Usmani:
I'm
not going into the details, but let me tell you that there is a minimum
mass which is required for explosion. Less than that it won't explode,
and above that, it, it will not go, it will automatically explode. So,
10-12 kilograms is normally assumed to be the minimum weight of
plutonium, if you can get hold of 239-plutonium, or pure 235, about
10-12 kilos. So what they do in a bomb is, that they... put six
kilograms on one side, and six kilograms on the other. Now, these are
sub-critical masses; they won't explode by themselves. There is a lead
window in the bomb, there is a lead window--which I'm describing it very
very, uh, outlines of a bomb. The bomb has the same casing, the same
mechanism, as you have in a conventional bomb, which is dropped from the
air, except that it has a lead window; we have two halves of the
so-called atom bomb... on either side of the window, and as soon as the
window is, as soon as the... the plane that is due to drop that bomb, is
on the target, the window is lifted, and the first half, of six
kilograms, rolls over into the compartment of the second half, and the
two together become 12. And the 12 is critical and they go. Why? Because
there is... a neutron source placed under them, which is emitting
neutrons, so you have a neutron source, two parts of the 239, or 235,
whichever it is. And they roll over, they come to a critical mass and
explode over the target. Now, in a reactor, this is not the case. The
two halves have to be 98 to 99 percent pure uranium or pure plutonium;
the reactor can live, and reactor is not a bomb. It's a controlled
fission reaction; 10 kilowatts, 50 kilowatts, 100 megawatts, 200
megawatts, 1,000 megawatts, we control the genie that is liberated at
any level by poisoning the fission, not encouraging the fission, but
poisoning the fission. So you add a certain poison in the reactor, which
inhibits the fission reaction, to a point which we would like to have,
namely, 10 megawatts, 100 megawatts, 1,000 megawatts; as the case may
be, the size of the reactor. So it is called a reactor, because there is
a reaction, which is controlled. In the case of the bomb, which is a
simpler mechanism, there is no control; the more energy liberated, the
better it is. So that is the difference.