The planned test was intended to prove that the plant could be controlled even in the event of a loss of coolant - water is used in this type of reactor - and a simultaneous failure of the external power supply. In nuclear technology, these scenarios are referred to as loss-of-coolant accident and emergency power supply accident respectively.

In the event of a loss-of-coolant accident, the reactor is automatically shut down immediately (emergency shutdown or scram) to prevent overheating. If an emergency power event were to occur at the same time, this would pose great challenges to the continuous power supply of the reactor cooling system as well as the monitoring and control instruments. Since these facilities must always be available and supplied with electricity for this purpose, a nuclear power plant is equipped with emergency diesel generators. Yet, the start-up time of these generators - about 40-50 seconds in the case of the Chernobyl plant - must be bridged by other means. After an emergency shutdown, the power plant turbine and generator will continue to produce electricity for a short period of time due to the remaining rotational energy. The experiment was intended to prove that they could cover the short-term power demand of the main feed water pumps until the emergency diesel generators started up and the emergency cooling pumps were available to continue cooling the reactor core.

The test was wrongly considered a purely conventional test in the field of electrical engineering, where no repercussions on the nuclear part of the plant were to be expected.

The safety test had been scheduled to happen during a planned shutdown of the reactor in the context of a regular overhaul. To prevent feeding in cooling water too early during the test, the automatic emergency core cooling system was deactivated in advance. The reactor power was systematically reduced for the shutdown; the test was to be carried out at about 25% of the maximum operating power.

Due to a mishandling or malfunction of the control system, the reactor power fluctuated greatly during shutdown, and fell to less than 1% of the operating power. Due to the instability of the reaction process at such a low value, an immediate shutdown of the reactor and a postponement of the experiment would have been necessary. However, the risk was not correctly assessed or not sufficiently considered. The experiment was not stopped at first.

To prevent a further drop in power, the control rods were raised almost completely, which made it much more difficult to readjust and shut down the reactor later. An emergency signal to shut down the reactor was ignored at this point to keep open the option of repeating the experiment.

When the main circulation pumps were switched off to initiate the experiment, the reduced coolant flow immediately resulted in an increase in power. This was recognised as dangerous, and the reactor shutdown was initiated manually.

A peculiarity of the design of the RBMK control rods caused a dramatic power surge as they were simultaneously inserted into the reactor. Due to the enormous release of energy, large quantities of the cooling water evaporated, the pressure in the pressure tubes increased dramatically and the reactor exploded.

At least two explosions destroyed the safety shields and the roof of the reactor building and hurled reactor inventory into the surrounding area. The graphite in the reactor core caught fire, resulting in the release of radioactive material far into the atmosphere. Large parts of Europe were subsequently contaminated with radioactive material.

Several factors contributed to the catastrophic accident. Reactors of the early RBMK design have specific physical and safety-related characteristics that have a decisive impact on their behaviour during operation and control processes. Particularly noteworthy here is a self-amplifying relationship between reactivity and temperature, as well as the property of the control rods to briefly amplify the nuclear reaction prior to its general suppression. Some of these peculiarities had not been considered by the operating staff. The design of the test programme was also inadequate, as possible effects of the experiment on the nuclear fission chain reaction had not been taken into account.

The operating staff carrying out the experiment was confronted with unexpected conditions from the beginning. The behaviour of the reactor and the control issues were not recognised as a high risk at first, and important safety measures were not initiated or initiated too late. Several operating regulations were actively violated.

Due to the explosion and subsequent fires, the released substances were emitted into the atmosphere. The prevailing air currents dispersed them over large parts of Europe and as far as Scandinavia and Great Britain.

The strength of the radioactive contamination varied locally and was determined not only by the prevailing winds during the ten-day release phase. The decisive factor was the intensity of precipitation during this period, which washed out and rained down the radioactive substances.

Accordingly, different locations saw very different levels of contamination. Areas in northern Ukraine, Belarus and western Russia were most severely affected.

Due to heavy local precipitation, the south of Germany was contaminated to a much higher degree than the north. Up to 100,000 Bq of caesium per square metre were deposited in the Bavarian Forest and south of the river Danube. In contrast, the deposition of this radionuclide in the North German lowlands rarely exceeded 4,000 Bq per square metre.

Radioactive caesium (Cs-137 and Cs-134) and iodine (I-131) were particularly significant in terms of human radiation exposure following the reactor disaster.

Today, it is mainly the long-lived Cs-137 that still plays a role in Central Europe. Due to its half-life of approximately 30 years, only about half of this radionuclide has decayed since 1986.

Cs-137 soil contamination in Germany after the Chernobyl reactor disaster

To reduce the release of further radioactive substances from the severely damaged building following the accident, a protective shell made of a steel/concrete construction - the so-called sarcophagus or shelter structure - was erected between May and November 1986. The construction was carried out under the most difficult conditions and under massive time pressure.

The sarcophagus had never been intended to be a permanent solution, but rather a temporary solution for a period of 20 to 30 years.

Steel and concrete sarcophagus above reactor block 4 of the Chernobyl nuclear power plant Source: Pixabay / Robert Armstrong

In 1997, the G7 countries, the EU and Ukraine agreed on the so-called Shelter Implementation Plan (SIP) to stabilise the old sarcophagus and build a new, larger shelter.

