Draft:Partial meltdown of Leningrad unit 1 and Chernobyl unit 1

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

RBMK reactor with fuel channel covers

Reactor hall of the RBMK-1500 at Ignalina Nuclear Power Plant, Lithuania—the upper biological shield (UBS) lies several meters below the floor of the reactor hall. There are no channel covers on the fuel channels of the reactor; the control rod drives are below the colored covers.

Schematic plan view of core layout, Chernobyl RBMK reactor No. 4. (Quantity of each rod type in parentheses):
  neutron detector (12)
  control rods (167)
  short control rods from below reactor (32)
  automatic control rods (12)
  pressure tubes with fuel rods (1661-1691)(1-2-nd generation cores(RBMK)
The numbers in the image indicate the position of the respective control rods (insertion depth in centimetres) at 01:22:30, 78 seconds before the reactor exploded.

A similar incident occurred at the Unit 1 of the Chernobyl nuclear power plant on the 9th September 1982 but was more severe than the 1975 Leningrad incident. These events were neglected by the nuclear industry in the Soviet union and were not conveyed to other RBMK power plants. Instead of improving the design of the reactors, manuals were revised and operating procedures were changed and made more strict. The Ministry of Medium Machine Building, along with the KGB, covered up these incidents, and were kept secret from the public. Instead of taking measures to avoid such incidents, the then Soviet minister of energy, Anatoli Mayorets's New energy policy reduced the time period of maintenance shutdowns and increased the gap between them at all RBMK power plants to maximize energy production. Hence, such incidents continued to occur, and culminated in the 1986 Chernobyl disaster, after which the design changes were implemented to improve the safety of RBMK reactors and they continue to operate safely even today.

Leningrad incident

Leningrad I nuclear power plant units 1 and 2

On the 28th November, during a reactor startup of Leningrad unit 1 after scheduled maintenance, the reactor was roughly at 2600MWth; turbine number 1, connected to the reactor , came across an issue while turbine number 2 was running. The operators decided to reduce to reactor power to 50% i.e. to 1600MWth and disconnect the turbine, repair it; increase the reactor power back to 3200MWth, and reconnect the turbine to the reactor, and synchronise the generator with the grid. As planned, the power was reduced to 1600MWth, but due to operator error, the Senior turbine control engineer(STCE), shut the still running generator, which resulted in shutdown of both turbines, leading to a reactor scram taking away 500MWe from the electrical grid. The deputy chief engineer of Unit 1, ordered to restart the reactor, and connect turbine number 2 to the reactor and synchronize the generator with the electrical grid. But as the Xenon-135 built up in the core, it made the reactor harder to control, leading to removal of almost all the control rods i.e. a violation of the Operating reactivity margin(ORM) i.e. the number of control rods to drop to less than 8. The reactor shutdown a second time. The operators began again and this time were able to raise the reactor power to 800 MWth. But they had made a too rapid power increase of almost 600MWth in just 20 minutes. This unstabilized the neutron field in the core, and alarms began to sound in the control room regarding no coolant water entering certain channels in the reactor, it was all steam. Hence the SRCE disconnected the automatic power regulators, and inserted all the automatic control rods back in. This helped stop those alarms. But then a new alarm sounds. There was moisture in the graphite. This indicates a fuel channel rupture and a meltdown of the fuel in that channel. The channel 13-33 had ruptured. The radiation in the surrounding area of the reactor and plant increased. Fragments of fuel, fission products started accumulating in the steam separator drums. The Senior reactor control engineer (SRCE) pressed the AZ-5 key, and shut the reactor down again. It took almost two months for the reactor to be repaired and brought back online. The graphite core stack, and filters of the gas circuit were cleaned. The radiation in the surrounding and Leningrad city lasted for about a month. Clean-up of the plants surrounding and Leningrad city was done to remove any contamination, but the accident was not reported in the media.

Chernobyl incident

Computer-generated synthesis drawing of all 4 units prior to the accident viewed from NW

On 9 September 1982, a partial core meltdown occurred in reactor No. 1 due to a faulty cooling valve remaining closed following maintenance. Once the reactor came online, the uranium in the channel 13–44 overheated and ruptured. 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, and no one was killed during the accident. However, due to the negligence of the operators, 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. This accident was somewhat similar to the 1975 Leningrad unit 1 accident. The accident was not made public until several years later, despite cleanups taking place in and around the power station and Pripyat. This incident was more severe than the 1975 Leningrad incident. 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 could never be used again.

Other known incidents

Many incidents occurred at various power plants operating the RBMK reactor. Most of them were covered up. Incidents such as thefts of materials, equipment malfunction, repeated shut down due to them, etc. occurred. The most serious incidents such as the Partial meltdown of Leningrad unit 1 and Chernobyl unit 1 were not taken seriously and the recommendations of the scientist and experts were not implemented, which paved the path to the 1986 disaster. These are some of the known incidents except the partial meltdown at Leningrad unit 1 and Chernobyl unit 1:

• Explosion of a tank holding radioactive gases at the Leningrad Nuclear Power Plant unit 1 in January 1975
• Power outage at the Kursk Nuclear Power Plant in 1980
• Discovery of the positive scram effect at Ignalina Nuclear Power Plant unit 1 in 1983 and at unit 4 of the Chernobyl Nuclear Power Plant
• Shifting of the concrete cross bars at Chernobyl Nuclear Power Plant units 3 and 4 in 1984
• Chernobyl disaster in 1986
• Turbine fire at Chernobyl unit 2 in 1991 resulting in its permanent shutdown
• On 28 December 1990, during refurbishment of Leningrad unit 1, it was noticed that the space between the fuel channels and the graphite stack (contaminated during the 1975 accident) had widened. The contaminated graphite was spilled, and the radiation levels in the space under the reactor increased. Radiation was detected 6 km away from the unit, but this was not reported in the media.

