Second Nuclear Apocalypse

The Second Nuclear Apocalypse, also known as the Death Wave, Second Praimfaya, or simply Praimfaya, was a cataclysmic event that occurred in 2150. It was caused by failing nuclear power plants around the world which resulted in a wave of radiation spreading across the world. It also caused a significant rise in atmospheric radiation, making the ground uninhabitable for everyone except Nightbloods for five years.

History
"After the second Fukushima disaster, Panacea Power patented a fail-safe system with multiple redundancies. By 2048, two dozen mega reactors were built to withstand any disaster, even a nuclear attack." However, "the plants were built to be self-sustaining for [only] a hundred years." After the first nuclear apocalypse, which occurred in 2052, no one maintained the power plants, so by 2150, the plants started to melt down. By the late February 2150, "there [were] more than a dozen at-risk plants around the world [with] 7 currently burning." "Global radiation levels [were also] rising." According to A.L.I.E.'s calculations, "96% of the Earth's surface will be uninhabitable [in less than six months] even for those born in space."

At the end of season 3, A.L.I.E. reveals to Clarke Griffin that a second nuclear apocalypse would be coming. A.L.I.E. had first detected the issue four months before using drones. The AI tries to convince Clarke that the best way to save the human race was to digitize them into the City of Light, but Clarke remains unconvinced and terminates A.L.I.E.

At the start of season 4, Raven Reyes confirms the rising radiation levels and that they have no more than six months before they would be dead. At first, black rain would start and there would be no drinkable water. Then people will start developing pre-cancerous lesions and the air would become toxic. And finally, "a wave of radiation [would] kill everything in its path." Afterwards, the atmospheric radiation would still be too high for five years for anyone except Nightbloods to live on the ground.

Throughout the fourth season, Sky People and Grounders work on several solutions on how to survive Praimfaya. They try to radiation-proof Arkadia, but Ilian blows it up. They search for a Second Dawn bunker and find one but it turns out to be a decoy.

When Luna and several of her clan come to Arkadia, suffering from acute radiation syndrome, only Luna survives. Abby Griffin realizes that her Nightblood helped her metabolize radiation and filter it out of her system. A group travels to Becca's lab to research and try to recreate Nightblood. They develop synthetic Nightblood from Luna's bone marrow and inject it into a Grounder scavenger. While at first it seems successful, it turns out to be insufficient and the scavenger dies during a test within the radiation chamber. Eric Jackson realizes the issue and fixes it, but they have to test the new serum. Clarke injects herself with it, but Abby refuses to run the tests on her own daughter and breaks the radiation chamber, leaving it unclear if the serum worked.

Meanwhile, Thelonious Jaha realizes where the real Second Dawn Bunker might be. With few others, he travels to Polis and finds the bunker under the temple. With all the clans converging to Polis and the bunker only being able to sustain 1,200 people, the clans agree to hold the Final Conclave. Octavia Blake wins the conclave and decides that each of the twelve surviving clans (including Sky People) would get 100 spots within the bunker and together they would form Wonkru. Everyone else is locked out of the bunker and left to their deaths.

A small group, consisting of Clarke, Raven, Bellamy Blake, Monty Green, Harper McIntyre, John Murphy, Emori, and Echo, decide to try a risky plan to try to survive Praimfaya in space. In Becca's lab, the group watches drone footage of the death wave moving through Polis, destroying everything in its path. They have ninety minutes to liftoff or the death wave will be too close. They successfully retrieve the oxygen generator and prepare the rocket, but Clarke gets delayed at the satellite tower. The other seven are forced to launch without her and hope that Clarke can successfully send the satellite signal to open the Ark's hangar door. Clarke manages to manual override the satellite dish to send the signal and runs back to Becca's lab with the death wave raging towards her.

Aftermath
The death wave ravaged the majority of the planet, leaving only one spot of green, Shallow Valley. However, even there, the atmospheric radiation is too high for anyone except Nightbloods to survive. The only people to have escaped the death wave were the 1,200 within the Second Dawn Bunker, the seven on the Go-Sci Ring, and two Nightbloods (one of whom is Clarke Griffin, proving the synthetic Nightblood serum a success).

46 days after Praimfaya, at the Second Dawn Bunker, Kara Cooper stages an unsuccessful coup with some of her fellow Skaikru members to take control of the hydrofarm, but were thwarted, and their punishment, issued by their new leader, Octavia Blake, was a gladiatorial fight for freedom, where only one emerged as the ultimate winner by being the last person standing. After the rebellion, Octavia took solo control of Wonkru and became Blodreina ("Red Queen" in Trigedasleng). Sometime during the six years in the bunker, Wonkru went through the "Dark Year", a period of time and troubles, in the Second Dawn Bunker. After those six years, there is only 814 people alive in the bunker, in which 386 people died under Octavia's rule.

Over the next five years, the radiation levels decrease to a survivable level. During that time, the two Nightbloods, Clarke and Madi, live in Shallow Valley, regularly trying but failing to make contact with the others. Even after five years, no one returns to the ground. Those within the bunker are trapped under Polis' debris and are unable to get out. The seven in space, lack the fuel to safely return to the ground.

Six years post Praimfaya, in 2156, Clarke notices a ship descending to Earth. At first, she thinks it is her friends, but soon realizes the ship is too big. The Gagarin, which was a prisoner transport vessel owned by the Eligius Corporation, lands in Shallow Valley. Later, the seven in space use fuel from Eligius IV to safely return to Earth. Bellamy Blake manages to strike a deal with the Eligius prisoners for them to open the bunker. However, now there are multiple factions wanting the only habitable land, Shallow Valley, all for themselves.