> or did they only form after ark fall?

**Yes**. **The power plants  started to melt recently, simultaneously and after the "Ark fall"**

To be exact, 4 months before the events in the 16th episode of the third season. As Alie says

> The nuclear power plants that were destroyed by the bombshave begun to melt down.

> My drones detected **the first of them 4 months ago**.

> There are more than a dozen at-risk plants around the world, 7 currently burning.

> Global radiation levels are already rising.

The Ark landed in the end of the first season. According to [this timeline](http://the100.wikia.com/wiki/Timeline), that will be, Oct 11, 2049

 Season 3 starts Jan 27, 2150, ~ three and half months later. And estimated to end by the end of February.

 So **season 3 ended after ~ four and half months from the "Ark fall"**. Means **the power plants started to melt ~ half a month after the Arc fall**.


And to explain **why the power plants started to melt at the same time**

When Clarke Asked Raven to check if Alie is telling the truth and here's what she found

> Raven: 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.


 > Harper : But that's good, right?

 > Jasper : There's always a catch.

> Raven : **The plants were built to be self-sustaining for a hundred years**.

> Monty : So the warrantee just ran out.