Decommissioning after a nuclear accident: Chernobyl NPP decommissioning strategy

The Chernobyl nuclear power plant (ChNPP), near the city of Pripyat in northern Ukraine, consisted of four RBMK-1000 reactors, each capable of producing 1000 MW of electric power (3200 MW of thermal power). The construction of the first Unit began in 1972: on August 15, 1972, the first cubic meter of concrete was put into the foundation of the main building. It was completed in 1977, followed by reactor No. 2 in 1978, No. 3 in 1981, and No. 4 in 1983. At the time of the incident, a fifth and sixth reactor units were under construction. Those projects were quickly halted in 1989, but Units 1, 2, and 3 kept producing power.
This beyond-design nuclear accident ⁽¹⁾ is among the most severe ever occurred, reaching the 7th level of the International Nuclear and radiological Event Scale (INES) ⁽²⁾, the international scale of nuclear accidents. The explosion in the reactor core of Unit 4 destroyed the protective barriers and safety systems: the unit lost all functional capabilities. Just after the accident, all the other units at Chernobyl were temporarily stopped.
The priority measures to reduce the accident consequences were carried out under very difficult conditions. As a result of the accident, a territory of 200000 km², including the industrial site and the facilities located on it, was heavily contaminated. About 50 MCi of radioactivity was released within 10 days.
To limit further release of radioactivity to the biosphere, a temporary cover was constructed over Unit 4 from May to November 1986: about 400000 m³ of concrete were placed and 7000 t of metal constructions were assembled tu build the cover, the Shelter Object (SO).
After the accident, the remaining three reactors, after a proper decontamination, continued to operate, as the Soviet Union could not afford to shut the plant down. In 1991, a fire broke out in the turbine of Unit 2, that was consequently shut down. Unit 1 was shut down in 1996, following pressure from foreign governments about the health effects radiation poisoning, including elevated rates of thyroid cancer among children.

The last functioning Unit 3 of Chernobyl NPP was stopped in 2000.

Figure 1 - A view of the Sarcophagus in 2005, IAEA Imagebank. Credit: Petr Pavlicek/IAEA - CC BY-SA 2.0

The Shelter Object was not created in accordance with the rules and norms of designing and construction, so a continuous monitoring to ensure nuclear and radiation safety was required. A complex of 8 measures of stabilization, in order to improve durability and reliability of steel structures and building constructions was performed within the period from 1998 to 2008 to reduce the risk of the Shelter Object critical structures collapse. Implementation of stabilization measures improved the SO safety level till 2023.
In 2010, the concrete and lead sarcophagus was followed by a large steel New Safe Confinement (NSC), thanks to the collaboration of 45 donors countries that combined their efforts together with Ukraine and pledged more than 1.5 billion euros. The new sarcophagus, constructed over the Shelter Object, performs the function of the primary physical barrier on the way of radioactivity release to the environment. The main functions of the NSC are:

• Restriction of radiation impact on general public, personnel and the environment: according to the results of gamma radiation levels measurements within the NSC construction zone, the radiation levels in average decreased by 10 times.

• Restriction of the spread of ionizing radiation and radioactive substances inside the Shelter.
• Creation of conditions for unstable structures dismantling, radioactive materials retrieval, accumulated water pumping, for ensuring implementation of measures to control and maintain the Shelter object and its industrial site. Indeed, the NSC covers the Shelter Object from atmospheric precipitation (radioactively contaminated water pumped from the Shelter decreased in average by more than 4 times). Besides, the release of radioactive aerosols through the Shelter Object openings is decreased. The NSC makes impossible direct impact of sunlight and wind on the Shelter, which created airflows inside the facility and transported radioactive aerosols outside. Total releases are decreased in average by 5 times.
• Monitoring of all the Shelter Object state parameters and technological processes management.
• Preventing unauthorized access to the radioactive materials and ensuring IAEA safeguards system functioning.

Then, after full removal (retrieval) of Fuel Containing Materials and Long-Lived RAW, decommissioning of Shelter Object will be carried out as the final phase of transformation into ecologically safe system.

Figure 2 – Information about the New Safe Confinement constructed from 2010 over the old Shelter Object. Credit: Berria CC BY-SA 4.0

Figure 3 - The Chernobyl Shelter Object in 2010, IAEA Imagebank, Photo Credit: Dana Sacchetti/IAEA - CC BY-SA 2.0

The decommissioning strategy
The first decommissioning concept was developed in 1992 by the Kiev Research and Design Institute. This concept was based on the decommissioning option which includes the safe storage of the unit’s equipment within the existing building constructions for 20–30 years with the minimum of dismantling works.
The “Chernobyl Decommissioning Plan” (‘CDP’) was developed in 1995. This strategy has been continuously improved over the years, taking into account the results of numerous R&D works, international experience in decommissioning, IAEA recommendations, comments and suggestions from the governmental and regulatory bodies in the fields of Nuclear Energy use and Radioactive Waste Management.
The final decommissioning strategy option for Chernobyl NPP, approved in 2008, was the deferred gradual dismantling – SAFSTOR (from SAFe STORage).
In accordance with this strategy, decommissioning will be carried out in four stages:

1. Shutdown (preparatory stage for decommissioning, from 2000 till 2015). During this stage, the nuclear fuel was removed and transported to the Spent Fuel Storage Facility designed for long-term storage.
2. Final shutdown and preservation of reactor installations (from 2015 approximately till 2028). The tasks of this stage are as follows:
   • Bringing the Chernobyl NPP Units to the state that excludes the possibility of using them for power generation, dismantling of the auxiliary components of the plant.
   • Removal of ionizing radiation sources.
   • Preservation of reactors, the primary circuit and the reactor compartment equipment (the most radioactively contaminated).
3. Safe enclosure (under supervision) of reactor installations for the period during which radioactive contamination should naturally decrease to the acceptable level (roughly till 2045).

4. Gradual dismantling of the primary circuit and reactor core. During this stage the equipment will be dismantled, and the site will be cleaned up to the established level, to ensure maximum release from restrictions and regulatory control (approximately till 2065).

At the moment Chernobyl NPP is at the Final Shutdown and Preservation stage. The permission for the implementation of this stage was obtained in 2015, after the removal of the Spent Nuclear Fuel from the Units was completed. The main task of this stage is reactors’ preparation to the long-term safe enclosure, under supervision.
The Decommissioning Strategy determines the final state of the Chernobyl NPP industrial site as "industrially developed site". The abandoned land will be restored for new economic activities. From radiological point of view, taking into account Exclusion zone specificity, the final state was established as "brown spot". This means that the level of contamination of the site and building will be made equal to that of the Exclusion Zone territory around the ChNPP.