Chernobyl Disaster

Chernobyl accident (April 26, 1986) was a nuclear catastrophe that took place in Chernobyl, Ukraine. It is considered one of the most regrettable events in history. On Saturday, April 26, 1986, a test was carried out to increase the safety of nuclear reactor No. 4 installed at the Chernobyl nuclear power plant.

The intention of the test carried out by the technicians was to find out how long the steam turbine could generate electrical power after the loss of electrical power supply. The conditions under which the test would be carried out had been previously agreed upon: A special team of electrical engineers was present to test the new voltage regulation system. At dawn, the programmed reduction of power began, reaching 50% of its capacity. A problem broke out: a regional power plant was taken offline, and the power grid controller in Kyiv ordered a halt to Chernobyl’s reduction in power production as it had to meet the peak demand in the afternoon.

The Chernobyl director followed orders and postponed the test. However, some tests were carried out that did not affect the power of the reactor. The emergency core cooling system, intended to provide water to the plant in the event of a loss of coolant, was disabled.

At 11 pm, the controller of the Kyiv grid allowed the power reduction to resume. The test, as they had planned, had to end in the daytime and at night it was only intended to control the remaining heat. Alexandr Akimov was the head of the night shift and Leonid Toptunov was in charge of the operational regime of the reactor. They witnessed a power reduction, due to the natural production of xenon-135: a very absorbent gas of neutrons. This process is known as “xenon poisoning.” Toptunov mistakenly inserted the control rods too fast, the power was over 500 MW. As a result, the power dropped to 30 MW, the control room staff decided to increase the power by deactivating the automatic system that moved the control rods and manually raising them to the stop. Although the power stabilized at 160-200 MW, xenon poisoning prevented the increase in power and, to counter this, more control rods had to be removed.

 

Low power operation, the presence of xenon-135, core temperature, coolant flow, and instability in the neutron flow, were the factors that triggered the alarms. The control room received multiple emergency signals related to the levels of the water and steam separators. Between 00:35 and 00:45, the alarms on the thermohydraulic parameters were ignored by the technicians, supposedly to maintain the power level. When the 200 MW power level was finally achieved, preparation for the experiment was resumed. At 01:05 additional water pumps were activated, increasing the water flow. The increase in the coolant flow rate caused an increase in the coolant temperature at the inlet of the reactor core.

 

Minutes later the limit was exceeded. Simultaneously, the additional water flow lowered the overall core temperature and reduced the existing steam voids in the core and steam separators. The operators decided to shut down two of the circulating pumps to reduce the feedwater flow rate to increase steam pressure and to remove other control rods to maintain power. In short, the automatic system that could do the same had been disabled to maintain the power level. These actions constituted serious violations of the Nuclear Safety Regulations of the Soviet Union.

 

For most of the experiment, the automatic control system successfully countered this positive feedback. However, the system only had 12-bar control. With the emergency systems disconnected, the reactor experienced such a rapid surge in power that the operators failed to detect it in time. At 01:23, the start of a SCRAM (emergency shutdown) of the reactor was recorded, which would trigger the explosion.

 

The SCRAM was started by pressing the AZ-5 button. Now the reason why the AZ-5 button was pressed is not known. There is an opinion that the SCRAM may have been ordered in response to the sudden and unexpected increase in power, although there is no documented data to prove it. Some have suggested that the button was never pressed, but that the signal was produced automatically by the emergency protection system (SPE), however, the SKALA computer registered a manual signal. This is a question that still does not have a clear explanation, just assumptions.

 

It only took three seconds for the power level to rise above 530 MW. According to some estimates, the reactor’s power increased to around 30,000 MW, ten times normal production. There was an explosion produced by the formation of a hydrogen cloud within the core, which blew the 2000t lid of the reactor, stoking a fire in the plant and a gigantic emission of fission products into the atmosphere. About 25% of the red-hot graphite and other overheated material was expelled from the fuel channels.

 

In short, the high temperature of the core created an airflow through it, and the hot air ignited the graphite. Minutes later, the disaster was dealt with. The flames affected several floors of reactor 4 and came dangerously close to the building where reactor 3 was located. The authorities managed to stop the threat of explosion. The spectators witnessed a true catastrophe: the fuel and other metals had turned into an incandescent liquid mass. The temperature reached 2500 ° C, and the radioactive smoke was driven off a chimney at a considerable height. On the other hand, an astronomical level of 2080 roentgen was recorded. It would take a human being fifteen minutes to die after breathing such content.

 

The inhabitants of the city of Prípiat were evacuated. This first evacuation began massively 36 hours after the accident and took three and a half hours. The Chernobyl evacuation took place until May 2nd. By then, there were already approximately 1000 affected by acute injuries produced by radiation. Several Red Army helicopters dropped a mixture of materials (sand, clay, lead, dolomite, and boron) on the core to avoid any emission.

 

By the end of the emergency missions on May 13, 1,800 flights had been used and 5,000 tons of materials were thrown into the nucleus. Later, it would be verified that its effect was opposite because it contributed to the release of radionuclides. The radiation spread to most of Europe, these areas had a high rate of radioactivity for several days.

For safety reasons, European countries instituted measures to limit the effect on human health of the pollution of fields and forests. International Humanitarian Aid Several countries collaborated, but Cuba has maintained since 1990 a medical program for the victims of this nuclear accident. Almost 24,000 patients, from Ukraine, Russia, Belarus, Moldova, and Armenia, have been treated at the Tarará Pediatric Hospital in Havana. Most of the patients are Ukrainian children affected by the disaster.

Around 67% of the children come from orphanages and schools for children without filial protection. They are evaluated and receive all kinds of treatments, including bone marrow transplants for those with leukemia. The Ukrainian Ministry of Health pays for the children’s travel to Cuba and all the rest of the funding for the program is covered by the Cuban government. On the other hand, the Galician NGO Asociación Ledicia Cativa temporarily shelters minors affected by Chernobyl radiation.