That the situation in Japan is serious should be a no brainer.
Here is an excerpt from the Greenpeace history of nuclear weapons testing to put things into perspective.
Yes, it's a Greenpeace document, but I believe it to be reasonably accurate.
"Of the 2,044 nuclear weapons tests worldwide, there have been 711 in the atmosphere or underwater: 215 by the U.S., 207 by the Soviet Union, 21 by Britain, 45 by France and, 23 by China.
The last atmospheric nuclear weapons test occurred on 16 October 1980 in China. The first was on 16 July 1945 in the U.S.
It is estimated that the total yield of all the atmospheric nuclear weapons tests conducted is 438 megatons.
That's equivalent to 29,200 Hiroshima size bombs. In the 36 years between 1945 and 1980 when atmospheric testing was being conducted this would have been equivalent to exploding a Hiroshima size bomb in the atmosphere every 11 hours."
Approximately 3,830 kilograms of plutonium has been left in the ground as a result of all underground nuclear testing and some 4,200 kilograms of plutonium has been discharged into the atmosphere as a result of atmospheric nuclear testing.
http://archive.greenpeace.org/comms/nukes/ctbt/read9.html