ability to function during a power outage. But this was no ordinary night; it was the night that would alter the course of Akimov’s life and the lives of countless others.

The reactor unexpectedly stalled during the test preparations, setting off a chain of events that would lead to disaster. Akimov, along with his colleagues, recognized the gravity of the situation. The reactor had become a ticking time bomb. In a desperate bid to prevent a catastrophic explosion, Akimov called for the emergency shutdown, pressing the AZ-5 (scram) button.

In the chaotic aftermath of the explosion, Akimov, along with his fellow operators, found themselves in a nightmarish scenario. The control room was filled with alarms, the reactor’s core was in ruins, and a plume of radioactive smoke billowed into the atmosphere.

Due to a design flaw, the descending control rods momentarily accelerated the nuclear reaction and caused the reactor to explode. The communications networks were suddenly flooded with calls and information. Akimov heard reports of massive reactor damage, but did not believe it, and as a result, relayed false information about the state of the reactor for hours thereafter. 

Akimov worked with his crew in the reactor building after he learned the extent of the accident. They tried to pump water into the exposed reactor core until the morning. He worked with Toptunov to manually open water valves in an attempt to increase water supply to the reactor. 

Their heroic efforts, however, came at a tremendous personal cost. The immense levels of radiation exposure began to take a toll on their health. Akimov, along with Leonid Toptunov, another operator, experienced the initial symptoms of acute radiation syndrome and were sent to the infirmary. 

Aleksandr Akimov was exposed during his work to a lethal dose of 15-20 Gy of radiation.

Akimov was admitted to the Pripyat Hospital but was quickly transferred to Moscow Hospital 6 due to the severity of his condition. By April 28, the initial symptoms of radiation sickness had somewhat subsided, offering a glimmer of hope. During his hospitalization, he had a heartbreaking conversation with his wife, acknowledging the possibility that he might not survive and expressing his intention to leave the nuclear industry.

Akimov, along with Toptunov, underwent a bone marrow transplant in a last-ditch effort to restore their compromised immune systems. However, the odds were stacked against them.

Tragically, Aleksandr Akimov’s condition deteriorated rapidly, and he succumbed to acute radiation syndrome two weeks after the Chernobyl disaster. He was just 33 years old at the time of his death. His family was informed that his death was the only reason he was not prosecuted for the accident.

In the Midst of Chaos: Leonid Toptunov

ImageLeonid Toptunov
Photo credits: Historical photography

In the annals of history, there are those whose names become etched in the collective memory of humanity. They are often remembered for their acts of courage, sacrifice, and heroism in the face of unprecedented disaster. Such is the story of Leonid Toptunov, a name less recognized than some but equally deserving of our remembrance.

Born on August 16, 1960, in Mykolaivka, Buryn Raion, Sumy Oblast, Leonid Toptunov’s journey into history began quietly in the embrace of a family touched by science and engineering. His father’s involvement in the Soviet space program meant that young Leonid was surrounded by scientists and engineers from an early age, nurturing his curiosity about the world and its intricate workings.

In 1983, Toptunov graduated from the Obninsk Institute for Nuclear Power Engineering, armed with a specialist degree in nuclear power plant engineering. Little did he know that this education would lead him to a pivotal role in one of the most catastrophic events of the 20th century.

At the Chernobyl Nuclear Power Plant

ImageRuined Reactor No 4 of Chernobyl NPP from above, 1986.
Photo: Unknown

Leonid Toptunov’s journey within the Chernobyl plant commenced in March 1983. Toptunov’s role at Chernobyl initially involved working as a unit control engineer and later as a senior reactor control engineer. But his fate was sealed when he found himself on duty on the night of April 26, 1986, in the control room at the reactor panel, alongside his colleague Aleksandr Akimov.

Toptunov only had two months’ experience in operating the reactor and this was his first shutdown as operator. 

The operators attempted to perform a rundown test before scheduled routine maintenance, during which reactor 4 exploded. The reactor’s power had been significantly reduced in preparation for the test, but it unexpectedly stalled due to a combination of factors, including errors in control rod placement attributed to Toptunov.

In preparation, Anatoly Dyatlov, the deputy chief engineer, ordered the power to be reduced to 700 MW, as the test plan stipulated. However, the reactor stalled unexpectedly during test preparations, dropping to a low 30 MW. Dyatlov ordered Toptunov and Akimov to raise the power to the requisite level for the test, a task made perilous by xenon poisoning and reactor design flaws unknown to the operators. 

