The nuclear bomb
It
is not difficult to construct a thermonuclear device. What is difficult
is to obtain enough 235U from the uranium ore to use as fuel
in the thermonuclear device. The isotope 235U must be separated
from the more abundant 238U isotope. In fact the 235 isotope
makes up only 0.7% of uranium metal. The 238U isotope is very
stable. It can absorb neutrons without undergoing a fission reaction.
This isotope mops up neutrons and prevents a chain reaction from occuring.
The presence of the 238 isotope prevents nuclear chain reactions from
occuring in Nature.
Above is a
simple representation of a fission bomb. TNT is used to drive two pieces
of fuel, of subcritical mass towards
a neutron source. As the two subcritical pieces unite they form a supercritical
mass of fuel. This supercritical mass can undergo a nuclear
chain reaction.