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From: https://www.commondreams.org/view/2014/01/12  (14 Jan 2014)

Yesterday I replied to a reader who emailed me asking why Fukushima was such a big deal when the Japanese cities of Hiroshima and Nagasaki were hit with US nuclear weapons but seem to be just fine 68 years later. I figure my reply is worth reposting here.

The Little Boy bomb dropped on Hiroshima had 141 pounds of enriched uranium of which less than 2 pounds actually underwent fission.

The Fat Man bomb dropped on Nagasaki had 13.7 pounds of plutonium, with only 1.5 pounds involved in the fission reaction.

In the 4 destroyed Fukushima reactors, each individual fuel rod contains 380 pounds of uranium. Each reactor has roughly 500 active fuel rod assemblies. That comes to roughly 42,000 pounds of uranium or 21 tons per reactor, times 4 ruined reactors, for 84 tons of uranium.

Add to that the 11,195 spent fuel rod assemblies which have less uranium but are loaded with plutonium and that adds 4 Million pounds or 2 thousand tons of potential fissionables.

Because the fuel rods do NOT blow themselves apart while fissioning a far larger percentage of the uranium and plutonium are available to a continuing fission process should one start spontaneously underneath the reactors. The neutron flux from underground fission, should it reach the adjacent spent fuel rods, can cause them to undergo fission as well.

And today another reader reminded me that “not only can the neutron flux from the fissioning material trigger more fissioning, it causes ‘activation’ of nonradioactive atoms, i.e. it turns them into radioactive ones.”