[apache]

Radiation decreasing "naturally"? Do you know how long that would take? This is way beyond the time scope of the game. A Hiroshima-style bomb takes about 60 years for radiation to decline to non-hazardous levels. The latest Hydrogen bombs take on the order of 1000 years for the radiation to decrease to non-hazardous levels.
To impart a high radiation level to a planet would make it totally uninhabitable for the rest of the game if the only way to decrease this radiation was by natural means. And destroying a planet then rebuilding it would have no effect on the radiation levels.
Besides, a civilization this advanced surely has developed technology to live with deadly radiation by some means. Maybe all their walls are 5 feet of lead, maybe something else.
Either way, I don't think there should be a way to make a planet completely uninhabitable. With enough resources and raw power, any area can be terraformed to liveable conditions, especially if a dome can be constructed.