A new federal study lays out a catastrophe of nightmarish proportions if a small, so-called dirty bomb were detonated in downtown Toronto.

The study says the explosion of a small device near the CN Tower would spew radioactivity over four square kilometres.

The result would be mass anxiety, a rush on Toronto's medical facilities and an economic toll of up to $23.5 billion.

Federal officials made that prediction based on detonation of a device containing a modest amount of americium-241, a silvery plutonium byproduct.

It's among several sobering projections quietly mapped out by officials to prepare for a terrorist attack in urban Canada.

The grim scenario is not that far-fetched.

A database of lost and stolen radioactive items compiled by the Canadian Press reveals that a device similar to the one in the study was stolen in Red Deer, Alta., in June 2003.

Though later recovered, the device was missing for five days before its owners even noticed it was gone.