Enrichment of Nuclear Fuel

A bomb powered by nuclear fission relies on the fissionable properties of elements such as uranium-235 and plutonium-239. Fuel usually must be enriched to 90% before it can be used in a nuclear weapon.

Uranium

Uranium is a very dense metal used in nuclear energy reactors and in nuclear fission bombs. It occurs naturally in rocks mostly as the isotope U-238 in concentrations of 2-4 parts per million. The radioactive decay of uranium inside the Earth's core at 0.1 watts per ton provides the main source of internal heat in our planet, which causes convection and drift. Because uranium is 18.7 times as dense as water and the heaviest of all naturally-occuring elements, it is also used in the keels of yachts and as counterweights in aircraft control surfaces (rudders and elevators).

Processing Uranium for Weapons Use

Extraction

The isotope of uranium used in nuclear fission reactions is U-235, which occurs in proportions of 0.7% in nature. After mining of the uranium ore, it is ground up and treated with acid to dissolve the uranium. The uranium is then recovered from the solution.

Alternately, uranium can be mined using a method called in situ leaching (ISL), in which it is dissolved from the matrix in its original position (in situ) and then pumped to the surface.

The end product of these two extraction processes is uranium oxide concentrate (U₃O₈), commonly known as "yellowcake." This is the raw form in which uranium is sold. It is still not usable as a fissionable energy source.

Conversion

The next step is to convert the uranium oxide into uranium hexafluoride gas (UF₆), referred to in the industry as "hex." This gas allows uranium to be enriched. (Iran's conversion of uranium oxide into UF6 has caused great controversy.)

Enrichment

Enrichment increases the concentration of uranium-235 from the natural level of 0.7% to 3-4% for nuclear energy reactors and 90% for nuclear weapons. Methods of enrichment include:

• Electromagnetic isotope separation (EMIS) - This was developed and used in the Manhattan Project to make the fuel used for the Hiroshima bomb. Ions of uranium-238 and uranium-235 are separated because they display varying radii when moving through a magnetic field.
• Jet nozzle process and Helikon vortex tube process - This is an energy-intensive process developed in the early days of nuclear enrichment. A high-speed gas stream containing UF₆ is made to turn a very small radius, causing the heavier isotopes to go towards the outside, similar to the way a centrifuge operates.
• Gaseous diffusion process - The diffusion process involves pressuring UF₆ gas through a serious of porous membranes. Since the U-235 molecules are lighter than U-238, they move faster and have a better chance of passing through the pores in the membrane. Thus the uranium that passes through the membrane is slightly enriched in U-235. The process is repeated many times in a series of stages called a cascade. Each stage consists of a compressor, a diffuser, and a heat remover. The gas must be processed through 1400 stages to obtain a product with 3-4% concentration. This method accounts for 40% of the world's enrichment capacity, but is slowly being fused out in favor of centrifuge technology.
• Centrifuge process - Centrifuges require relatively little power to enrich uranium when compared to the other methods (50kWh/SWU). UF₆ gas is fed into a series of vacuum tube cylinders, each containing a rotor 1-2 meters long and 15-30cm in diameter. The rotors are spun at speeds between 50,000 rpm and 70,000 rpm. The outer wall moves at a speed of 400-500 meters per second, yielding a force of a million times the acceleration of gravity. As in a roller coaster, centrifugal force is exerted on the uranium gas molecules. The heavier U-238 accumulates near the edges, and the lighter U-235 accumulates in the center.

A bank of centrifuges at the Urenco plant in Capenhurst, United Kingdom

• Laser processes - This is the leading edge of technology. Currently, No laser processes exist for commercial enrichment, though they are in the advanced development stages. A powerful laser is used to ionize atoms in uranium gas. Atoms can be ionized by light of a specific frequency. The lasers are tuned to ionize U-235 but not U-238. The positive U-235 ions are then attracted to a magnetized plate and collected. This method can also be used to enrich plutonium. Laser techniques promise lower energy requirements and lower capital costs.

After enrichment, the concentrated UF₆ is converted into UO₂ and made into fuel pellets. The ceramic pellets are encased in metal tubes to form uranium fuel rods for nuclear reactors or processed for nuclear weaponry.

Plutonium

Plutonium is a synthetic element made from uranium. The plutonium metal has a silvery appearance and tarnishes yellow when oxidized. The element is chemically reactive. A large piece of plutonium will feel warm due to the energy given off in alpha decay. (It would be dangerous to be near radioactive plutonium at any time.) If the sample is large enough, plutonium can produce enough heat to boil water.

One kilogram of plutonium results in an explosion equivalent to 20,000 tons of chemical explosive. Every year, twenty thousand kilograms of plutonium are produced by nuclear reactors throughout the world. As of 1982, experts estimate that 300,000 kg of plutonium exist worldwide.

Plutonium is a very hazardous radioactive element. Special equipment and handling procedures must be employed when processing plutonium. Handlers must be very careful to prevent the unintentional formation of a critical mass. Liquid plutonium is more likely to go cortical than plutonium in solid state.

"Plutonium Breeders" - Production of Plutonium for Weapons Use

Plutonium exists in trace quantities in naturally occurring uranium ore, but for purposes of weapons production it is produces in the laboratory by the bombardment of uranium-238 (natural uranium) with neutrons. The reaction occurs in two steps with two beta decays, as depicted below.