Second Nuclear Apocalypse

The Second Nuclear Apocalypse (also known as the Second Praimfaya or Praimfaya) was a catastrophic event that was caused by failing nuclear power plants around the world. In 2156, a year after Praimfaya ended in the year 2155, many people on Earth survived by going into the Second Dawn Bunker and others went to space in a rocket to the Go-Sci Ring, while Clarke and Madi survived due to their Nightblood.

History
A.L.I.E. told Clarke Griffin that a second nuclear apocalypse would be coming, caused by melting nuclear power plants around the world, the radiation spread across the planet in the form of a giant wall of fire and pyroclasticks wind call "death wave" who burn everything in his pass. In Season Four, many people worked on solutions on how to survive Praimfaya. When the Sky People discovered Luna's Nightblood allowed her to metabolize radiation and filter it out of her system, they decided to use Nightblood to produce a serum for everyone. However, their attempts failed when Abigail Griffin destroys the radiation test chamber. After this failure, Thelonious Jaha and a few others found the Second Dawn Bunker that could hold 1,200 people. 364 Sky People were culled to provide food and oxygen as to make room for all the grounders and the remaining 100 Sky People. When Praimfaya arrived, Polis was destroyed, with the 1,200 people inside the bunker surviving. As Praimfaya was about to hit Becca's Island, Bellamy Blake, Raven Reyes, Monty Green, Harper McIntyre, John Murphy, Emori and Echo flew a rocket to the Go-Sci Ring. Clarke Griffin sacrificed herself to move the satellite dish so a signal could be sent to open the hangar door, but survived due to becoming a Natblida.

Aftermath
In 2156, six years after Praimfaya, Clarke and Madi are the only survivors above the ground as Earth began to be habitable again for non-nightbloods. During this time, there were 7 survivors in space, and 1,200 people still in the bunker. Meanwhile, the Gagarin, commissioned by the Eligius Corporation, descended towards the Green Valley.