Exactly 32 years ago, the largest accident in the history of nuclear power engineering occurred at the Chernobyl nuclear power plant in Ukraine. As a result of the explosion and subsequent destruction of the fourth power unit of the power plant, a large amount of radioactive substances were released into the environment. The 30-kilometer zone around the power plant has been declared an exclusion zone. But since then much has changed. It has become safer both in the exclusion zone and inside the destroyed power unit, where, however, a radioactive, non-cooling formation is still preserved, which cannot be approached even with strict safety measures. First discovered in December 1986, for its appearance and color, it was called the “elephant’s foot”.
On the night from April 25 to April 26, the Chernobyl nuclear power plant was scheduled to shut down at the fourth power unit for preventive maintenance and routine and non-standard tests. Then it was planned, in particular, to check the run-on mode of the rotor of the turbogenerator. This mode was proposed by the general designer as another emergency power supply measure – the rotor of the turbogenerator, which continues to rotate due to inertia, could theoretically provide electricity for the power supply and main circulation pumps for some time. The work of the latter is necessary even on a stopped reactor for its cooling, since the power drop in the installation occurs smoothly; even an hour after stopping, the residual energy release is about 1.4–1.5 percent. Tests of the run mode were carried out before
On the night of April 26, 1986, during the experiment, a power jump occurred, the protection system and emergency shutdown of the reactor worked, however, it was likely that the reactor did not turn off. Attempts by the staff to compensate for the jump were unsuccessful, and the capacity continued to grow uncontrollably. According to one of the versions explaining the causes of the accident, during an uncontrolled increase in power, fuel elements (containing nuclear fuel) melted, which, in turn, damaged the cooling circuit. Because of this, water and steam got into the reactor space, the pressure in which was maintained at the atmospheric level. As a result, the first steam explosion occurred that threw up the reactor lid. This structural element weighing about two thousand tons pierced the roof of the power unit. Water from the cooling circuit, which continued to flow into the reactor, still turning to steam; a second explosion occurred, releasing large quantities of radioactive elements into the environment.
In November last year, Swedish scientists came to the conclusion that during the accident at the Chernobyl nuclear power plant there actually was a nuclear explosion. Its capacity was about 75 tons in TNT equivalent. Before arriving at this conclusion, the researchers analyzed the concentrations of 133Xe and 133m Xe isotopes in samples from the Cherepovets air-liquefaction factory, and also modeled the weather conditions after the disaster using the recently published 1986 detailed data.
As a result of these explosions, the cooling stopped – dehydration of the reactor core led to a sharp set of power and an abrupt rise in temperature. The accident at the Chernobyl power plant went into a difficult phase, which is informally called the melting of the reactor core. The international organizations for nuclear safety do not recognize this term, but, nevertheless, are used to describe uncontrollable events that occur at an emergency nuclear power plant if fuel elements begin to melt there. In the entire history of nuclear power, the reactor core melted five times: during the accident at the second unit of the Three Mile Island power plant in the USA in 1979, at the Chernobyl nuclear power plant and three times at the Fukushima nuclear power plant in Japan in 2011.Because of the explosions, the reactor of the Chernobyl NPP was seriously damaged, including in the steam distribution system. The melted radioactive mass began to flow into the formed holes — fuel elements that turned into lava. Spreading out, this lava, containing uranium and zirconium (fuel and fuel element coating), was mixed with fragments of concrete, sand, and metal elements of the structure. According to the cooling and steam distribution system, this melt, called “Chernobyl”, spread for almost a week, until it reached the pool-bubbler. This is the name of a special cold water tank, where in emergency situations excess steam from the cooling system is discharged, which then condenses. The accident liquidators feared that lava from the reactor core could melt its bottom,
To protect against melt flow, a concrete tray with metal pipes for water cooling was installed under the reactor. However, the flow of melted Chernobyl stopped in the pool-bubbler, and the concrete pan was not useful. In May 1986, emergency workers who worked in the fourth power unit discovered a source of high radiation — during the inspection of the premises, the Kirzhach-3 device, which measures the intensity of gamma fields and has an x-ray of three thousand per hour, is oversized and out of order. According to the recollections of one of the liquidators, research engineer George Popkov, later in one of the suspicious rooms, where ordinary equipment went off scale, they pushed the baby horse on wheels, on which the measuring sensor was fixed. The device showed 14.5 thousand X-rays per hour.To penetrate into the pool-bubbler, where a very high level of radiation was recorded, the researchers managed only in December 1986. There they saw a huge bloated drop with a gray, sometimes black, shiny surface. Measurements showed radiation of about eight thousand X-rays per hour on the surface of a drop, which, for appearance, was called the “elephant’s foot.” At first, the researchers decided that it consists of molten lead – the first time after the accident at the fourth power unit, lead blocks were dropped from helicopters into the reactor zone. It was assumed that lead will take away some of the heat and, melting, increase the heat exchange area. This was supposed to quench the power of the molten core. Some time after the discovery with great difficulty, short approaches literally for a couple of seconds,
