Nuclear Fission & Fusion

1. Nuclear Fission & Fusion Objectives: • Describe what happens in a nuclear chain reaction. • Explain the use of water in the storage of spent fuel rods. • Distinguish fission from fusion reactions. Vocabulary • Fission, neutron moderation, neutron absorption, fusion

2. Fission reactions In a fission reaction the nucleus splits apart into smaller fragments. There are 2 fissionable isotopes:U-235 & Pu-239. Nuclear chain reactions make use of the fissionable isotopes to generate enormous amounts of energy for the use in nuclear reactors and atomic warheads. • U-235 is hit with a neutron and becomes U-236 (very unstable)

3. Storage of U-235 • the fissionable isotope is contained in a 3 meter fuel rod • spent fuel rods are a major source of radioactive waste • spent fuel rods can spend a decade in holding tanks as they cool

4. Neutron Moderation • Moderation is the process of slowing down the reaction by having the neutron pass through a barrier before contacting the radio isotope • Neutron moderation slows down the neutron using water or powdered graphite to a speed where it will fuse with a new neutron • energy is released as the U-235 nuclei splits forming Kr-91, Ba-142 & 3 neutrons • if not moderated the 3 neutrons will set off 3 more fission reactions • Neutron absorption occurs as cadmium rods are lowered into the reactor core. • the absorption of neutrons allows for the slower release of energy and the management of reactor heat levels

5. Nuclear waste anyone?

6. Nuclear Fusion • Fusion reactions that occur in the center of stars create much more energy than that of a fission reaction. Fusion reactions occur when small nuclei fuse to make a single atom of greater mass. The reaction takes place at temperatures in excess of 40,000,000oC. The hydrogen bomb makes use of a fission reaction to generate enough heat to fuse a H-2 nucleus with a H-3 nucleus in an uncontrolled reaction.

7. Sun's Nuclear Reaction • 4H g He + 2positrons + energy