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, that will be, Oct 11, 20492149
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 (The scientists, the stations and every one was already on the ground).
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, 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.