The last episode is going to go through the accident inquiry and show what happened in flashbacks as they look for the guilty parties.
The following is my breakdown of what happened... kind of long but maybe interesting to some, I try and explain what happened for someone with little or no nuclear physics experience.
The Chernobyl RBMK reactor was hiding two design flaws that made it dangerous, a positive void reactivity coefficient, and graphite tipped controll rods. When uranium fissions, it produces two fission fragments plus 2 to 3 free fast neutrons that go on to cause more fissions and thus a chain reaction. Reactivity- which is a measure of what happens to the neutron population from generation to generation of neutrons produced, measures the change in population of neutrons per life cycle. On average, each fission produces 2.42 neutrons. In the neutron life cycle, this 2.42 average must get past a bunch of obstacles in order to make another atom of uranium split and give up another 2 to 3 neutrons to keep the cycle going. When the number of fissions per unit time is constant, Reactivity is 0. That means that for every 2.42 neutrons produced, at least 1 fission is created by its neutron offspring, so 1.42 neautrons are lost (or made up for) due to a whole bunch of reasons.
The positive void reactivity coefficient dirrectly speaks to one of these losses of neutrons. When water is in the core, the water acts like an absorber, it slows neutrons down which is a good thing if you are the moderator- which is a material that slows fast neutrons down to thermal energy levels that let the neutrons be absorbed and prevents them escaping the core before being absorbed by an uranium atom. In this reactor design, they use graphite blocks as the moderator, so water in the core acts like a poison to the nuclear cycle, and steam (void space) is way less absorptive and causes reactivity to increase due to providing more fast neutron flux that increases reactor power. With almost all boron control rods removed from the core to maintain “measured” reactor power above 200 megawatts (for the safety test they were conducting), at low power levels and high rod heights, actual reactor power can be a lot higher at the core bottom where the nuclear activity is shielded from the detectors due to physical location. Hot spots in the core, where steam voids made reactor power swing wildly due to the positive void reactivity coefficient, caused power to suddenly start to spike and the operators, seeing the unchecked power increases SCRAMMED the reactor, which immediately starts inserting all the boron control rods back into the core.
This is when all hell broke loose, because the rods, which had been pulled completely out of the core, initially start displacing the water below the rod channels, replacing it with graphite, because the rod tips (bottoms) where made out of graphite because when fully inserted, they provided flux shaping that helped overall core flux densities at the bottom of the core. This water displacement adds positive reactivity due to the positive void coefficient, the graphite increases reactivity by being a more efficient moderator than water.
In a nuclear reactor, to make it stable and controllable, we do not rely soley on first generation neutrons to make up for all neutron population losses in the life cycle. Some of these neutrons come from the decay of radioisotopes called delayed neutron precursors. These are fission product particles that decay releasing neutrons when they do. If during the operation of a nuclear reactor, if you increase neutron population to the point that these delayed neutrons are no longer needed to sustain the chain reaction, the reactor can go on a runaway power excursion- basically a nuclear atom bomb.
If allowed to continue (due to design) a nuclear bomb goes instantly bang. In a reactor, the density of the fuel, the enrichment of the fission fuel, added poisons and control rods are all designed for safe power generation, not instantaneous power production, so the fuel rods start heating up really quickly, flashing the water in the core to steam (remember that positive void coefficient?) and now the reactor literally blows itself apart due to both power generation, but mainly due to steam pressure.
This is what happened. The added positive reactivity of the initial rod insertion due to scramming the reactor made up for the delayed neutrons from precursors and the reactor went super critical and on an uncontrolled power excursion until physics took over and blew it apart. The graphite caught fire, and 60 tons of radioactive waste was vaporized and spread all over Russia and the rest of the world.
Now you know why I am glad I got out of nuclear power.