Why are thermal out of control reactions dangerous
Start of a series of experiments
On April 25, 1986, the reactor team started a series of tests in Block 4 of the Chernobyl nuclear reactor. It should be checked whether, in the event of a power failure, the rotational energy of the turbine would be sufficient to produce enough electricity until the emergency power generators are running. The reactor should remain in operation for this time. The emergency cooling system and other safety systems have been switched off. Gradually the power of the reactor was reduced.
After a short time, the experiment had to be interrupted due to a power demand. It could only be continued at night. The emergency cooling system remained - contrary to the safety regulations - switched off in the meantime.
Shortly after 11 p.m., preparations for the actual experiment were resumed. The test protocol called for the reactor to be shut down to around 25 percent of its capacity. But the output sank within a very short time to less than one percent of the nominal output. The reactor was no longer stable in this area. The reason for this drop in performance is still unclear.
Instead of shutting down the reactor, the technicians tried to increase the output again. For this purpose, the control rods, which were used to control the output of the reactor, were pulled out of the reactor core. The output then stabilized at around seven percent - still too little for safe operation of the reactor.
Reactor out of control
The attempt was started anyway. The technicians closed the turbine safety valves, thereby reducing the water supply to the reactor. The reactor power increased rapidly within seconds. The shift supervisor tried an emergency shutdown - without success. The output increased further - to an estimated 100 times the nominal output of the reactor.
In order to brake the reactor, the control rods would have had to be retracted completely into the reactor core. A process that takes 18 to 20 seconds. The team didn't have that much time left. The pressure that had built up in the reactor as a result of the heated and evaporating water was already too high. Due to the extreme temperatures, the rods had bent and no longer fit into the intended insertion holes.
There was a first explosion in which parts of the reactor and the 64-meter-high reactor building were destroyed. The graphite jacket of the reactor began to burn. Radioactive material was thrown into the atmosphere. A second detonation occurred a few seconds after the first explosion. The fire spread to the roof of Block 3. The smoke carried tons of radioactive material into the atmosphere.
The reactor had to be extinguished
In order to contain the extent of the disaster, the reactor had to be extinguished. On the night of April 26th, the firefighters began to pump cooling water into the reactor core. When that failed, military helicopters were organized, with the help of which lead, boron, sand and clay were thrown into the fire from above - a total of around 5,000 tons of material.
However, this led to the opposite effect: the cover increased the temperature. More radioactive materials were released. Only when the reactor - ten days later - could be cooled with nitrogen, was the fire under control.
So that the reactor could no longer release any radioactivity into the environment, it was decided to completely encase and cover it. A so-called sarcophagus made of concrete was therefore built around the reactor by autumn 1986. It was designed for a period of 20 to 30 years, but serious damage became apparent after a few years.
In 1997, at an international conference in which the G7 countries, Russia, Ukraine and the European Union participated, the decision was made to build a new shell that would safely surround the destroyed reactor for at least 100 years. The new protective cover was completed in 2015.
In total, an estimated 600,000 to 800,000 men, so-called liquidators, were involved in the clean-up work and the construction of the first sarcophagus.
They were called liquidators because they were supposed to liquidate the consequences of the catastrophe, that is, to eliminate them. Robots that were supposed to be used for the clean-up work simply stopped because the electronics failed in view of the high radiation exposure.
So those responsible sent men to the roof of the reactor blocks while the reactor was still burning. They were supposed to shovel the catapulted graphite and other shiny chunks that had been hurled outward by the explosion back into the crater.
Nobody told them what mortal danger the liquidators were in due to the unimaginably high dose of radiation at the reactor.
Insufficient protective measures for the liquidators
As a protective measure, the men should only stay on the roof for 45 seconds and wear lead protection in front of their chest and back. But what are 45 seconds to climb a ladder onto a meter-high roof equipped with lead in order to work there?
The young men wanted to lend a hand - they noticed nothing of the invisible, deadly radiation. So they stayed longer and sometimes took off their protective clothing because it was too hot.
Even when a tunnel was dug under the reactor to pull a layer of concrete under the reactor core, which threatened to eat its way down into the groundwater, the men worked without any further protection.
300 millisievert was set as the limit value for the radioactive contamination of the liquidators. Radiation physicians such as Munich professor Edmund Lengfelder assume that many men have received ten times as much. That is 300,000 times as much as the average annual dose is considered harmless in Germany.
Evacuation and establishment of a restricted zone
Although those responsible were aware of the danger, their main concern was to contain the disaster, but not to inform the population and educate them about the dangers.
A day and a half passed before the Chernobyl reactor area was evacuated. Almost 50,000 people lived in the city of Pripyat alone, most of whom worked in the power plant. On April 27, 1986, the population was then taken away by bus.
A total of 135,000 people were resettled by the authorities. Another 300,000 joined because the exclusion zone, which was then drawn around the reactor in a 30-kilometer radius like a circle, ran through the middle of villages. Communities and economic units broke up. The city of Pripyat, only a few kilometers away from the scene of the accident, is now a ghost town.
Even outside of the 30-kilometer radius, many areas were highly contaminated. After 1990, radiation doses as high as those in the immediate vicinity of the reactor were measured in some cases. For this reason, a restricted zone was subsequently set up near the Belarusian city of Gomel. For five years the people there had lived unsuspecting.
The regional catastrophe turned into a global problem that severely scratched the image of nuclear power. Even today people in the affected regions in Ukraine and Belarus are suffering from the consequences of radioactive contamination.
Serious illnesses, especially of the thyroid gland, and a cancer rate 30 times higher than that before the disaster make the extent clear.
The so-called radiation sickness means that the body cells and red blood cells are destroyed and the mucous membranes dissolve. Extreme radiation destroys the skin. The low radiation exposure also attacks the body: malfunctioning of the thyroid gland is still a major problem in Ukraine and Belarus.
It is difficult to determine exactly how many people actually died as a result of the accident. The liquidators' medical files are kept under lock and key and are not available to treating doctors.
Radiation doctors assume that more than 50,000 liquidators died as a result. The International Atomic Energy Agency (IAEA), on the other hand, declares almost cynically that only around 30 people died from direct exposure to radiation.
The search for the causes
Even decades after the accident, it has not been conclusively clarified what actually happened in Chernobyl. When reconstructing the accident and looking for the causes, the scientists relied primarily on eyewitness reports.
From the damage caused, they tried to draw conclusions about the events. Even so, it is still unclear what caused the original drop in performance and ultimately the two explosions.
Unsafe reactor technology?
Scientists agree that many different factors led to the worst-case scenario. Above all, the design of the reactor is still considered a safety risk today.
In order for a controlled chain reaction to take place within the reactor core, a so-called moderator is required, which slows down a neutron before it splits a uranium atom. If the neutrons are unbraked, nuclear fission does not occur and no nuclear energy is released.
Most nuclear power plants use water as a moderator. They are either boiling water or pressure tube reactors. There are also reactors of this type in Germany.
In the so-called RBKM reactor or boiling water pressure tube reactor in Chernobyl, graphite served as a moderator. This is where the decisive disadvantage lies: graphite is flammable. Although the fuel rods are cooled with water, the water supply was switched off in Chernobyl. The fuel rods heated up to around 2000 degrees within a few seconds. The graphite started to burn.
After the Chernobyl disaster, Russia, Lithuania and Ukraine started retrofitting their RBMK reactors. Some are still online today.
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