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TW, there is no such thing as "weapons grade" U238. U238 makes up better than 99% of all Uranium mined, which is why it was so difficult to come up with the amounts of U235 necessary to make a bomb. Developing processes to separate out the U235 were a major part of the Manhattan Project. U235 is the rapidly fissionable material used for bomb making. When you introduce an extra neutron to U235, the result is unstable and breaks down into two lighter atoms, such as Cesium and Strontium. There are also additional neutrons released, which go off and fission other atoms of U235. This is the "chain reaction". Energy is also liberated in the fissioning which is where the big boom and destruction and radiation comes from. When you throw an extra neutron at 238 it absorbs it, which is what makes P239, which is weapons grade plutonium. The graphite and uranium pile at the Squash Court was a primitive precursor to today's breeder reactors, if I remember my Manhattan Project history correctly.
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