The fourth power unit of the Chernobyl nuclear power plant remains one of the most discussed and tragic facilities in the history of nuclear power. It was here that on April 26, 1986, a planetary catastrophe occurred, forever changing the attitude of mankind to the world. nuclear safety. Studying what was inside the reactor hall before the explosion allows engineers and physicists to understand the mechanism of the destruction and prevent similar scenarios in the future.
Inside the block was located RBMK-1000 reactorIt was distinguished by a unique design with a graphite moderator and the possibility of overloading fuel on the go. Although the facility is now mothballed and sheltered with new safe confinement, technical data on the internal structure of Unit 4 are available thanks to the surviving drawings and commission reports. These documents reveal the complexity of engineering decisions that on a fateful night were fatal.
Understanding the internal structure of the reactor is essential to analyze the causes of the accident. Design features RMKSuch as positive vapor reactivity and the design of the rods of the CVC have played a key role in the development of an unmanaged process. In this article we will analyze in detail how the 4th power unit was arranged, what was happening in its bowels at the time of the disaster and what is the state of the object today.
Design features of the RBMK-1000 reactor
The heart of Unit 4 was a type reactor. RBMK-1000 (The High Power Canal Reactor). It was a powerful installation capable of producing a thermal power of 3200 MW. The main feature of the design was the absence of a traditional sealed shell, typical for Western reactors. Instead, the active zone was a cylinder of graphite blocks permeated with thousands of channels.
Through these channels passed pipes through which water circulated, turning into steam under the influence of heat from a nuclear reaction. Graphite served as a neutron moderator, allowing the chain reaction to be maintained. This was the design of the reactor. large-scale It is relatively easy to maintain, but it has a number of hidden shortcomings that manifested themselves under certain operating conditions.
The control and protection system (CS) consisted of many rods containing boron carbide - a neutron absorber. The introduction of the rods should have stopped the reaction. However, the design of the rods RBMK had a critical defect: their lower part (the tip) was made of graphite. When the rod was introduced into the active zone, the graphite terminal first displaced water, which for a short time reactivatedbefore the boron began to lower it. This effect, known as the “end rod effect,” played a fatal role.
Importantly, the reactor operated in a steam circuit where water and steam were separated directly in the reactor shaft. This created a complex hydraulic system, sensitive to pressure differences and coolant flow. The engineering solutions applied at Unit 4 were advanced for their time, but required strict compliance with the regulations.
The state before the disaster: the course of the experiment
On the night of April 25 to 26, 1986, planned preventive repairs were planned at Unit 4. In preparation for it, it was decided to conduct an experiment to run out of the turbogenerator. The goal was to test whether the inertia of the turbine’s rotation could generate electricity for the station’s own needs (circulation pumps) within seconds of shutting down the steam until it started. diesel generators.
To conduct tests, the reactor had to be brought to low power, about 700-1000 MW of thermal power. However, in the process of reducing power due to operator error and the behavior of the reactor at low power ("xenon pit") power fell to almost zero. Operators, trying to raise power, removed almost all control rods from the core, violating the operational reliability limit.
The situation was aggravated by the fact that the reactor was a large amount of xenon-135, which absorbed neutrons and interfered with acceleration. To penetrate this poison, the operators raised power, but the reactor was in an extremely unstable state. The parameters of the work went beyond the rules, but the experiment was decided to continue. At this point, the conditions for the 4th block were created. heat-explosive.
The Destruction Mechanism: What Happened Inside
At 01:23:40 local time, the experiment began. The turbine generator started running, the pumps began to work slower, the water flow through the reactor fell. Due to the decrease in flow, the water in the channels began to actively boil, forming steam stoppers. Since water is a neutron absorber and steam is not, the replacement of water with steam has led to a sharp increase in reactivity. Unmanaged power acceleration has begun.
Operators tried to emergency shut down the reactor by pressing a button AZ-5. All the rods of the Souz went down. But it was at this point that the "end rod effect" worked. Graphite tips entered the core, displacing water and causing a local power surge at the bottom of the reactor. Power jumped to 100 times the nominal level in a split second.
There was a heat explosion. The pressure in the channels increased so much that it broke the fuel channels and destroyed the reactor cover. A huge amount of radioactive substances were released into the atmosphere. Graphite masonry caught fire, which contributed to the spread of radionuclides to great altitudes and distances. Unit 4 ceased to exist as a functioning system.
Details of the explosion
The first explosion was thermal, caused by a sharp increase in vapor pressure. He destroyed the reactor structure. A second explosion, presumably chemical (hydrogen), occurred seconds later and scattered debris of graphite and fuel across the station.