The new protective shell - New Safe Confinement (NSC) - is an arch-shaped construction. Construction next to the damaged unit 4 began in March 2012. In November 2016, the shell was then moved over the old sarcophagus on rails.

The NSC is about 260 metres wide, 165 metres long and 110 metres high, and is intended to safely isolate the radioactive material it contains from the environment for up to 100 years.

Protective shell NSC under construction before it was moved over the sarcophagus in 2016 Source: picture alliance / Photoshot

Upon completion of the construction, trial operations began in May 2019. They were followed by an "industrial operation experience " phase of the NSC starting in August 2019. In the summer of 2020, the NSC was handed over to the operator, the Chernobyl nuclear power plant. Under the protection of the NSC, the radioactive waste is to be removed and the old sarcophagus dismantled.

The plan is for the remaining building structures as well as the three other reactor units at the Chernobyl site to be dismantled by 2065.

Parallel to the new NSC shelter, the construction of the safety-relevant long-term interim storage facility - Intermediate Storage Facility 2 (ISF-2) - was initiated. This is where the spent fuel elements from all Chernobyl reactor units are to be stored.

Intermediate Storage Facility 2 (ISF-2): Dry interim storage for spent fuel elements from the Chernobyl nuclear power plant Source: pa / dpa | Andreas Stein

Following the accident at unit 4, the last of the three remaining reactor units at the Chernobyl site was finally decommissioned in December 2000. All spent fuel elements that had been used for electricity generation at the four units of the Chernobyl nuclear power plant since the commissioning of the first reactor unit in 1977 and up until 2000 (a total of more than 20,000 fuel elements) were stored in a wet storage facility (ISF-1 - Intermediate spent fuel storage facility 1) for the time being. This interim storage facility was commissioned in 1986, and has an operating licence until 2026.

The construction of a dry interim storage facility, ISF-2, began in 2001. After completion, all spent fuel elements from ISF-1 are to be transferred there. ISF-2 is designed for an operating time of 100 years.

In April 2021, the Ukrainian supervisory authority granted the necessary operating licence for ISF-2. The transfer of the spent fuel elements to the ISF-2 interim dry storage facility began in June 2021. By January 2022, just under 1700 spent fuel elements had been transferred from ISF-1 to ISF-2.

The RSK concluded that the accident at Chernobyl could not be transferred to the light water reactors of German design. Nevertheless, the RSK performed a safety review of all operating nuclear power plants as well as those under construction in the Federal Republic of Germany at the time.

In the time following the catastrophic accident of Chernobyl, the results of these reviews contributed significantly to the improvement of safety in nuclear power plants in the Federal Republic of Germany.

One of the most important results of the safety review was the recommendation to introduce in-house emergency protection measures at all plants. Today, in-house emergency protection is an integral part of the safety concept of German nuclear power plants and has been anchored in the "Safety Requirements for Nuclear Power Plants" since 2012. Furthermore, it was also included in the Atomic Energy Act in June 2017.

Founding of the Ministry of the Environment in the Federal Republic of Germany

As an organisational consequence of Chernobyl, the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (then BMU, as of 2021: Bundesministerium für Umwelt, Naturschutz, nukleare Sicherheit und Verbraucherschutz BMUV (Federal Ministry for the Environment, Nature Conservation, Nuclear Safety and Consumer Protection)) was founded in the Federal Republic of Germany to bundle competences in the field of nuclear safety.

After the Chernobyl reactor disaster, the Federal Government also promoted international cooperation about safety in nuclear power plants. Its initiative contributed significantly to the drafting and subsequent adoption of the Convention on Nuclear Safety in 1994.

The signatory states to this convention undertake to comply with basic requirements for reactor safety and submit to regular review processes. The Nuclear Safety Convention is thus an element of safety improvement in nuclear power plants in Germany and abroad, which emerged indirectly from the catastrophic accident at Chernobyl.

Some countries abandoned nuclear energy soon after the Chernobyl reactor disaster. Italy, for example, stopped plans to build more nuclear power plants following a referendum in November 1987. As a result, the last two Italian nuclear power plants were shut down and officially decommissioned in 1990.

Consequences for the use of nuclear energy in the GDR

As far as environmental groups in the GDR were concerned, the catastrophic accident at Chernobyl had sent a powerful signal. The GDR government's information policy was criticised, sometimes harshly so. The population demanded more transparency on the health implications of the nuclear disaster as well as the safety of the reactors in operation in the GDR. As a result, the GDR witnessed a broad debate about the use of nuclear energy for the first time, and voices in favour of a phase-out grew louder.

Nevertheless, the reactors of the Greifswald nuclear power plant remained in operation until the time of the German reunification. During this period, various expert reports concluded that considerable upgrades to the plant would be necessary to meet West German safety requirements in the future. With the shutdown of all reactor units in 1989 and 1990, the GDR's last nuclear power plant finally went out of operation.

Nuclear phase-out in the Federal Republic of Germany

In the Federal Republic of Germany, the Chernobyl nuclear disaster was not followed by an immediate abandonment of nuclear energy. It was in the years between 2000 and 2002 and following long public debates that the federal government first decided to gradually phase out nuclear power. The final decision came following the serious accident at the Fukushima nuclear power plant in Japan on 11 March 2011.

In mid-April 2023, the remaining three operating nuclear power plants in Germany were finally shut down.