The Chornobyl accident

Measurement of the ambient dose rate with a handheld measuring device at the Chornobyl reactor as part of a measurement exercise in 2016. At the time of the accident, the measured values were much higher.

On 26 April 1986, the worst reactor accident in history took place at Unit 4 of the Chornobyl (Russian: Chernobyl) nuclear power plant in Ukraine. The far-reaching and protracted consequences of this accident for the environment, physical and mental health, and the economy presented considerable challenges for the former Soviet Union and subsequently for Russia, Belarus and especially Ukraine – and these challenges persist to this day.

How the accident happened

The Chornobyl nuclear power plant used a reactor type that was built exclusively in the former Soviet Union. Significant differences between this reactor type and Western reactors are that it uses graphite to reduce the speed of neutrons in the nuclear fission reaction and lacks a pressure-tight concrete and steel "containment" shell around the reactor core.

During a scheduled slow shutdown and a simultaneous testing programme to verify various plant safety characteristics, an uncontrolled nuclear chain reaction occurred. This led to an explosion in the reactor, lifting the approximately 1,000-tonne roof of the reactor vessel. Due to the lack of containment, the powerful explosion left the reactor core exposed, so that radioactive substances from the reactor could enter the atmosphere unhindered.

The graphite used in the reactor caught fire. During the firefighting and clean-up operations, many reactor workers and firefighters, as well as emergency service and clean-up personnel known as "liquidators", were exposed to extremely high levels of radiation. Of these individuals, 134 developed acute radiation syndrome. The impacts of the reactor accident on physical – and mental – health are still being investigated today.

It took 10 days to stop the release of radioactive substances by dropping about 5,000 tonnes of sand, clay, lead and boron onto the reactor plant from military helicopters and by blowing in nitrogen to cool the melted core.

In 1986 and 1987, over 240,000 people were deployed as liquidators within a 30-kilometre exclusion zone around the damaged reactor. Further clean-up operations continued until about 1990. In total, some 600,000 liquidators were enlisted.

The Federal Office for the Safety of Nuclear Waste Management (BASE) provides information on the course of the accident and long-term plans for dismantling the facility on its website.

Release of radioactivity into the environment

As a result of the accident, significant quantities of radioactive substances entered the environment from 26 April to 6 May 1986. The 10-day-long reactor fire generated an enormous amount of heat. For days, the thermal lift carried large quantities of radioactive substances out through the destroyed roof of the reactor hall and up to altitudes of many thousands of metres. Various air currents (winds) spread the radioactive substances across large parts of Europe. The substances contaminated an area in excess of 200,000 square kilometres, of which 146,000 square kilometres were in the European part of the former Soviet Union.

A sign in the exclusion zone warning people about the “Red Forest”, an area that received the highest levels of contamination following the accident in Chernobyl.

Among others, the following substances were released:

• radioactive noble gases, such as xenon-133
• highly volatile substances, such as radioactive iodine, tellurium and radioactive caesium, which the wind spread widely across the northern hemisphere and especially Europe
• low-volatility radioactive nuclides, such as strontium and plutonium, which were deposited primarily within an approximately 100-kilometre radius of the damaged reactor in Ukraine and neighbouring areas of Belarus.

Due to their relatively short half-lives, radioactive iodine and xenon-133 had practically disappeared from the environment three months after the accident. caesium-137 and strontium-90, on the other hand, have half-lives of about 30 years and contaminate the environment for a significantly longer period of time: 30 years after the accident in Chornobyl, the activities of these radioactive substances have roughly halved. As plutonium-239 and plutonium-240, which can be found in the immediate vicinity of the damaged reactor, have half-lives of several thousand years, these radioactive substances have practically not decayed to this day and their activities are at approximately the same level as in 1986.

In late April/early May 1986, the radioactive air masses from the Chernobyl reactor accident arrived in Germany. Due to heavy local precipitation in Southern Germany, this area received a significantly higher level of contamination than Northern Germany. Among other locations, the radioactive substances were deposited in forests and on fields and meadows – including on vegetables that were ready for harvesting and on pastureland.

The article "Environmental impacts of the reactor disaster of Chornobyl" provides information on the environmental impact in the immediate vicinity of the reactor and in Germany as a whole.

Civil protection measure

The accident at the Chernobyl nuclear power plant not only affected the environment but also had a massive impact on the health and lives of the population in the worst-affected areas in northern Ukraine, in Belarus and in western Russia.

An amusement park was to be opened in Pripyat on 1 May 1986, but the town was evacuated on 27 April 1986. A Ferris wheel still stands at the site.

Evacuations

The town of Pripyat was evacuated on the day following the accident and remains deserted to this day. The area within a 30-kilometre radius of the Chernobyl nuclear power plant was subsequently designated as an exclusion zone in order to protect the population against high levels of radiation.

The areas within the exclusion zone were evacuated and abandoned – in 1986, this included not only Pripyat but also Chernobyl, Kopachi and other municipalities. The spatial characteristics of the exclusion zone were later adjusted based on contamination levels. In total, several hundred thousand people were relocated (either forcibly or of their own accord).

Protection against radioactive iodine

After 1986, there was a significant increase in the number of cases of thyroid cancer in the populations of Belarus, Ukraine and the four worst-affected regions of Russia. This is largely due to exposure to radioactive iodine during the first few months after the accident.

The radioactive iodine was primarily ingested by consuming milk from cows that had previously eaten contaminated pasture grass. This is considered the principal cause of the high rate of thyroid cancer in children. Radioactive iodine was also absorbed via contaminated food as well as from the air by inhalation. Following absorption into the body, it accumulates in the thyroid gland.

If non-radioactive iodine is taken in the form of a high-dose tablet at exactly the right time, it can prevent radioactive iodine from accumulating in the thyroid (this is known as iodine thyroid blocking). However, the respective authorities in the affected states of the former Soviet Union did not notify the population of this, especially in rural areas, nor did they warn them not to consume potentially contaminated foodstuffs or only to consume them in limited quantities. In addition, the affected population often lacked access to alternative products for food intake.

Protective casing around the reactor

Protective cover over the damaged Chornobyl reactor (New Safe Confinement) Source: SvedOliver/Stock.adobe.com

In order to safely enclose the radioactive substances inside the destroyed reactor unit and limit the further release of radioactive substances into the environment, a steel and concrete structure known as the "sarcophagus" was constructed around the destroyed reactor in the period from May to October 1986. Given the urgency of the situation, there was no time for detailed planning.

In 2016, with international support, a protective casing (the "New Safe Confinement") measuring some 110 metres in height was slid into place over the original sarcophagus. In 2019, with the casing now ready for operation, responsibility for the structure was handed over to Ukraine. The New Safe Confinement is some 165 metres long and has a span of approximately 260 metres; its projected service life is 100 years. The next challenge is to dismantle the old sarcophagus, recover the radioactive material it contains, and ensure the safe final disposal of this material.

Consequences for emergency preparedness in Germany

For information on the consequences of the Chornobyl reactor accident for the organisation and implementation of radiological emergency protection in Germany, please refer to the article "Development of emergency preparedness in Germany".