PFE020: Nukes

Nuclear weapons are probably the most famous invention/discovery from the physics community. Never mind things like the atom leading to all modern chemistry. Or the laser. Not to mention that nukes were only used in wartime twice. Of course, those two uses killed a ton of innocent people which is kind of hard to forget.

Avoiding the political aspects both past and present still leaves enough interesting history that it's worth discussing.

We know the theory behind it, that we can get energy from mass by E=mc². The actual mechanics are interesting too.

The first thing to cover is that everything radiates energy. Every atom, in my body, is giving off energy. Photons are radiating away from us so fast!  This is from a notion known as "black-body-radiation."

 Physicists just got chills. Everyone else got bored.

Some particles aren't content to just throw away photons, some want to do more for those around them. So they toss out big particles (alpha particles, or He nuclei), with a big mass, changing the particle to something else altogether. When a radioactive atom ejects a particle like this, it decays into smaller atoms, but more importantly, the total mass also decreases creating a huge amount of energy.

The rates at which particles do this vary wildly. Some atoms will break apart in less than a second, while others will hang around for thousands of years. To make a bomb, you need something in between. For a given atom, there is a "critical" set of conditions (amount of the element and how close together they are) that will lead to a nuclear reaction.

All of these processes can be sped up. As atoms eject alpha particles, these alpha particles can hit other atoms and make them decay too. So if you pack a bunch of these highly radioactive particles together, they will create a huge explosion from the chain reaction.

There are a number of different bomb designs and I won't go into too much detail on them as they are still classified in some sense of the word and I don't want any budding terrorists pointing the finger at me.

The most basic design (and believe me, it is anything but easy to build) is a standard fission bomb. In this case, you take a sub-critical radioactive sphere (one that won't blow up in your hands, although carrying it around with you as you eat lunch is probably still a bad idea) and place explosives all around it, and set them to all go off at exactly the same time. This squeezes the ball in, it goes critical, and boom. This is the design of the fat man bomb dropped over Nagasaki on August 9, 1945.

The first bomb dropped on Japan, over Hiroshima on August 6,

was called the little boy, and was an example of the gun assembly model. In this case, there are two radioactive parts, one is a sphere with a hole bored half way through it, and the other the bullet. At the opportune moment, the bullet is shot down the length of the bomb into the core sending the bomb past the critical requirements.

Most nuclear bombs today are what is known as "hydrogen bombs", although this is a bit of a misnomer. They are typically two stage devices, the first of which is a standard fission bomb like the fat man. The second stage uses a fusion rod, along with a fission core, to greatly expand the effect of the blast. The name hydrogen bomb comes from a "booster" that is used to increase the effectiveness of the standard fission reaction. The problem is that if the fission material explodes outward too rapidly, it won't all undergo fission. The booster material is typically "heavy" hydrogen - deuterium or tritium.

For comparison, the fat man was between 50% and 100% more powerful than the little boy. A hydrogen bomb (depending on the model) is 450-600 times more powerful than the fat man.

That's nukes.