I depends on what is meant by "disaster". Ask any good union man what loosing his union wages would be. "It would be a Disaster, OMG!"
As far as radiologically active materials escaping confinement is concerned there will come the time when Chernobyl is considered trivial.
If you're going to rank disasters by order of magnitude, then pretty much any disaster, save the biggest, could be considered trivial, though it would be best if you weren't relying upon the possibility of some future event to make your comparison.
At any rate, I think it's safe to say that any event which results in the imposition of a 30 km radius "exclusion zone" qualifies as a disaster.