The reactor accident at the Chernobyl nuclear power plant

The sequence of events and causes of the nuclear accident

At the time of the accident, four reactor units of the Soviet RBMK (reaktor bolshoy moshchnosti kanalnyy) design were in operation at the Chernobyl nuclear power plant. RBMK reactors are graphite-moderated boiling water pressure tube reactors with difficult physical properties that were already known at the time.

The accident at the Chernobyl nuclear power plant happened when Unit 4 was to be shut down for an overhaul. During the shutdown, a commissioning test was scheduled to take place. The purpose was to check certain safety features of the emergency core cooling and residual-heat removal system.

During the test, an increase in power occurred at 1:23 a.m. on 26 April 1986 due to unforeseen inadmissible plant conditions that could no longer be compensated by the control system. Owing to the special features of the RBMK core, the initiated manual shutdown led to an extremely rapid increase in the release of energy in the fuel elements, which completely destroyed the reactor core. The energy released in the fuel also caused the surrounding coolant to evaporate abruptly. The resulting extreme increase in pressure in the reactor core led to the destruction of the reactor and the surrounding building.

Graphite fire and first actions taken

The graphite used as a moderator in the reactor ignited and a graphite fire broke out. Various fires also started on the adjacent buildings due to the debris that was thrown out. After about four hours, the fires outside the reactor were extinguished. The attempt to cool the reactor core with water was abandoned after ten hours. The other three reactors were shut down. The next day, helicopters began to fill the reactor with various materials. This was to stop the chain reaction, smother the graphite fire and at least partially shield the radiation emanating from the destroyed reactor unit. 

Direct releases of radioactive substances into the environment

The explosion completely destroyed the reactor hall. As a result of the graphite fire, there was a massive release of radioactive substances into the environment. The extreme heat of the graphite fire caused these substances to be transported to high altitudes. During the entire release period of about ten days, the weather conditions in the nearer and more remote surroundings of the site changed considerably.

The released radioactive materials were initially transported in a north-westerly direction across Belarus to Finland and the central and northern parts of Sweden. On 27 April, the wind turned to a westerly direction. The path of the radioactively contaminated air masses led via Poland, the Czech Republic and Austria to southern Germany, where they arrived between 30 April and 1 May. The plume then spread in a north-westerly direction over the western part of Germany and north-eastern France, reaching Great Britain and Scotland on 2 May.

During this time, another plume formed at the site of the accident and dispersed in an easterly direction, causing weaker contamination as far as the area south of Moscow. The major city of Kiev, which is close to the power plant, remained outside the main dispersion paths. The level of contamination that occurred locally in each case resulted from the respective intensity of the rainfall (leaching of radioactivity from the air), so that locally there were differing levels of contamination.

A Sarcophagus as a temporary protective cover

Already in 1986, a provisional protective cover made of concrete and steel was erected over the destroyed reactor: the so-called Sarcophagus. Due to the high radiation levels, not all components could be welded or bolted together. Some of them were therefore merely placed on top of each other. 

An exclusion zone was also set up within a 30-kilometer radius around the nuclear power plant, which even today can only be entered with a permit. 

New Safe Confinement as new protective cover

The Sarcophagus was designed to last for about 30 years. The G7 countries, the EU and Ukraine therefore agreed in 1997 that a new protective cover should be erected over the destroyed reactor.

The so-called "New Safe Confinement" (NSC) was placed over the old Sarcophagus in 2016 to contain the escape of radioactive substances for at least 100 years and at the same time to create the conditions for controlled dismantling. 

In August 2021, regular operation of the NSC began after the nuclear regulatory authority SNRIU had granted the operating licence. The plan was to dismantle all the unstable structures of the old Sarcophagus located under the hull of the NSC by the end of 2023. The remaining building structures and the three other reactor units are to be dismantled by 2065. Initial work and plans to dismantle the Sarcophagus as well as visual and instrumental inspections and radiation measurements have been started but are delayed due to the war. 

Conditioning and waste management facilities 

Interim Spent Fuel Storage Facility  (ISF)

A major challenge in the remediation of the site is the safe management of radioactive waste from operation and dismantling. The site has a wet storage facility for spent fuel elements - the Interim Spent Fuel Storage Facility (ISF-1). This is to be replaced by a new Interim Fuel Storage Facility (ISF-2). The new interim storage facility received its nuclear licence in April 2021. More than 21,000 spent fuel elements from Units 1 to 3 are to be stored here in concrete modules in dry conditions for at least 100 years. About 2,000 of the spent fuel elements currently stored in the ISF-1 wet storage facility have already been transferred to ISF-2. 

Liquid Radwaste Treatment Plant (LRTP)  

The Liquid Radwaste Treatment Plant (LRTP) also received its official operating licence in 2021. The plant treats the liquid wastewater generated during the operation of the nuclear power plant. In 2021, more than 4,000 containers of treated liquid wastewater were produced. After a waiting period of at least 28 days, the waste that has been prepared in compliance with the requirements for disposal can be transferred to the Engineered Near Surface Disposal Facility (ENSDF) for low- and medium-level radioactive waste. The operating licence for the facility was already granted in December 2014. The operating licence of the LRTP was suspended between 26 April and 15 August 2022 due to the war. 

