Fuel enrichment Two naturally occurring isotopes of Uranium – 235 U and 238 U. 235 U makes up only.7% of the Uranium found in nature. For self-sustaining reactions, 3% is needed – for nuclear weapons, 90% is needed. The enriched fuel is then prepared in bundles of FUEL RODS that make up the fuel core.
Neutron moderation For 235 U to capture the neutron, the neutron must be moving very slowly – at Thermal energy =.025 eV A MODERATOR is used to slow down the neutrons coming from prior fissions so that they can be caught by other 235 U nuclei.
Types of moderators and plant design A substance consisting of light weight atoms. Pressurized water (nuclear subs) Boiling water (most plants in US) Graphite (Chernobyl plant in Russia) Liquid Sodium – For Plutonium breeder reactors.
Control Rods The IDEAL RATIO of fission events to capturable neutrons released is 1:1. Plant operators need dynamic control of the number of neutrons to maintain that ratio. CONTROL RODS are made of neutron- absorbing material, such as Boron, and are inserted or removed from the fuel bundles to control the number of neutrons available. The accident at Chernobyl (1986) was the result of a failure to lower the control rods quickly enough.
Cooling and plant operation In-class computer simulation.
Loss of coolant accident When water is the coolant AND moderator, a loss of water means the fission reaction shuts down. Residual radioactivity produces enough heat to melt the core and breach the reactor. The exposed super hot steam produces enough pressure to blow out the containment shell. Three Mile Island (1979) was a loss of coolant accident.
Nuclear waste Fuel rods begin with 3.2% 235 U, 96.8% 238 U. After 3 years of operation,.85% 235 U,.55% 239 Pu and 4% other fission products, plus 238 U. Reprocessing on site? Permanent storage in Nevada?
A finite resource? 150 years of operation at todays level. Breeder reactors would produce plutonium which could supply 1,000 years worth of energy. A few in France and Japan. Fears of too much bomb-grade material being present.
Nuclear Fusion Just as energy is released from mass lost by splitting very large nuclei, so energy is released from mass lost by joining very small nuclei! Deuterium and Tritium join to form Helium + 17.6 MeV Energy source of the Sun. Difficult to produce the conditions needed for fusion to be sustained.