Lets borrow some ideas from Robert L. Forward's Camelot 30K. Now, as with most of his aliens I see no way they could evolve, but lets go with something much simpler:
We have a spacegoing plant (It originally evolved in atmosphere and managed to make the transition as it's planet of evolution slowly lost it's atmosphere and then was destroyed by passing within the Roche limit of a gas giant.) It's seeds are incredibly tough, long-lived and radiation resistant. When one comes to rest in a sufficient (which would be quite low) gravity field the plant sprouts. There are plants that throw seeds but this plant took the idea much farther. It sent down a big taproot that is made out of an explosive. At the end of it's life cycle the plant dies, the root dries out and eventually detonates. The seeds are cast a considerable distance this way, sometimes even at escape velocity (the plant normally grows on carbonaceous asteroids.)
At this point a variation arose that used a block of metal in the root to direct the blast better. Denser metals work better, a version evolved that used uranium. At this point it encountered a very young asteroid belt containing uranium that has a far higher percent of U-235 than we have.
It still built the uranium blocks in it's taproot, but now it's actually possible for them to go critical. A critical mass does us no good but as it approaches criticality it gets warm, the root dries and detonates. If the blocks are arranged properly they are blown together and you get a nuclear yield. At first it will be small but it throws the seeds much farther than before (and dispersing it's seed is the big issue for this plant), it's a big reproductive help. Better bombs evolve as well as tougher seeds to survive the launch. Interstellar travel becomes likely rather than requiring quite a fluke.
Now, one of these seeds has fallen on the planet your heroes are exploring. You need fresh uranium and a decent amount of it but you don't need the sort of concentrations that others are talking about which would almost certainly cook your planet.
Now, trapped under an atmosphere the plant can't throw it's seeds offworld but they get dispersed.
Now, this plant was adapted for life in space and conservation is it's driving principle, it's life cycle is going to be slow and it's not going to look like a traditional plant. Your heroes will probably mistake it for a mineral formation until the biologists figure it out--and note that messing with a mature one is asking to get yourself nuked.