Just one problem: it is impossible to make a bomb with it. It would decay before you could make a bomb. Finally, it would not destroy the Earth. You would have to fabricate a lump of francium, build the bomb mechanism, and keep the francium from overheating, all within 20 minutes.
It still would be impossible to make a bomb, as that 20 minutes that I mentioned is the half-life of the longest lived isotope of francium, francium-223. By the time that 20 minutes has passed, you will have lost half of your francium, it having decayed into either astatine or radium. (It has a 99.99% chance of beta decaying into radium and a 0.01% chance of decaying into astatine.)
So you took the time to clarify that extra 2 minutes, but then follow it up with "it's just a game"? A little inconsistent there.
TPT just being a game is completely irrelevant. If it is not going to act like francium, why call it francium? You could call it literally something else, and satisfy both the people who want it in the game, and the people who want accurate names.
But that is beside the point anyways, the real point is that even if it is just a game, that does not mean you should be throwing ideas at it willy nilly. A game should still strive to be fun, and this doesn't seem to actually add anything to the game. Since it is not adding fun, why add it at all?