Withdrawing a dangerous number of control rods, the operators could only reach 200 MW due to xenon poisoning. Despite their valiant efforts, which included pressing the AZ-5 (scram) button to shut down the reactor, the control rods’ design flaw caused a catastrophic explosion.

In the aftermath of the explosion, Toptunov and Akimov, along with non-essential personnel, were initially dismissed from the control room. Yet, a deep sense of responsibility compelled Toptunov to return. In a desperate bid to cool the reactor, he and Akimov manually opened water valves, a task that exposed them to acute radiation. They were found by other workers and taken to the infirmary. 

ImageMeasured exposure rates in Roentgen in air on 26 April 1986 in the local area of the Chernobyl reactor.

Despite medical intervention and bone marrow transplants aimed at restoring their immune systems, Toptunov’s condition deteriorated rapidly. His exposure to a fatal radiation dose of 1300 rem was beyond salvation. 

On May 14, 1986, a mere few weeks after the disaster, Leonid Toptunov succumbed to the devastating effects of acute radiation poisoning. He was just 25 years old at the time of his death, a stark reminder of the immense sacrifices made by those on the front lines of the disaster.


In the end the initial Soviet investigation put almost all the blame on the operators, later findings by the IAEA found that the reactor design and how the operators were informed of safety information was more significant. Nonetheless, the operators were found to have deviated from operational procedures, changing test protocols on the fly, as well as having made “ill judged” actions, making human factors a major contributing factor.

Of course, they deviated from established protocols and conducted a risky safety test under less-than-ideal conditions. This decision set off a chain reaction that ultimately led to the reactor explosion. 

While the operators’ actions were undoubtedly a contributing factor, it’s crucial to recognize the systemic failures that placed them in this perilous situation. The RBMK reactor design, employed at Chernobyl, had inherent flaws, including a positive void coefficient that could lead to a sudden power surge when coolant was lost. This design flaw was not adequately communicated to the operators.

The Chernobyl plant, like many Soviet facilities, suffered from a pervasive lack of safety culture. Workers often faced pressure to meet production targets and cut corners to do so. This culture discouraged questioning authority or raising safety concerns, even when operators had reservations about the test.

Some operators at Chernobyl were relatively inexperienced, including Leonid Toptunov, who was working at the reactor control panel for the first time that night. This lack of experience played a role in the mismanagement of the reactor’s conditions.

ImageRadioactive cloud over Europe in 1986.

The Chernobyl disaster occurred within the context of the Soviet Union, where the emphasis on secrecy and a desire to maintain an image of technological superiority often trumped safety concerns. This culture of secrecy hindered the flow of crucial safety information – read more in our blog “WHAT IS THE COST OF LIES: VALERY LEGASOV – CHERNOBYL HERO?”

In the immediate aftermath of the explosion, the Soviet government sought to deflect blame away from systemic issues. Operators Anatoly Dyatlov, Aleksandr Akimov, and Leonid Toptunov were heavily scrutinized and, in some cases, scapegoated. Dyatlov, in particular, faced harsh consequences for his role in the events.

Blaming the operators for the Chernobyl disaster is an oversimplification of a much broader issue. While their actions played a role, the disaster was the result of a combination of factors, including reactor design flaws, inadequate training, a deficient safety culture, and a socio-political context that prioritized secrecy over safety. Recognizing this complexity is essential to prevent similar disasters in the future and to honor the memory of those who suffered the consequences of Chernobyl.

The Legacy

The legacies of Aleksandr Akimov and Leonid Toptunov are not merely stories of tragedy and “human factor” but also tales of courage, duty, and the indomitable human spirit. These two engineers, alongside their colleagues, confronted a disaster of unprecedented magnitude with resolute determination.

As we reflect on their lives and sacrifices, we must also consider the broader lessons of Chernobyl. While initial blame often fell on the operators, subsequent investigations highlighted a complex interplay of factors, including reactor design flaws and inadequate safety information dissemination. The Chernobyl disaster serves as a stark reminder of the importance of stringent safety measures, constant vigilance, and transparent communication in the realm of nuclear energy.

Aleksandr Akimov and Leonid Toptunov’s stories deserve to be told and remembered. They are the embodiment of those who, in the face of unimaginable adversity, stood firm at their posts, sacrificing their health and, ultimately, their lives for the greater good. In their legacy, we find inspiration and a solemn reminder that, even in the darkest of hours, the human spirit can shine with unwavering resolve and heroism.





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