NUCLEAR ENERGY

• There are two paths to deriving energy from nuclear reactions Nuclear Fission and Nuclear Fusion.

  First, what is a Nuclear Reaction ? Remember that the nucleus of every atom is composed of protons ( ) and neutrons ( ). If you add or subtract a nucleon to a nucleus this is a nuclear reaction.

• The Forces of Nature

1. Gravity is the force the prevents the Sun from expanding despite the fact that it is a hot gas and keeps the Earth forever falling towards the Sun in it's orbit and keeps all the people in this room in their seats. One way to look at gravity is that it is an attractive force between objects with mass.

2. Electric Force is a force felt between charged particles like electrons (e-) and protons (p+). There are both attractive and repulsive electrical forces. Like charges repel, opposites attract. The Electrical Force holds atoms and molecules together and is useful for all kinds of nifty gadgets that have been built in the last 150 years.

   Fun note: The electric force is what keeps you from falling through the chair and floor and earth and being cooked in the center of the Earth. You are suspended above your chair by the repulsive force of the protons in your body against the protons in the chair.

3. Nuclear Force. If you think about it for a minute, nuclei of atoms which contain protons (positively charged) and neutrons (no charge) should not be stable. All those ``like'' charged protons should be repelling one another and fly right out of the nucleus. Since nuclei ARE stable, it can be inferred that there is another force that is stronger than the electrical force which glues those nuclei together. This is the Nuclear Force or the Strong force. The Nuclear force would right now be binding EVERYTHING together except for the fact that it acts over only very tiny distances.

Nuclear Fission

• For the ``heavy'' elements in the Periodic Table there are some that have nuclei that spontaneously fall apart - these are the radioactive elements.

  

  The wild thing about this is that if you weigh the initial nucleus and weigh the bits after the ``fission'' they don't add to the same total.

• The Missing Mass in the fission reaction has been converted into energy via the only equation that 8 out of 10 Americans know:
  

  This equation says that Mass and Energy are interchangable.

  

• The equation above as applied to this reaction (and also to chemical reactions) is:
  

  where is the mass difference between the Uranium nucleus and the fission product nuclei.

• This is a pretty efficient source of energy. Some fission reactions release as much as ergs/gram of fissionable material. The problem is, the Sun has only a tiny fraction of its composition in radioactive materials.

THE ANSWER: NUCLEAR FUSION

• There is another way to derive energy from nuclei. Think about a gas of protons. At ``low'' temperatures, the electrical repulsion between them prevents the close approach of any two protons. But, as the temperature of the gas increases, the minimum approach distance decreases and occasionally two protons ( meters) so the short-range but very strong nuclear attractive force will bind or fuse the protons together.

  

• When two protons fuse, a chargeless, perhaps massless particle called a neutrino is spit out, and one of the protons emits an anti-matter particle called a positron - like a positively charged electron. Having lost its electrical charge, the proton becomes neutron.

  The shorthand for this reaction is:

  

There is a short string of similar reactions that go on in any hot gas of hydrogen atoms. This is called the proton-proton cycle or the P-P cycle.

Step I:

Step II:

Step III:

The whole thing can be summarized with one net reaction:

• Here is the amazing thing:
  

  So, grams of matter disappear in the P-P chain.

• Of course this mass has been converted into energy (gamma rays). For one reaction, the Energy released is:

  

• For every 4 hydrogen atoms ( grams) we get this much energy so we can calculate the efficiency of the P-P chain:

• Now let's suppose the Sun is made of Hydrogen and it is hot enough to run the P-P Chain. How long would its lifetime be at as a nuclear-fusion powered object?

• This is the available energy from hydrogen fusion
  

• This is the lifetime of the Sun with fusion power:
  
  

• Looks pretty promising!

• Is Hydrogen Fusion for real or just another theory? It's for real - don't forget Bikini Atoll.

• WHAT ARE THE REQUIREMENTS FOR A FUSION-POWERED SUN?
  1. Lots of protons - no problem, the Sun is made mostly of hydrogen.