Nuclear Reactors, Atomic Energy & Uranium Chemistry

Principles of Nuclear Reactors

• Nuclear fission = large nucleus splitting into 2 highly energetic smaller nuclei + neutrons
• Sufficient neutrons of suitable energy can induce fission of further nuclei Þ Chain reaction

  to sustain chain reaction a critical mass of uranium must be achieved (prevents neutron loss)

• Kinetic energy of main fragments is converted to heat (10⁶x energy of same mass of coal)
• Only naturally-occurring fissile nucleus is ²³⁵U (0.72 % natural abundance)

  

• ' Fast' neutrons produced by fission are not very effective in producing further fission

  Þ either increase proportion of ²³⁵U in fuel

  or slow down (moderate) the fast neutrons

  ~ necessary anyway to balance the chain reaction & prevent explosion

• First self-sustaining nuclear 'reactor' built in 1942 at University of Chicago

  NOT the first nuclear reactor on Earth!

  The Oklo Phenomenon

  in the Oklo Uranium mine in Africa it is observed that ²³⁵U levels are lower in those parts where the total uranium content of the rock is highest and fission product elements are found in the deposits

  • when the rocks were formed 2 x 10⁹ years ago ²³⁵U was 3% Þ critical mass
  • water in the clay mineral moderated the neutrons
  • criticality was possibly maintained for 10⁶ years

Modern Nuclear Reactors

Fuel

• Current nuclear reactors use UO₂ fuel ~ less reactive than U metal
• Enrichment is by fractional gaseous centrifugation of UF₆ (easily sublimed)
• Breeder reactors generate new fuel during operation.

  Neutron capture by ²³⁸U results in formation of ²³⁹Pu, which is fissile. Significant amounts of Pu will only be produced in an unmoderated reactor (fuel reprocessing more dangerous!)

Cladding

Fuel container is usually stainless steel or zirconium alloy (resistance to corrosion)

Moderators

Best moderators are light atoms;

• ¹H in water (so efficient at moderation that enriched fuel must be used)
• ²H in heavy water
• ^10,11B in boron-steel control rods,
• ¹²C in graphite (must be highly purified)

Coolants

• Water/Heavy Water ~ to keep it liquid it must be pressurized (PWR)
• CO₂ gas ~ in the Advanced Gas-Cooled Reactor (AGR)
• Liquid-Na needed for the more severe cooling problems in breeder reactors

Nuclear Fuel Reprocessing

Fission products 'poison' the fuel (by absorbing neutrons themselves) before it is spent

• 'spent' fuel = uranium / plutonium / trans-uraniums (small amounts) / ~30 fission products (inc. 2^nd row transition metals / lanthanides as alloys or complex oxide phases)
• reprocessing exploits the different redox/complexation chemistries of

  uranium vs plutonium vs fission + capture products

Bibliography