Laboratory tests showed that there was no lead in this formation. It contained about ten percent of uranium, a complete set of radionuclides of nuclear fuel, a melt of silicon dioxide and some other elements. Such a melt is otherwise called “Corium” (from the English word core– core), since it is formed as a result of melting of the reactor core. Corium was formed during the accident at the Three Mile Island reactor; it was also formed during the accident at the Chernobyl nuclear power plant. At the same time, the fourth power unit remains to this day the place with the largest accumulation of lava-like alloy in the world, which is not found in ordinary nature. “Elephant’s foot”, whose initial mass was estimated at 11 tons, is only the largest formation of corium – in the reactor zone, molten Chernobyl has formed drops, stalactites and stalagmites with different radioactivity, which usually rarely exceeded 600-700 X-rays per hour.According to the measurements of researchers, the area of the base of the “elephant’s foot” was about six square meters, and the area of the base of stalactite extending from the “foot” was 1.7 square meters. Some time after the accident, the “elephant foot” began to crack. In addition, the teams of liquidators regularly poured water on it and chipped pieces off the “leg”. But it is believed that this education has not cooled down to the present. The initial temperature of corium at the Chernobyl nuclear power plant, according to various estimates, was about two thousand degrees Celsius. Today, the temperature of this object remains a few degrees above the ambient temperature, since the decay of actinides and other radioactive elements in it still continues. It is believed that if today you are close to the “elephant foot” for 500 seconds, then death from overexposure will occur within two days. According to various estimates, the influx of Chernobyl will be radioactive for another hundred thousand years.
In order to prevent the spread of radioactive elements from the destroyed fourth power unit of the Chernobyl nuclear power plant, by November 1986 an insulating structure was quickly built over it. It received the official name “Shelter” and the unofficial name – “Sarcophagus”. The construction was erected in just 206 days. Its service life was predicted to be 20–40 years old. In 2007, construction began on Shelter-2, an arch protection structure, which in 2017 was pushed onto the fourth power unit over the Sarcophagus. Fully construction and sealing work on the design is scheduled for completion in 2018. From now on, it will be impossible to penetrate under the “Sarcophagus” (initially, technological inputs for researchers and controlling engineers were provided for in the construction of the first shelter).
And although now double shelter should prevent the spread of radioactive substances by air, it probably will not provide complete safety. Some experts suggestthat the “elephant foot” in the future may be the cause of a new catastrophe. It is assumed that it continues to melt the concrete base beneath itself and eventually pass through the concrete into the soil, from there it can reach the water layers. This, in turn, will lead to poisoning of underground water sources and even a new explosion. All this resembles the so-called “Chinese syndrome”. So in the 1970s, it was decided to mock the fears of scientists who warned that the melting of the reactor core could go beyond the protective shell of the reactor. The jokers then said that the melt from the core would pass through the Earth and would go somewhere in China.
This fantastic script in 1979 even inspired an American director to shoot a film with the same name – “ Chinese Syndrome ”, which was released a few weeks before the Three Mile Island accident. In the 1980s, this syndrome was forgotten, but remembered again, and already seriously, in 2017. Then, during another survey of the Japanese nuclear power plant Fukushima-1, which was affected by an earthquake in 2011, it turned out that the active zone of the reactor of the second unit of Fukushima-1 melted and leaked beyond the outer contour. In particular, the researchers found that the section of raised floor beneath a one-square-meter reactor completely melted. Research at the Japanese power plant continues.
But back to the “elephant foot.” With the construction of the Chernobyl nuclear power plant more than 40 years have passed. Of these, for 32 years, a thorough inspection and repair of the station’s structure has not been carried out at all. The time and consequences of the accident could lead to cracking of the concrete foundation. Some experts believe that Sarcophagus and Shelter-2 created additional pressure on the ground around the fourth power unit and in the future the foundation may simply collapse. As a result, the “elephant foot” and other formations from Chernobyl will fail, fall into the soil and eventually begin to leach out, poisoning, first of all, underground water sources. However, other experts are skeptical of this version, believing that the concrete foundation and the surviving metal structures of the fourth power unit are quite reliable and will not allow the corium to penetrate the soil.
In the 1990s, researchers published a report on the post-accident condition of the fourth unit of the Chernobyl NPP, which stated that it was necessary to saturate the leakages of Chernobyl with neutron-absorbing solutions and close them. It was assumed that this would prevent the conversion of the “elephant foot” and other influxes into dust, the main source of radiation infection in Chernobyl. However, later Ukrainian researchers announced, that the treatment with solutions will not give anything, and in the worst case, it will even lead to “loss of control over the neutron flux”. In addition, these measures will not prevent the formation of radioactive dust. For this reason, Shelter 2 is considered the most reliable way to protect against the spread of radioactive elements from the fourth block. The further fate of lavoobraznyh fuel-containing materials in the Chernobyl nuclear power plant, scientists intend to determine the calculation and analytical method.