Re: The difference between a nuclear reactor and a nuclear bomb.

The biggest difference between a nuclear reactor and a bomb is the rate at 
which energy is released.  In a nuclear reactor, as in a fire, energy is 
released at a controllable rate.  In a nuclear weapon, as in a 
conventional weapon, energy is released at a very high and uncontrollable 
rate.  Everything else is more or less just explaining the details.  Some 
of those details are important, however!  I'll go over a few of them.

Uranium "enrichment"
Uranium has two primary isotopes, each with different properties.  U-235 
is lighter, splits (fissions) more readily, and constitutes only 0.72% of 
all uranium found in nature.  U-238 is harder to fission and makes up 
99.2% of all natural uranium.  You can't make natural uranium into either 
a bomb or a reactor because there just isn't enough of the fissionable 
uranium present (unless, that is, you're using heavy water and a lot of 
neat engineering).  This means that you have to process the uranium to 
increase the amount of U-235 so that it will work.

Nuclear criticality
First, to dispel some misinformation - ALL nuclear reactors are critical 
when they are operating.  "Critical" simply means they are running at a 
constant power level.  To make a reactor critical, you have to assemble 
the uranium fuel correctly, add water, and pull control rods so that 
fissions will start and stay at a constant level.  In general, between 2 
and 3 neutrons are released from each fission, and 1 of them will go on to 
cause another fission.

A nuclear reactor will operate for months or years in a critical state.  
This means it's doing just what it's supposed to do.  During criticality, 
neutrons are emitted during nuclear fission (splitting the U-235 atoms), 
and these neutrons go on to cause new fissions.  Some atoms are absorbed 
by U-238, some are absorbed by the metal of the reactor, and some just 
escape from the core altogether.  Even with enriched uranium fuel (most 
civilian reactors operate with fuel containing from 3-8% U-235) it takes a 
lot of engineering to make a reactor work.  In other words, it's not easy 
to create a chain reaction, and any number of factors can interrupt the 
reaction so the reactor shuts down.  Without power, even if the core melts 
down, a civilian nuclear reactor CANNOT explode in a nuclear explosion.  
Chernobyl suffered from a huge steam explosion, but it was physically 
impossible for it to have a nuclear explosion.

In a nuclear weapons, the components are arranged so that fission is much 
more efficient.  They are made of very highly enriched uranium (over 90% U-
235) or with plutonium (Pu-239), which also fissions easily. The Pu or U 
is violently compressed by high explosives, and the manner in which this 
is done makes it possible for up to 2 of the neutrons from each fission to 
cause another fission.  This means that the power is increasing incredibly 
quickly because each fission will cause two more, and each of those causes 
two more, and so on.  And this all happens very quickly - millions to 
billions of "generations" per second.  All in all, you get an uncontrolled 
release of great amounts of energy because a nuclear weapon 
is "supercritical".  If you add tritium (heavy hydrogen) to the mixture, 
the heat and temperature are sufficient to cause hydrogen fusion, which 
releases even more energy.

The bottom line is that you need to have enough material to sustain a 
critical or supercritical reaction (critical mass) and you have to arrange 
it in such a way that the reaction continues (critical geometry).  No 
matter how much U-235 you have, if it is in a puddle that is never more 
than 0.25 inches deep, it will never become critical because too many 
neutrons escape.  That's the critical geometry.  Similarly, if you only 
have 5 grams of Pu-239, you will never get a nuclear reaction because 
there just aren't enough Pu atoms to capture the neutrons needed to 
sustain a chain reaction.  That's critical mass.

For more information, you can try any of the following sources:

The Making of the Atomic Bomb (Richard Rhodes)
Dark Sun:  The Making of the Hydrogen Bomb (Richard Rhodes)
The Los Alamos Primer (Richard Serber)
The Curve of Binding Energy (John McPhee).
Nuclear Engineering (Ronald Knief)

Good luck!