Perhaps if Uranium does come in then world length would be inversely proportional to the amount of uranium found? In real life uranium has a half life of about 4 billion years. That's too long for DF. Lets say Uranium has a half life of 100 years(while still having the same radioactivity). Then, every 100 years into worldgen the crust contains half as much uranium. The deposits would simply turn into the rock that's around them "laced with lead" and every 100 years there would be half the amount of uranium in the world.
Significantly less half-life while 'having the same radioactivity' is an oxymoron. Half-life is what denotes radioactivity, as Uranium decays it releases radiation, when it's not decaying it is not releasing radiation. Unless Toady wants to go and completely break physics there is no way to not have something with a half-life of 100 years be extremely deadly and likely cause melting flesh.
Physics is already broken. Besides, in DF you can make it so Half life does not denote radioactivity, by simply having 2 tags, [HALFLIFE] [RADIOACTIVITY]
It's not physics, it's realism. Really. They are inherently linked. Besides, I'd rather see decay chains implemented: have a [DAUGHTER_PRODUCT:inorganic mineral] for minerals and use metals for other metals. The only problem is that unless you want to track percentages of radium/polonium/lead/radon/so much more per bar, the entire bar will turn into something else at once. Just imagine the squad brandishing uranium bolts that suddenly turn to polonium, killing everyone. (Or assemble a not-so-critical mass of uranium and wait for a few bars to turn to polonium, which would probably get so hot that the entire mess would melt and kill everyone not underground like the dwarves)
EDIT: Or the "lacing" system, but I don't know how many levels of contamination it does (one? two? like spatter/pool of blood?). The uranium bolt could be laced with thorium-234, then laced with thorium-234 and proactinium-234, then laced with thorium-234 and proactinium-234 and uranium-234, and then it becomes mostly uranium-234 while also being laced with thorium-230. Amounts would be difficult if they aren't concrete.