#rbmk

Leonid Toptunov. - "The man who blew up Chornobyl." - "The man who pressed the AZ-5 button."

Leonid Toptunov was 25 years old, and pressed a button meant to fix things, but because of many outside factors, this button ended up doing more damage.

He, along with everyone else in the control room, were evacuated to safety. Leonid, however, along with some other men such as Akimov, returned to the site in order to try and minimize damage.

The RBMK-1000 Boiling Water Reactor is a Soviet-designed nuclear reactor capable of generating 1,000 megawatts of electricity. The core of the reactor is a short, wide cylinder. The active zone is contained inside a large metal drum, known as the core shroud. The reactor assembly is supported by a large metal disk known as the Lower Biological Shield. This sits on top of a larger metal cross labeled “Structure S”. On top of all this rests the 2,000 ton Upper Biological Shield of the reactor, known as "Structure E". The reactor sits in a large reinforced concrete shell which provides structural support and shields plant personnel from radiation.

The core region of the reactor is a large pile of graphite 14.52m × 9.7m. This pile is composed of graphite blocks 25cm by 25cm, with a height of between 20 and 60cm depending on its location in the reactor. Drilled through these blocks is a 11.4cm diameter hole, through which a zirconium alloy tube (known as a ‘technological channel’) is inserted. These contain either a fuel assembly, a control rod, or reactor monitoring equipment. These channels can be opened in situ or removed completely to replace any fuel or equipment inside them. Zirconium is used due to its high melting point and because it allows the neurons that produce the fission reaction in the core to pass through it far easier than other alloys such as stainless steel.

These metal technological channels have water pumped into them from the bottom by the Main Circulation Pumps. The entire reactor vessel is pressurized with a helium-nitrogen mixture, to prevent the oxidization of the graphite. Graphite is flammable in oxygen, but removed from it it can become quite an efficient thermodynamic conductor.

Below: A photo of RBMK technological channels at Chernobyl Unit 2. The length of these gives a good idea as to how massive the core of the RBMK is.

This picture is a screencap from this video.

The fuel of an RBMK is small uranium oxide pellets, stacked into small metal pipes and bundled together into fuel assemblies. Uranium oxide is a ceramic material composed of Uranium 235. This element, under special conditions, can create a nuclear chain reaction which generates heat. The RBMK has three primary components that help create these special conditions to create the controlled fission reactions in the core. These are graphite, water, and boron.

Graphite is used in the core of an RBMK as a moderator. Basically, it slows down the neutrons discarded by U-236 atoms (a U 235 atom which a neutron has collided with) when they split apart. When they are released they are travelling at a tremendous speed, and have very little chance of coming into contact with another atom of uranium. Slowing them down, however, creates a higher chance of the neutrons coming into contact with an atom of U-235, creating the unstable U-236 and then pulling itself apart, thereby creating more neutrons (as well as several other elements) and sustaining a nuclear chain reaction. This sustained reaction is what creates the heat in the core of a nuclear reactor. The more neutrons there are in the core, the more reactivity (and therefore heat) is created. It should be noted that graphite is combustible at high temperatures. The core contained 1,700 tons of graphite.

Water in the core of an RBMK serves as a coolant. Because the core of a nuclear reactor gets extremely hot, it becomes necessary to cool its components if you wish to avoid destructive melting within the core region. Water is the most common coolant in nuclear reactors, as it is cheap and abundant. The water is pumped in under high pressure at about 265 C by the Main Circulation Pumps from the bottom of the reactor up into the technological channels containing the fuel and other components of the reactor. After passing through the channels and heating up to about 284 C, the water is piped out of the top of the reactor. Some of the coolant water heats up so much that it forms into steam bubbles inside the reactor. When the water is pumped out of the core it is then sent into four steam separator drums, where the steam is separated from the water. The water is then pumped back into the reactor, while the steam is sent to the turbine generators of the plant to create electricity. After this, the steam is condensed back into water using cool water from the plant cooling pond and recirculated into the cooling system.

Below: A model showing the circulation system of an RBMK-1000 reactor. Coolant water is in blue and hot water/steam is in red. The yellow structures are the main cooling pumps, and the green structures are steam turbines. This model is spatially to scale, essentially what you would see if you removed every part of the reactor except for the coolant circuit.

Some of the channels in the RBMK contain control rods (large boron carbide rods) that move up and down in the channel as necessary to keep the reactor within operational limits. Boron is a neutron sponge. It absorbs neutrons and can effectively eliminate a chain reaction. It functions as the brakes on a human made nuclear reaction, useful both in making sure a chain reaction does not become a runaway criticality and also in being the off switch on a nuclear reactor. The RBMK has 211 of these control rods, some of which are under operator control and some of which are under the control of a computer. A design quirk of the RBMK is that at the end of each standard control rod was a 14ft 9in graphite displacer. When a control rod was withdrawn out of the core it left behind a space that would be filled with water, a neutron absorber. Since more water in the core would kill reactivity, the designers of the reactor hung this displacer from the control rods to replace the space left by the control rod with something that would increase reactivity rather than kill it. This was a sound design choice, but it was a major factor in the events of the accident at Chernobyl.

Below: An illustration of the control rod displacers in an RBMK.

Below: A top down view of the channels of an RBMK. You can see the layout of the control rods (green), neutron detectors (blue), shortened control rods inserted from below the reactor (yellow), automatic control rods (red), and the fuel channels (grey). The number on the green, yellow, and red squares are the last recorded insertion depths of control rods in Chernobyl Unit 4 1m 30s before the explosion. Only one is fully inserted.

Below: A cutaway of the RBMK system layout.

Two additional factors also come into play regarding the water. Water is naturally a neutron absorber, albeit a far less effective one than boron. The more water in the core, the less neutrons are present and therefore the lower the reactivity. However, when transformed into steam, water loses nearly all of its neutron absorbing properties. The more steam in the core, the higher reactivity is. This is called a ‘positive void coefficient’, and it was a known quirk of the RBMK and indeed several other reactor designs. However, the RBMK had a much higher level of this effect in its core due to its design. This is important to the accident sequence.

It is also important to note that the RBMK is an enormous construction. It is temperamental, unstable unless operating at full power, and requires constant monitoring and guidance from its operators. It requires three operators just to run it normally, and it was notoriously difficult to operate. The core region is so large that the equipment used to monitor it could not accurately read a large portion of it, and hotspots of reactivity would often form resulting in alarming and unexplained jumps in power output and temperature. While in theory not a bad design, the RBMK was a deeply flawed machine.

An enormous thank you is owed to @nicotinebeige , who was extremely helpful in the creation of this post. If you like film photography, you should check out their blog!

After months of construction, unit 2 is nearly online! Expect it to be operational before December of 2024

Enjoy! ♥️

(don’t forget to find Рutin 😉)

https://www.rosenergoatom.ru/stations_projects/sayt-leningradskoy-aes/3d-excursion/rbmk_new/