webform test

Friday, November 12, 2010

Fusion vs Fission

What is the difference? Atomic bombs and H-bombs? Are they the same thing?

There are two options available when it comes to nuclear reactions, Fusion and Fission. Fusion is what takes place on the sun and has therefore been around as long as the sun. Fission was developed first in America under the name Manhattan Project during world war II. Scientists were able to divide elements with large nuclei into two elements with smaller elements.

This actually helps me remember the difference between the two. Fission rhymes with division. And fusion could rhyme with glue-sion if such a word existed. I gave you a hint about fusion. It combines the nuclei of two smaller elements to create a larger element. Like Hydrogen to Helium. Here is a quick summary of the pros and cons of fission and fusion.

Fission Pros-
Relatively easy reaction to initiate
Usability in power plants
Atomic bomb(?)

Fission Cons-
Relatively rare fuel
Radioactive byproduct

Fusion Pros-
Relatively abundant fuel source (Hydrogen)
Non-radioactive byproduct (Helium)
Hydrogen Bomb

Fusion Cons-
Requires high amount of energy to start reaction
Uncontrollable to use as energy 

Which is stronger? And why? Enter Albert Einstein.

You've heard of E=mcor, energy equals mass times the speed of light squared, but what does it mean? Basically mass can be converted into energy and vice versa.

Earlier I mentioned nuclei as being larger and smaller, I was referring to the number of protons and nucleons in the nucleus of an atom. Now I would like to talk about the size of an individual neutron or proton. Not all nucleons (protons or neutrons) are created equal. For the purpose of this explanation think of three sizes, bowling ball, softball and baseball. 

Hydrogen has one big nucleon, when you smash two together they collapse in size. So think of two bowling balls. When they touch each other, or fuse (think fusion) they collapse to the size of two baseballs. That reduction in mass is converted into energy and becomes a nuclear bomb. 

Uranium on the other hand has a lot of medium size nucleons. There is a technical explanation for this, but for simplicity, the more nucleons that are packed together they become larger in size. When an extra neutron is fired into uranium it divides (think fission) into two parts. Because the parts are smaller groups of nucleons, each nucleon in the group reduces in size. So think of a bunch of soft balls that reduce to the size of baseballs. Similar to fusion, the reduction in mass is converted to energy. This is an atomic bomb.

In the graph below you see number of nucleons on the horizontal axis and amount of energy released/reduction in size on the vertical axis. Hydrogen (H) moves to Helium (He) and Uranium (U) only moves a short distance vertically to Xenon (Xe).

Referring to the pros and cons above, we would prefer to use fusion if we could because it releases a lot more energy with an abundant fuel source without leaving a radioactive byproduct. The draw back is it requires an atomic bomb to have enough force to smash to hydrogen nucleons together and as of yet we haven't been able to control the resulting explosion, it is just too big.

Eventually we will though and it will be exciting.


  1. Great post on fusion ad fission. I like your conclusion. It would be nice if we didn't have to leave a radioactive byproduct. I wonder if we will ever be able to control the explosion of an atomic bomb. Hmmmm...

  2. Thanks for writing this. As a physics major it bugs me sometimes that people mix up fusion and fission. Fusion is great - that's what powers solar panels (indirectly...)

  3. Fusion has the letters s u n in it...another memory trick!