The accident at Chornobyl NPP Unit 4 has occurred on April 26, 1986, during design testing of one of the safety systems.

This safety system stipulated the use of residual rotation energy from turbine generators being shut down to generate electric power under the conditions of two simultaneous emergency situations.

Situation 1 – complete loss of NPP power supply, particularly main circulation pumps and emergency core cooling system pumps.
Situation 2 – ultimate design-basis accident which is considered in the design as a break of the large-diameter pipeline of core circulation circuit.

The design anticipated that if external power supply is lost, the electric power being generated by the turbine generators due to residual rotation to be supplied to start up the pumps included into the emergency core cooling system. This would ensure dependable cooling of the core.

Except for Chornobyl NPP, such design testing has not been performed at any NPP with RBMK-1000 reactors since commissioning.

The testing at Unit 4 was scheduled for April 25, 1986, in the afternoon at reactor thermal output of 700 MW following which it was planned to shut down the reactor for scheduled maintenance. Therefore, the testing had to be carried out under reduced power mode characterized by increased, relating to nominal, loss of coolant through the reactor, minor subcooling of coolants to the boiling point at core inlet and minimum void fraction. These factors had the direct impact on the accident scale.

At 01:23:04, the testing has been started, and four main circulation pumps started operation from residual rotation of turbine generator by virtue whereof water consumption began decreasing and steam void fraction increasing.

At 01:23:43, a rapid increase of power began following which the explosions occurred. The reactor was destroyed, and huge volumes of radioactive substances came out into the atmosphere from its rupture. A fire has started at Unit 4.

Elimination of the accident has been initiated that night at once.

During several hours following Unit 4 destruction, the firemen and personnel of ChNPP succeeded to suppress numerous fires that prevented fire spread to other Power Units. Right away after the accident, firstly Unit 3 was stopped located in common building with Unit 4, and then Units 1 and 2 were shut down.

On April 27, backfilling with absorbers started from the helicopters to the Unit 4 rupture. The downthrown materials for almost 2 weeks covered the central hall with a layer ranging from 1 to 15 meters, hereby isolating the reactor from the environment.

Simultaneously, the actions were carried out relating to water removal from a pressure-suppression pool, cooling of the core, and liquid nitrogen supply to it. The water from the pressure-suppression pool was removed on May 6. Till that time, the experts concluded that there is no potential for self-sustained chain reaction within destroyed reactor, and radioactive release into the atmosphere decreased several thousand times.

On April 27, Prypiat town located 3 km from ChNPP was evacuated completely. On May 2, it was decided to evacuate habitants from 30-km area of ChNPP and other settlements exposed to radioactive contamination. Later, up to the end of 1986, about 116,000 people had been evacuated from 188 settlements (including Prypiat).

At once after the accident, an issue came up regarding long-term preservation of Unit 4 by means of construction of a structure limiting the radioactive release and ionizing radiation outside the destroyed Power Unit.

On May 29, 1986, it was decided to construct a facility called the Shelter of ChNPP Unit 4 for burial of ChNPP Unit 4 and related structures.

The building structures of the Shelter are an integration of old structures of the destroyed Unit 4 and new ones constructed after the accident. The basis of physical barriers is the external protective structures assembled after the accident, namely Cascade Wall, Buttress Walls, a cover over reactor unit, Deaerator Stack, and Turbine Hall. Relatively undamaged structures of Unit 4 constitute a support contour for bearing elements of a cover over Reactor Hall and Deaerator Stack.

Due to such combination, a unique structure was created, building structures of which perform critically important function of physical barrier on the way of radioactive release and ionizing radiation into the environment.

About 90,000 people were involved in the construction of the Shelter facility. It was constructed in record-breaking short time – 206 days.

Construction of the Shelter Object under extremely complicated radiation condition required the development and implementation of such organizational and technological decisions that would maximally ensure radiation protection of the builders.

The main actions for radiation protection of the personnel consisted in radiation investigation of work performance areas, in using different shielding tools and applying remotely controlled techniques of work performance under the most radioactively hazardous conditions.

An assembly technique using enlarged structures mounted within “clean” area and that can be assembled remotely was the most efficient one. The structures were designed having the supporting and connecting assemblies not requiring operations associated with people presence within mounting area.

During construction of the Shelter Object, about 345,000 cubic meters of concrete were placed, and 7,000 tons of steel structures were assembled. Except building and assembling operations, a significant scope of work were performed relating to creation of necessary systems for safe operation of the Shelter Object (ventilation, power supply, fire-extinguishing system, control systems, etc.).

On November 30, 1986, an Act of the State Commission on the Shelter Object Maintenance Acceptance was signed.

However, the Shelter Object facility was not created in accordance with rules and standards of designing, construction, commissioning and operation. Its building structures do not correspond to the requirements of safety-related regulatory-technical documents in terms of structural integrity and reliability and have an indefinite period of operation. All these require a continuous survey of safety-related structure condition of the Shelter Object and interference in case of a threat of unsafe deviation of their condition from the stable one. Therefore, right away after completion of the Shelter Object, the investigations of its building structure condition were initiated and are being continued and the urgent measures on its reinforcement are being implemented.

In course of time, in 1990’s, a strategy of Shelter transformation into environmentally safe system was developed and is under implementation.