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NUCLEAR ENERGY: FISSION

NUCLEAR ENERGY: FISSION. CONVERSION OF MASS TO ENERGY = mc 2. Nuclear Fission (1). Neutron + Radioactive Heavy Nucleus  Stable Lighter Nuclei + ENERGY 1 0 n + 235 92 U  142 56 Ba + 91 36 Kr + 3 1 0 n

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NUCLEAR ENERGY: FISSION

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  1. NUCLEAR ENERGY: FISSION CONVERSION OF MASS TO ENERGY = mc2

  2. Nuclear Fission (1) • Neutron + Radioactive Heavy Nucleus  Stable Lighter Nuclei + ENERGY • 10n + 23592U  14256Ba + 9136Kr + 3 10n • This is one fission pathway for U-235. There are nearly 400 fission pathways for U-235. τ1/2 = 7.04E+8 yrs. • Sum of mass number and sum of atomic number must be conserved. Note Ex.17.4

  3. Prentice Hall Slide

  4. Nuclear Fission (2) • 10n + 23592U  14256Ba + 9136Kr + 3 10n • The 3 product neutrons can induce 3 more fission rxns. These 3 rxns can cause 9 more fission rxns. This is called a Chain Reaction • If amt of U-235 is large enough (critical mass = 56 kg), the chain rxn can be self-sustaining. I.e. an external neutron source is not needed.

  5. Nuclear Fission (3) • U-235 is naturally occurring in UO2 ores at 0.7% abundance level. It is the only isotope of U that can undergo fission. • In a controlled fission rxn, the energy (1.68E+10 kJ/mol) produced can be captured and used for generating electricity. • Nuclear power require fuel rods enriched to 3.0% U-235.

  6. Nuclear Fission (4) • If the amt of U-235 is in a small enough volume, the energy release is so rapid as to create a nuclear explosion or atomic bomb. • Nuclear bombs require the fuel to be enriched to 90.0% U-235. • Current world concern about Iran and North Korea’s ability to achieve this enrichment level.

  7. Plutonium - 239 • The most abundant isotope of uranium is U-238 (99.3%). It is not fissionable but is radioactive. • U-238 in fuel rods absorbs neutrons to form highly radioactive U-239 which decays to give highly radioactive Pu-239. (p 639) • Pu-239 has a long τ1/2 = 2.4E+4 yrs, is radioactive (emits α particles) and is fissionable. • Pu-239 is not naturally occurring. • If enriched to 93%, Pu-239 can be used in an atomic bomb.

  8. Nuclear Waste • Nuclear plant waste is highly radioactive and must be secured, stored and monitored. • The big challenges are: How, where, for how long? • Recent news story about storing waste in Utah, but there are additional challenges including transporting waste to Utah.

  9. Other Issues • Reprocessing spent fuel to obtain Pu-239. • The world supply of Pu-239 is growing. • Breeder reactors maximize production of Pu-239 to be used for nuclear power plants instead of U-235. • The last nuclear energy plant in the US was built in 1979. • Public acceptance of nuclear energy is low • Pres. Bush’s 4/29/05 press conference.

  10. Prentice Hall Slide

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