Industrial complex for solid radioactive waste management (ICSRM)

At the industrial complex for solid radioactive waste from the decommissioning of the Chernobyl nuclear power plant, solid waste is to be processed and packaged so that it is ready for disposal in the ENSDF. The final commissioning phase, the so-called "hot tests", for the ICSRM started at the end of 2021. During this phase, the facility is operated in a mode that comes as close as possible to future conditions. For example, radioactive materials are already being processed on a test basis during this phase. The work was completed on 26 August 2022.

As soon as all the necessary documents have been prepared, the final operating licence for the ICSRM is to be issued.

Centralised Spent Fuel Storage Facility (CSFSF)

In 2021, the construction of the centralised interim storage facility (CSFSF) for spent fuel from the Rivne, South Ukraine and Khmelnitsky nuclear power plants was also completed within the exclusion zone. The Zaporizhzhya nuclear power plant has its own interim storage facility. So far, no casks with spent fuel elements have been emplaced at the CSFSF. One reason for this is that the railway line needed to transport the casks from the NPP sites to Chernobyl has not yet been completed. Another reason is that the nuclear supervisory authority has not yet given its approval.

War in Ukraine: Situation on the plant site and in the exclusion zone

From 24 February to 31 March 2022, the site of the former nuclear power plant in Chernobyl and the surrounding 30-km exclusion zone were under the control of the Russian military. When the Russian troops entered the exclusion zone, the local dose rate registered at various measuring points increased in some cases by up to 30 times the usual values but did not pose any immediate danger to those present there. The Ukrainian supervisory authority attributed the increase to the fact that Russian military vehicles had stirred up radioactively contaminated dust. As the International Atomic Energy Agency (IAEA) and others reported, there was no major fighting or damage at the plant site itself.

Owing to the armed conflicts in the wider vicinity of the exclusion zone, the site's connection to the external power grid failed from 9 to 14 March. The installations on the power plant site that depend on external power supply - for example the wet storage facility for spent fuel elements (ISF-1) - had to be supplied by emergency diesels for a longer period of time. 

The operating personnel at the power plant site worked under extremely stressful conditions. The first change of personnel after the plant was occupied only took place after four weeks of continuous operation on 21 March. Since the end of the occupation, the regular exchange of the operating personnel has resumed.  However, this continues to be difficult because the direct route between the power plant and the place of residence (Slavutych) of the power plant workers has been destroyed and also leads through Belarus. Therefore, a long diversion has to be taken. As a result, the travel time between the power plant and the workers’ places of residence is said to be about 7 hours.

In the course of the occupation, the laboratory in the exclusion zone has been looted and destroyed. The Ukrainian regulatory authority suspended several permits on 26 April, mainly for decommissioning and conditioning work. It justified the decision by saying that compliance with nuclear and radiation safety standards and regulations could not be guaranteed due to the impact of the Russian occupation. In August 2022, all licences for the management of radioactive waste at Chernobyl were reissued after expert review.

Chernobyl: Work of GRS

GRS has been dealing with the Chernobyl reactor accident and its consequences since 1986. This includes both the scientific reappraisal of the accident and the support of the authorities on site.

GRS experts collect data on the radiological situation and forest fires

Since 2006, GRS has been developing the "Shelter Safety Status Database" (SSSDB) together with Ukrainian scientists. The database collects data on the radiological situation on site, which are collected in cooperation with Ukrainian experts. With the help of the database, geographically correct overview maps with measured values as well as three-dimensional views of the site including the Sarcophagus and the New Safe Confinement can be viewed. The data includes information on radionuclide activities in ground and surface water, near-ground air, waste dumps, and forest fires. 

The collected data on waste dump sites could be used for an initial classification when reports emerged of increased radiation doses received by Russian soldiers after digging trenches during the occupation of the Chernobyl nuclear power plant. 

In the exclusion zone, forest fires have occurred repeatedly in the dry seasons in recent years, becoming particularly severe in 2020. These fires can lead to radioactive particles being stirred up and released into the atmosphere. A total of over a thousand forest fires from the period 1993 to 2020 are recorded in the database. The activity values are stored in the database together with other parameters in order to better investigate possible connections between the fires and polluted air. 

Project on the Handling of Nuclear Fuel Containing Materials at Chernobyl

In a project on which GRS is working together with the Ukrainian technical expert organisation SSTC/NRS, the behaviour of the nuclear fuel-containing materials inside the Sarcophagus and the associated radiological effects are to be systematically analysed.

This work follows up earlier joint investigations into the monitoring of unshielded nuclear fuel in the Sarcophagus until it can be recovered and disposed of. It is also intended to continue the work on analysing factors that have a significant influence on the condition of the nuclear fuel.