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
It was mentioned by A.L.I.E. that a second nuclear apocalypse would be coming, and it was caused by melting nuclear power plants around the world. 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 others found the Second Dawn Bunker that would hold 1,200 people. 364 Sky People were culled to provide food and oxygen and make room for all the grounders and remaining Sky People. When Praimfaya arrived, Polis was destroyed and the 1,200 people in the bunker are believed to have survived. 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 help move the satellite dish towards the Go-Sci Ring in order to allow her friends to turn on the space station and open the hangar door.

Aftermath
Five years after Praimfaya ended in 2155, and one year later, in 2156, Earth was already habitable for non-nightbloods. Clarke Griffin and Madi are the only known survivors outside on the ground, with 7 people in space, and 1,200 people still in the Second Dawn Bunker. Shortly thereafter, the Gagarin, commissioned by the Eligius Corporation, lands by the Green Valley.