Ok, so #ChernobylHBO is over. Let's talk reactor kinematics! This will be a summary of the INSAG report, the most authoritative work on the matter.

So, let us start with the beginning. #Chernobyl is unlike many reactors. It uses both graphite AND water.
In a nuclear reactor, we need neutrons to cause a chain reaction. When born from fission, neutrons are going way to fast for low enriched uranium to handle. We have to slow them down. This is done with water for most modern reactors, but sometimes also graphite.
The first reactors ever made used graphite because it is easy to pile up. It isn't easy, however, to extract heat from this system. This is where the water comes in. Water in most reactors is used both to slow down neutrons AND remove heat from the system.
#Chernobyl was different. It used graphite to slow down neutrons primarily, and used water to remove the heat.This allowed them to use lower levels of enrichment in their reactor systems because water is a neutron thief! When water is primarily used,you have to enrich the uranium
This ended up being a bad design decision for this specific implementation. The #Chernobyl reactor has an instability. If the water started to form voids (bubbles), it would cause fewer neutrons to be absorbed by the water and they would be moderated by the graphite instead.
This causes the water to heat up more because more neutrons are being absorbed by the fuel, which causes more water to boil and more voids to form, repeat until the operator intervenes or there is an accident.
A second problem was the design of the control and safety rod system. The problem of inserting a mass into water causes displacement of the water. As such, there were certain graphite displacers inserted in the core to manage reactivity issues as the control rods were inserted.
However, this resulted in excess reactivity being injected into the bottom of the core. It was believed this would never be a problem. But if you have a power excursion in that domain of the reactor already, you can have a serious runaway effect.
(that visualization is tricky, so here is the picture of the displacer)
The last compounding factor was the fact that because they were attempting to use natural or extremely lowly enriched uranium along with graphite, the core itself was HUGE. Parts of the criticality chain in the reactor could be completely divorced from the other parts.
This problem was compounded by the poor ability for operators to know the entire power domain of the core. You wouldn't know if your top was subcritical and bottom supercritical in certain situations. If you got outside the operation envelope, you were playing guesswork.
The most damning shortcoming of it all, though, was the need for the reactor to be regularly accessible because of its dual role as a potential weapons fabrication facility. To make weapons-grade Pu, you need to regularly access your core. This was made easier in the RBMK
But that ease of access came at the cost of primary containment systems. If the reactor ever had a problem, the only thing standing in its the building and the "plug" for the pressure vessel. When combined with the thermal runaway properties, the stage was set for disaster.
In the final moments, before the reactor exploded, it is believed the power spiked to something like 30GW in a few seconds. This is over 10x the rated capacity. This was due to the bottom of the core being pushed to prompt supercritical due to the insertion of the control rods
Due to the flaw mentioned earlier with the control rods, it was possible to inject excess reactivity to the bottom of the core. The bottom of the core was likely already unstable. This was the final insult to the system which it could not withstand.
This is what is known as the positive scram effect. It is only possible in certain core configurations. But once the core is in that configuration, operators are doomed. Increasing the flow rate was not possible....the reactor is now certain to explode.
At this power level, the water in the reactor flashes to steam. Probably most of it. This causes the entire reactor to rupture. The first explosion of the day happens. Now the lid to the reactor container comes smashing down on the core, exposing anything that wasn't already.
Now chemistry takes hold. The high-temperature steam reactors with the zirconium cladding producing hydrogen gas. The second larger explosion of the day now happens, ripping the rest of the top of the building away and starting a graphite fire.
This second explosion and subsequent fire now take this from a local disaster to a regional one. Plumes of fresh fission products now leave the reactor system. If it wasn't for the heroic work of the liquidators, the contamination could have been far greater.
This concludes the cliff notes version of Chernobyl from a reactor point of view. The far greater problem was, of course, safety culture. Accidents like this are just a series of events from the choices of specific people.
It is possible to operate a reactor like this and not have problems.If crews were better trained, if monitoring systems were more robust,if safety drills prepared crews for all possible emergency scenarios etc.While I have focused on the technical this accident was entirely human
People are afraid of nuclear because of the choices of a decided few number of people. That is a lot of trust to have for something you have little say in. If you wonder why people have a fear of nuclear, this lack of control plays into it.
I think that is what this show attempted to show more than anything. More than Chernobyl being a problem reactor (it was) it was also a problem of people making horrible assumptions, communication poorly and lying to survive in communist Russia.
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