Fusion and Fission Reactions Atomic Physics

Fission splitting of a nucleus into two smaller nuclei nucleus must have an atomic mass A > 120 the energy difference of binding energy is the energy released in the reaction

Energy released

Fission Many fission reactions are initiated by a free neutron colliding with a nucleus to form an unstable isotope. The unstable nucleus breaks up almost instantly into smaller nuclei.

Eg) Fission of Uranium-235 High energy neutron collides Barium -141 Krypton - 92 + energy This energy is released by each atom that reacts.

Chain reaction Note that the example fission reaction emits 3 n°. Each n° can initiate additional fission reactions. This can produce a chain reaction.

Chain Reaction Video http://www.youtube.com/watch?v=Pmy5fi vI_4U http://www.youtube.com/watch?v=z5TMU 6HGGU0 Big Boom http://www.youtube.com/watch?v=Gzk8- 8RSVIs&feature=related

Slow boom A nuclear reactor slows the chain reaction so that it does releases energy in a slow steady rate. This is usually done by absorbing neutrons to prevent one reaction from initiating several new reactions. http://www.youtube.com/watch?v=NW9q B2dN_o8 http://www.youtube.com/watch?v=G1jtW R_tcX4&feature=related

Fusion – process of combining smaller nuclei to produce a larger one with the release of energy Involves two low mass nuclei, A < 60, resulting in a single nucleus being formed Energy released is the difference in mass of the nuclei and individual parts http://www.youtube.com/watch?v=uOxuGzYX HSQ&feature=related

Power of the Sun

Reaction The proton-proton chain is a series of fusion reactions (see page 821). The overall reaction is: Energy released: 24.67 MeV

Example: Fusion of Hydrogen Isotopes Temperature of 106 ⁰C Energy could be used to produce steam to drive a turbine and create electrical energy through the generator effect Energy is needed to have the reaction occur to overcome the electrostatic force of repulsion between protons Inefficient because the energy in is greater than the energy out

Fission vs. fusion Fission: 1 big --> 2 small nuclei Fusion: 2 small --> 1 bigger nucleus Fission can occur spontaneously. Fusion requires pushing two positive nuclei together and overcoming electrostatic repulsion. (high temperatures: very fast moving atoms)

Nuclear power Energy from fission reactions used to vaporize water, steam used to turn turbine, which generates electricity. No green house gases. Need to contain radioactivity. Waste product radioactive, often with very long half-lives. Must be stored.

Power from fusion Sustained, controlled fusion reaction needed. Not (yet) able to do this. Much less radioactive waste (almost none usually). Potential is almost unlimited in theory.

Nuclear weapons A-bomb: uses fission of uranium or plutonium H-bomb: uses fusion of H (way bigger boom) Hazards: huge explosion, scatters radioactive waste all over the place.

How to build a bomb Just kidding…. Obtain at least critical mass of weapons grade U or Pu. Mold into two half spheres. Use TNT to drive two half spheres together. Boom (if you are lucky).