Table of technical parameters of the 4th power unit
To understand the scale of the facility, we will give the main technical characteristics of the reactor installed on the 4th unit before the accident. These data allow us to estimate the enormous energy contained within.
| Parameter | Meaning | Unit of measurement |
|---|---|---|
| Type of reactor | RBMK-1000 | - |
| Heat power | 3200 | MW |
| Electrical power | 1000 | MW |
| Number of fuel channels | 1661 | Shh. |
| Graphite masonry mass | ~1700 | tons |
As you can see from the table, Unit 4 was a giant energy object. The mass of the active zone and the graphite moderator created a huge thermal inertia. After the destruction of the hull, cooling the debris became one of the most difficult tasks. Radioactive fuel, melted, glass in the lower floors of the building, forming the so-called "elephant leg" - frozen lava from a mixture of uranium, graphite, concrete and metal.
Now the processes are continuing inside the destroyed block, albeit at a slow pace. Fuel residues require constant monitoring. The design, which was supposed to serve for decades, has become a source of danger, requiring constant monitoring and control of the system. radiation protection.
When studying the diagrams of the RBMK reactor, pay attention to the lack of a sealed envelope – this is a key difference from modern Western reactors, which allowed radioactive substances to freely enter the atmosphere.
Elimination of consequences and creation of the Sarcophagus
Immediately after the accident, work began on the localization of the hearth. The primary objective was to extinguish fires and prevent radioactive materials from entering groundwater. For this purpose, a concrete slab was built under the 4th block. Work then began on the creation of a shelter known as the "Shelter Object" or sarcophagus.
The construction was carried out in extreme conditions of high radiation. The designs were installed remotely or with minimal time for people to stay in the danger zone. The sarcophagus was a complex engineering structure of steel and concrete, covering the destroyed reactor. It was supposed to isolate radioactive emissions for up to 30 years.
⚠️ Warning: Being inside the perimeter of the old Sarcophagus without special protective equipment is deadly due to the high level of radiation and instability of structures. Only special robotic systems and trained personnel in exoskeletons have access to it.
By 2016, a New Safe Confinement (NSC) had been installed over the old Sarcophagus. This is a giant arched structure that allows you to carry out work on the dismantling of unstable structures of the old shelter and the extraction of fuel-containing materials. The NSC has created conditions for safe operation of equipment for 100 years.
Current status and monitoring
At present, Unit 4 is in a state of long-term conservation. Inside the confinement, automated monitoring systems are working, monitoring temperature, radiation background and structural integrity. The main task is to prevent the re-entry of radioactive dust into the environment.
The process of extracting fuel from the bowels of the reactor is a challenge for future generations. Fuel-containing materials (FCMs) are highly radioactive and unstable. Any mechanical action can lift a cloud of dust. All operations are planned with the use of robotics and remote control.
☑️ Key stages of the decommissioning of the 4th unit
Scientists are still studying the behavior of SCI. It turned out that in some parts of the reactor are still weak nuclear reactions that need to be controlled. The Block 4 disaster management has been a lesson for the global nuclear industry, leading to a review of safety standards and reactor design.
Chernobyl lessons for modern energy
The tragedy of the 4th power unit of the Chernobyl nuclear power plant showed that neglect of safety rules and design flaws can lead to global consequences. After the accident, the design of RBMK was significantly improved: the composition of absorbers was changed, control systems were improved, and the speed of emergency protection was increased.
Modern nuclear power plants are designed with the "principle of deep-echeloned protection". It is assumed that even if all systems fail and personnel fail, the physics of the process will not allow a disaster to happen. Security culture It has become the industry’s top priority.
⚠️ Attention: Analysis of the accident showed that a combination of design flaws in the reactor and the actions of the personnel who violated the regulations led to the explosion. No single factor alone would guarantee disaster, but their combination was fatal.
The study of the 4th block is continuing. This is not just a monument of engineering thought and human error, but also a testing ground for the development of technologies for accident elimination. The experience gained in Chernobyl is used in the design of new nuclear power plants and the recycling of old ones.
The main lesson of Chernobyl is that safety is the absolute priority over economic or production indicators. No amount of energy generated is worth the price of a mistake made on Block 4.
FAQ: Frequently Asked Questions
Can you see the 4th power unit from the inside?
Visitors can see Block 4 only from the outside, from a safe distance, as part of organized tour groups. Access to the reactor hall or the Sarcophagus is strictly forbidden for tourists due to high levels of radiation and the risk of collapse of structures.
Is Block 4 Dangerous for Kiev and Europe Today?
Thanks to the installation of the New Safe Confinement (NSC) in 2016, radioactive emissions are virtually eliminated. Monitoring systems are operating normally. The threat of a repeat explosion or a large-scale release is minimal, provided the integrity of the PSC is maintained.
What happened to the fuel from Reactor 4?
Most of the fuel was thrown away by the explosion. About 95 percent of the fuel remained inside the destroyed reactor, melting and mixing with the structures to form radioactive lava. Retrieving this material is a challenge for decades to come.
Is it true that the reactor is still smoldering?
Active burning of graphite has long been gone. However, in fuel-containing masses, weak self-sustaining fission reactions occur, which require constant monitoring of neutron levels and the addition of inhibitors to prevent heating.