• On 3 December 1991, at Leningrad Nuclear Power plant, due to faulty equipment and a lack of safety rule compliance, 10 new fuel rods were dropped and damaged. The staff tried to conceal the accident from the plant's management.
• Melting of cables at Chernobyl unit 1 in 1991 while testing of the ion chambers during a maintenance shutdown and the automatic control rods didn't respond during the AZ-MM signal, the low power protection system fails
• Radioactive water was released when a seal plug of one of the Main Circulation Pump of Chernobyl unit 1 failed in 1992
• Chernobyl unit 3 scammed following high level of water in the steam separator drums in March 1993
• Chernobyl unit 3 scammed in 1994 following a short circuit resulting in pumping of the Emergency core cooling system (ECCS) water into the steam separator drums
• Chernobyl unit 3 scammed following detection of a steam leak in one of the fuel channel due to defective welding during assembly in 1981
• Chernobyl unit 1 scammed after the refueling machine got stuck in one of the channels in 1995
• On 27 August 2009, the third unit of the Leningrad Nuclear Power plant was stopped when a hole was found in the discharge header of a pump. According to the automated radiation control system, the radiation situation at the plant and in its 30-kilometre (19 mi) monitoring zone was normal. The plant's management refuted rumors of an accident and stated that the third unit was stopped for a "short-term unscheduled maintenance", with a restart scheduled for 31 August 2009.

Reasons behind the incidents

Schematic diagram of an RBMK

Schematic side view of the layout of an RBMK reactor core

The reactor hall and piping systems of the RBMK reactor.

The reasons contributing to the incidents were many. A flawed design, with poor safety culture, and miscommunication between the Soviet nuclear industry, irresponsible policies, etc. contributed to the disaster. The RBMK reactor design had many short comings. As an early Generation II reactor based on 1950s Soviet technology, the RBMK design was optimized for speed of production but sacrificed redundancy. Several of its design characteristics would prove to be dangerously unstable when operated outside their design specifications. The decision to use a graphite core with natural uranium fuel allowed for massive power generation at only a quarter of the expense of heavy water reactors, which were more maintenance-intensive and required large volumes of expensive heavy water for startup. However, its unintended consequences would not reveal themselves fully until the Chernobyl disaster in 1986. While a part of the incidents occurred due to the design flaws in the reactor, a part of them was also caused due to unhealthy safety culture and undertrained and irresponsible staff and management.

High positive void coefficient

Light water (ordinary H₂O) is both a neutron moderator and a neutron absorber. This means that not only can it slow down neutrons to velocities in equilibrium with surrounding molecules ("thermalize" them and turn them into low-energy neutrons, known as thermal neutrons, that are far more likely to interact with the uranium-235 nuclei than the fast neutrons produced by fission initially), but it also absorbs some of them. In the RBMK reactors, light water functions as a coolant, while moderation is mainly carried out by graphite. As graphite already moderates neutrons, light water has a lesser effect in slowing them down, but could still absorb them. This means that the reactor's reactivity (adjustable by appropriate neutron-absorbing rods) must take into account the neutrons absorbed by light water.

In the case of vaporisation of water to steam, the place occupied by water would be occupied by water vapor, which has a density vastly lower than that of liquid water (the exact number depends on pressure and temperature; at standard conditions, steam is about 1⁄1350 as dense as liquid water). Because of this lower density (of mass, and consequently of atom nuclei able to absorb neutrons), light water's neutron-absorption capability practically disappears when it boils. This allows more neutrons to fission more U-235 nuclei and thereby increase the reactor power, which leads to higher temperatures that boil even more water, creating a thermal feedback loop. This partially led to the incidents at Leningrad unit 1 and Chernobyl unit 1. In RBMK reactors, generation of steam in the coolant water would then in practice create a void: a bubble that does not absorb neutrons. The reduction in moderation by light water is irrelevant, as graphite still moderates the neutrons. However, the loss of absorption dramatically alters the balance of neutron production, causing a runaway condition in which more and more neutrons are produced, and their density grows exponentially. Such a condition is called a "positive void coefficient", and the RBMK reactor has the highest positive void coefficient of any commercial reactor ever designed. A high void coefficient does not necessarily make a reactor inherently unsafe, as some of the fission neutrons are emitted with a delay of seconds or even minutes (post-fission neutron emission from daughter nuclei), and therefore steps can be taken to reduce the fission rate before it becomes too high. This situation, however, does make it considerably harder to control the reactor, especially at low power. Thus, control systems must be very reliable and control-room personnel must be rigorously trained in the peculiarities and limits of the system. Neither of these requirements were in place at most of the RBMK power plants: since the reactor's actual design bore the approval stamp of the Kurchatov Institute and was considered a state secret, discussion of the reactor's flaws was forbidden, even among the actual personnel operating the plant. Some later RBMK designs did include control rods on electromagnetic grapples, thus controlling the reaction speed and, if necessary, stopping the reaction completely. All RBMK reactors underwent significant changes following the Chernobyl disaster. The positive void coefficient was reduced from +4.5 β to +0.7 β, decreasing the likelihood of further reactivity accidents, at the cost of higher enrichment requirements of the uranium fuel. As of October 2025, 7 RBMK reactos continue to operate safely at 3 sites.