d20 Future puts the damage on a tactical nuclear missile (for near-future PL6 starship combat) at 16d8.
Now, that sounds low, and it is if you think about what we know nuclear damage to do on Earth. However, much of the damage on Earth comes from the interaction of the nuclear weapon with the atmosphere, like the overpressure wave that is created, while in space most of the energy is simply radiated away from the target and it's a burst of incredibly powerful radiation, but without an atmosphere to help transmit the heat and pressure of the blast. In an atmosphere it may well be doing x5 damage or so.
The Nuclear Toxyderm (essentially a radiation elemental) in Urban Arcana has a breath weapon of 20d6 (radiation damage) and when it is reduced to 0 HP it's "Chain Reaction" ability activates doing 400 points of damage to everything 400 feet within a mile and 100 points of damage to everything within a mile (reflex save DC 25 for half), and explicitly says it creates a mushroom cloud and generally leaves a highly radioactive crater a quarter-mile across
Both items, are OGC even.
The Nuclear Toxyderm is also where the radiation poisoning rules were reprinted, after originally being in the d20 Modern Web Enhancement, and were made OGC as part of the Nuclear Toxyderm entry..