READING QUIZ Which of these isotopes is a common fuel for nuclear reactors? uranium-235 uranium-236 uranium-237
Lecture 41: Fission, Fusion, power plants, bombs Discovery of fission E=mc2, nuclear vs chemical energies Chain reaction, critical mass Isotope enrichment Power plants, control rods “Atomic” bombs Fusion “Hydrogen” thermonuclear bombs
Lecture Quiz 41 - Question 1: Suppose you had a beam of neutrons aimed at a target of Uranium (Z=92). Your goal is to artificially create an element with a larger atomic number, like Neptunium (Z=93). How could this happen? Neutrons hitting a Uranium nucleus can be captured by it, and adding one neutron turns it into Neptunium. One neutron is captured by a Uranium nucleus, which then undergoes beta decay and turns into Neptunium. One neutron is captured by a Uranium nucleus, which then undergoes gamma decay and turns into Neptunium.
Lecture Quiz 41 - Question 2: You aim a beam of neutrons at a Uranium target, and then see if any Neptunium is present. Instead, you find Barium (Z=56). What happened? The neutrons fused into alpha particles, which then fused into larger nuclei, until Barium was formed. The neutron beam caused the Uranium nucleus to emit alpha particles, until what was left was a Barium nucleus. One neutron added to the Uranium nucleus caused it to break into pieces, one of which was Barium. Uranium spontaneously decays into Barium with some characteristic half-life, even if there is no neutron beam.
FISSIONBarium-142 and krypton-91 are two possible fission fragments produced when a neutron is absorbed by uranium-235, causing a fission reaction.
Nuclear energy comes from E = mc2. Nuclear reaction: He4 + Be9 -> C12 + n1 Initial mass = 13.014 u Final mass = 13.008 u Dm = 0.006 u ~ 10 me So DE = 9 x 10-13 Joules ~ 6,000,000 eV ~ 6 million chemical bonds!!! Similar results obtained from FISSION: n + U235 -> Kr91 + Ba142 + 3n
A chain reaction involving nuclear fission. Neutrons are produced in each fission of a uranium-235 nucleus, which, in turn, can initiate more fission reactions.
ISOTOPIC ENRICHMENT Natural Uranium: 0.7% U235 99.3% mostly U238 Only U235 is fissile, releasing neutrons spontaneously. These neutrons are too fast to cause a chain reaction at this concentration. If they are slowed down, chain reaction is adequate for a nuclear power plant. If the U235 is enriched to maybe 7%, then the fast neutrons can cause a chain reaction. If there is a critical mass, a nuclear bomb is possible.
ISOTOPIC ENRICHMENT However, enrichment is very difficult! It takes huge factories to produce a few pounds of bomb grade uranium. (Or many, many small factories.) UN inspectors in IRAQ (and Iran) were looking for the industrial infrastructure necessary to enrich uranium.
A diagram of a modern pressurized-water nuclear reactor. Hot water coming from the reactor is converted to steam when the pressure is reduced in the steam generators. The steam turns the turbines, which power the electric generator.
The gun concept used in the Little Boy uranium bomb design. A subcritical-size cylinder of uranium-235 is fired into the hole in a subcritical sphere of uranium-235 to make a supercritical mass of uranium-235.
The Fat Man plutonium bomb used chemical explosives arranged around a subcritical mass of plutonium-239. When imploded by the explosives, the increased density makes this mass supercritical.
FUSIONA deuterium nucleus and a tritium nucleus combine to form a helium-4 nucleus and a neutron. The difference in the masses is converted to the kinetic energy of the emerging particles.
FUSION mass difference ~ 0.019 u ~ 30 me energy gain ~ 20,000,000 chemical bonds!!!
THERMONUCLEAR (HYDROGEN) BOMB A fission bomb is exploded around the fusion fuel to produce the high temperatures and density required for a fusion chain reaction in a thermonuclear bomb.
Lecture Quiz What is the purpose of building a Graphite moderated reactor? power usage produce weapon grade material produce weapons grade material and power production
Lecture Quiz • What fraction of natural Uranium is used as the fuel in a commercial nuclear reactor? • 95% • 50% • 25% • 10% • Less than 1%
Lecture Quiz What is the “Plutonium” economy? • U(238) is converted to Pu(239) for power production • Plutonium is made into bombs to threaten other countries for their natural resources. • Plutonium is turned into gold by nuclear reactions • Everone will be watch by “big brother” because everyone will have Pu at their basement for energy production.