NUCLEAR ENERGY: FISSION

CONVERSION OF MASS TO ENERGY = mc2

Nuclear Fission (1)

• Neutron + Radioactive Heavy Nucleus Stable Lighter Nuclei + ENERGY

• 10n + 235

92U 14256Ba + 91

36Kr + 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

Prentice Hall Slide

Nuclear Fission (2)

• 10n + 235

92U 14256Ba + 91

36Kr + 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.

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.

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.

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.

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.

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.

Prentice Hall Slide