Keeping the "Tiger" at Bay; July 1996; Scientific American Magazine; by Zorpette; 2 Page(s)

One of the most dreaded mishaps involving nuclear materials is an accidental criticality-- the inadvertent accumulation of enough fissile materials, typically plutonium or highly enriched uranium, to cause a spontaneous shower of deadly radiation. Since the mid-1940s, there have been 34 known cases of accidental criticality in the U.S. Five of them killed a total of seven people; those five and others also injured workers or subjected them to significant doses of radiation.

Seven of the U.S. incidents, and one that happened in England in 1970, were "process" accidents, meaning they occurred at places where plutonium or highly enriched uranium was being created or processed. Last autumn it was revealed that 12 such episodes took place in the Soviet Union between 1953 and 1978. In the late 1980s, after the cold war, the U.S. and the Soviet Union stopped making plutonium and enriched uranium for weapons. Ironically, however, some experts say the threat of criticality is perhaps greater now than ever.