[FONT=Book Antiqua]There’s an ideal ratio of protons to neutrons for an atomic nucleus to be stable. Even when their protons and neutrons are at the right ratio, big atoms have a nucleus that is kind of wobbly and unstable to begin with. Over time those big atoms, left to their own devices, will tend to split into fragments. This is called radioactivity. Exactly what fragments an atom splits into depends on what kind of atom it is (which is determined by how many protons and neutrons were there in the first place).
When a big atom gets hit by a neutron, an already-unstable atom has its proton-to-neutron ratio suddenly be “wrong.” Which causes it to become extremely unstable and split into fragments almost instantly. A bunch of binding energy is released which causes the fragments to fly apart, like what happens when you cut a tight-stretched rubber band. How fast atoms are moving determines temperature, so when an atom is split, it creates heat as well as fragments.
Leo Szilard, a physicist who left Germany when Hitler became chancellor, realized that if we could find some atom that splits into fragments that include multiple neutrons, you could create a chain reaction. In a chain reaction, one atom breaks and releases two neutrons, which breaks two more atoms, which releases four more neutrons, which breaks four more atoms, which releases eight neutrons…Szilard convinced other influential physicists, including Einstein, to advise the US government that this was possible and might be able to be used to create a powerful bomb.
If you have only a small amount of the right kind of atom, the chain reaction can’t get properly started – too many neutrons escape off the edges of the sample. But once you have enough atoms – a condition called critical mass – the chain reaction will be able to sustain itself. If you’re just over critical mass, the sample will start to heat up and get hotter and hotter until it melts itself and its container, this is called meltdown (the flow will eventually spread it out enough that it stops being critical and cools down, but before that it can cause lots of damage in places that are difficult / impossible to safely repair because of radioactivity). If you have way more than critical mass, then incredible amounts of heat and neutrons will be produced very quickly, which causes an enormous explosion.
One use of this chain reaction phenomenon is to build a power plant: Use neutron absorbing rods which can be inserted/removed from a sample to keep it near critical mass, enough so that it heats up enough to boil water to create steam and generate power, but not to the point of meltdown. Another use is to build a bomb: Use regular explosives to smash two samples together to create a single sample far above critical mass, which will cause a very powerful explosion.
The right kind of atom for this process, a kind of uranium, turns out to be fairly common all over Earth, but since it is unstable, a lot of it has split into fragments over the billions of years that have passed since it was formed. The remaining amounts of useful uranium are mixed with large amounts of useless uranium that doesn’t release multiple neutrons. So the useful uranium needs to be separated from the useless uranium.
Normally when scientists or engineers want to separate things, they will rely on different properties of the things they want to separate. For example, you can melt the sample and let the liquid settle into different layers (taking advantage of different liquid density). Or by making the sample hot enough for only one of the mixed things to boil away (taking advantage of different boiling points). Or by adding chemicals which react differently with the things they want to separate (taking advantage of different chemistry). But none of this works for uranium, because the two kinds of uranium are very similar and have almost identical properties.
The best method they found for separating uranium involves vaporizing the uranium into gas and then spinning the gas really fast with a machine called a gas centrifuge. The two different kinds of atoms weigh different amounts, allowing the right kind of uranium to be separated and purified. This process is incredibly expensive and inefficient, which is a good thing. Hobbyists or terrorists can improvise regular bombs fairly easily, but can’t feasibly build a nuclear bomb in their backyard. Even for a government, in the age of satellites it’s not really feasible to hide the enormous facilities needed, so countries basically know which other countries can / are trying to build bombs, and can pressure each other not to create weapons.
This is the Class 6 Explainer. [/FONT]
[SIZE=3]c+p from depthhub[/SIZE]
