Trying to think of where I read this - there was a society that put actual plutonium in the coinage, as a means to encourage people not to be too greedy; if one hoarded too many coins, they'd go critical and generally ensure the distribution of wealth.
It's not a story, but a short essay by Larry Niven, Yet Another Modest Proposal: The Roentgen Standard (the link is to Niven's own web site).
"Self-Limiting" by Robert L. Forward is a short short story which was first published as a "Probability Zero" story in Analog Science Fiction and Fact, May 1992 and reprinted in Forward's collection Indistinguishable from Magic. It tells the fate of Queed, a greedy xanit (inhabitant of the planet Xanax):
He entered his dwelling, and leaning over the money pit, he gloated as he dropped the bag of coins into the hole that was waiting for it. Queed could hardly wait until it was night-time and he could take out all the coins and count them. He was now certainly a millionaire!
As the heavy bag of silvery-grey coins made of pure plutonium 239 dropped into the waiting hole, the money pit went critical and exploded . . .
There are no millionaires on Xanax.
Forward's 1992 story could possibly have been inspired by Larry Niven's 1984 essay "Yet Another Modest Proposal: The Roentgen Standard" (see Keith Thompson's answer).
The King was pleased to see so many gleaming ducats come into his treasury, for he did not know that they were of uranium and not lead. That same night the cosmogonist melted the bars of the prison and freed Pyron, and as they walked through the valley in silence, in the light of the radioactive mountains, which was as though a ring of moons had fallen and encircled the horizon on every side, suddenly a terrible radiance poured forth, for the pile of uranium ducats in the royal treasury had grown too great, setting off a chain reaction. The detonation blew the palace and the metal hulk of Archithorius sky-high; its force was such, that the tyrant's six hundred dismembered hands went flying into interstellar space.
I had an old Amiga game called Weird Dreams that came with a Novella. They had a similar element.