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as SuperCritical Mass. The actual mass needed to facilitate this chain
reaction depends upon the purity of the material, but for pure U-235, it is
110 pounds (50 kilograms), but no Uranium is never quite pure, so in reality
more will be needed.

Uranium is not the only material used for making atomic bombs. Another
material is the element Plutonium, in its isotope Pu-239. Plutonium is not
found naturally (except in minute traces) and is always made from Uranium.
The only way to produce Plutonium from Uranium is to process U-238 through a
nuclear reactor. After a period of time, the intense radioactivity causes the
metal to pick up extra particles, so that more and more of its atoms turn into
Plutonium.

Plutonium will not start a fast chain reaction by itself, but this
difficulty is overcome by having a neutron source, a highly radioactive
material that gives off neutrons faster than the Plutonium itself. In certain
types of bombs, a mixture of the elements Beryllium and Polonium is used to
bring about this reaction. Only a small piece is needed. The material is not
fissionable in and of itself, but merely acts as a catalyst to the greater
reaction.



============================================================================


- Diagram of a Chain Reaction -
-------------------------------



|
|
|
|
[1]------------------------------> o

. o o .
. o_0_o . <-----------------------[2]
. o 0 o .
. o o .

|
\|/
~

. o o. .o o .
[3]-----------------------> . o_0_o"o_0_o .
. o 0 o~o 0 o .
. o o.".o o .