Lecture 6: The Gadget
1. To Build a Nuclear Bomb: “The Gadget”
A. To Use Fusion?
Teller pushes it. Two problems in the 1940s, scientists were unsure you could make the bomb.
They were more
sure the fission bomb was possible.
Second, in order to make a fusion (hydrogen) bomb, you needed a fusion
bomb already.
You needed an atomic bomb to set off a hydrogen bomb.
When they started collecting at Los
Alamos, they wanted to do fission bomb first.
They gave Teller money, but low priority.
B. To Use Fission?
What isotope to use?
That is the first question. Groves answers the question and decides to make machines for
both.
i. To Use U235?
Absorbs neutron, two or more neutrons on average. The energy that is released is the strong force of the nuclear
reaction.
ii. To Use Pu239?
With plutonium, it's more stable.
Fissiong of Plutonium spreadsheet.
iii. To Use Some Other X???
Each ounce of uranium gives you something, but if you have billions of it happening every second, you have a
huge chain reaction. You have to have enough nuclei packed together that the probability that enough neutrons will
find the nucleus.
THe odds are that the neturons are going to escape.
Los ALamos wants to make sure that
neutron generation works (slide).
Multiplier effect: each generation doesn't double, there isa
factor.
One of the
things that Los Alamos figures out, is that the standard process goes through 80 generations.
Each generation goes
through a billionth of a second.
B. “Critical” vs. “Supercritical” Chain Reactions: Reactors vs. Bombs
If people are just talking about bombs, they are talking about "critical."
Nuclear reactors involve critical mass and
chain reaction, and a bomb involves a supercritical mass and supercritical chain reaction.
Critical mass, is a mass
density (amount of mass in matter, in a certain amount of density).
If you increase the density, you need more
mass.
Fermi was trying to discover the critical mass for his pile.
Supercritical mass is the next step up.
It's going to be a mass volume relation so that once you have that and it gets
neutron in it, the chain reaction will be uncontrollable.
It will explode.
One of the things Los Alamos has to do is
to figure out how much uranium is standard volume, how much to work with?
Same with plutonium.
Looking for
the minimum amount you need, once you get that, the chain reaction will explode.
Suppose the figure out, with a
certain dense of uranium, you get a supercritical mass.
You find out the supercritical mass of U235.
If you bring a
supercritical mass to class, once you determine that mass, you can't store uranium and plutonium together.
Once