Partial meltdowns at Leningrad and Chernobyl

On 28 November 1975, the Unit 1 of the Leningrad nuclear power plant suffered a fuel melting event. A channel of the RBMK reactor of the first power unit of the Leningrad nuclear power plant starved off coolant, overheated, ruptured and partially melted away, degrading the graphite core and releasing radiation into the atmosphere.

A similar incident occurred at Unit 1 of the Chernobyl nuclear power plant on the 9th September 1982 but was more severe than the 1975 Leningrad incident.

Soviet policy The exclusively autonomous Ministry of the USSR, the Ministry of Medium Machine Building, along with the KGB, covered up these incidents very well, and they were kept secret from the public as well as other power plants. Practically the same accident occurred in Unit 1 of the Chernobyl Power Station in 1982. After the disaster, design changes which were being recommended by scientists from all over the Soviet union and abroad were finally implemented to improve the safety of RBMK reactors and the small EGP-6 reactors at Bilibino Nuclear Power Plant which was shutdown in December 2025.

Leningrad incident

On the 30th November 1975, a year after the reactor 1 reached its full power, it was being brought back online following scheduled maintenance. The AZ-5 protection key was tripped, following which a partial meltdown occurred, damaging 32 fuel assemblies and releasing radiation over the Gulf of Finland. It was the first major accident involving an RBMK reactor. The Ministry of Medium Machine Building set up a commission to investigate what had gone wrong. Later on, the official conclusion was that a manufacturing defect had led to the destruction of a single fuel channel. But the commission was aware that the accident was the result of the design faults inherent in the reactor and caused by an uncontrollable increase in the steam void coefficient. The commission made several important recommendations, to be applied to all RBMK-1000 reactors: "develop new safety regulations to protect them in the event of coolant loss; analyze what would happen in the event of a sharp rise in steam in the core; and devise a faster-acting emergency protection system." On the next day of this accident, approval was given to construct 2 more reactors at Chernobyl.

Chernobyl incident

On 9 September 1982, a partial core meltdown occurred in the reactor of the first power unit of the ChNPP due to a faulty cooling valve remaining closed following maintenance. Chernobyl unit 1 was already coming out of a very difficult and complicated maintenance period. Once the reactor came online, the fuel channel overheated and ruptured and the fuel in it partially melted down. This occurred due to a so-called valving error by the operators, while some reports claimed that it occurred due to the plant manufacturing its own channels on site, which were not competent with the design requirements laid down by the designers of the RBMK at Kurchtov institute and NIKIET. The extent of the damage was comparatively minor as compared to the 1986 disaster at Unit 4, and no one was killed during the accident. However, due to the negligence of the operators and faulty equipment of the control system, the accident was not noticed until several hours later, resulting in significant release of radiation in the form of fragments of uranium oxide and several other radioactive isotopes escaping with steam from the reactor via the ventilation stack while others accumulating in the steam separator drums. This accident was somewhat similar to the 1975 Leningrad unit 1 accident. The accident was not made public until several years later due to the policies of the Soviet government and the Central Committee of the Communist Party of the Soviet Union, despite cleanups taking place in and around the power station and Pripyat. The roads of Pripyat had to be resurfaced with tar. This incident was more severe than the 1975 Leningrad incident and caused permanent damage to the reactor core. The reactor was repaired and put back into operation after eight months with its capacity reduced by 20% to 800MWe as the damaged part of the core and graphite could never be used again as fuel channels and only after spending a large volume of money.

Other known incidents The RBMK power plants were one of the most dangerous due to design flaws, safety culture issues, political issues, and due to faulty equipment and this was known to the Soviet Government. From the startup of the first power unit of the Leningrad nuclear power plant in 1973, and many other units including those at other sites being commissioned later, various incidents and accidents occurred from time to time, which were ignored and operations continued In the same way. Many scientists from the Kurchtov institute, and NIKIET had already warned about the underlying threats and had suggested design changes, but they were ignored. Instead of dealing with the design flaws, manuals were revised. These are some of the known incidents are listed below :

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