Fast Breeder Reactors

Uranium contains a fissile isotope ²³⁵U, as 0.7% of the naturally mined and chemically refined metal. Current commercial power plant designs, which are remarkably successful and reliable, use only the fissile isotope, and in the USA they essentially throw away the rest. A fast breeder reactor (actually it's a fast-neutron, breeder, reactor) uses neutrons which have not been slowed down by a moderator, to turn some of its non-fissile isotope ²³⁸U into ²³⁹Pu, a fissile isotope of plutonium. This plutonium is a neutron-capture product. In a fast breeder reactor, the fuel atoms which have not yet split may capture more neutrons, forming americium, curium, and perhaps more. These elements are also called transuranic.
Strictly speaking, this is not the only possible design.
²³²Th --thorium --can be made into ²³³U, a different fissile isotope of uranium, in the same way.

In the long run, therefore, a fast breeder reactor can extract about 140 times as much energy from a ton of fuel as the non-recycling designs. This is pretty impressive, considering that about 50 tons of uranium suffice to provide as much energy in today's reactors as 1.25 million tons of coal in coal burning plants.

There have been several designs tried for constructing a viable breeder reactor for civilian use.
The best was done at Argonne National Labs., twenty years ago. It was called the Integral Fast Reactor, IFR for short. It worked, and in the very month that the Chernobyl reactor failed, the project's EBR II (Experimental Breeder Reactor II) had already proven itself immune, by deliberate test, to that kind (and the TMI kind) of failure. It was cancelled in 1994 in response to people who think themselves environmentalists. There exists now (July 2010) a proprietary plan to build packaged 100 MWe reactors using the proven technology of the EBR II reactor.

How the IFR differs from current nuclear reactor designs

Fast neutrons can consume all of the transuranic, neutron capture products.
The fuel rods are metallic, and conduct heat better than the ceramic metal oxides and nitrides of current designs.
Unlike even most other breeder reactors, the reprocessing of the spent fuel rods is done on-site, without their leaving the containment dome, and its lethal (to malefactors) radioactivity. The processing is done by machines resistant to the radioactivity.
The coolant is liquid sodium, which does not tend to eat its steel pipe containment as water does by causing rust.
The coolant is not under pressure, because its boiling point is far higher than even uncontrolled operation of the reactor can reach.
It uses fast neutrons, so the matter of the moderator is moot.
The configuration of the reactor makes it passively immune to meltdown. The chain reaction is set to use the neutrons produced by fission, just enough to maintain itself. Excess heat from loss of coolant or of control power changes the geometry of the core enough to let neutrons escape (into the shielding) so that too many escape for the reaction to continue.
Once the initial enriched fuel rods are installed, the supply of fissile fuel needs no more of the difficult and energy expensive process of uranium enrichment.
It can consume the long-lived 'nuclear waste' plutonium (half-life 25 millennia) that presently is scheduled to be buried at Yucca Mountain.
It can consume 'depleted' uranium, and 'surplus' atomic weapons plutonium and uranium.
The nuclear waste, which is now only the fission products, can be stored on-site for half a century. Its radioactivity declines quite rapidly, and the annual production from a one-gigawatt reactor would be about the weight of a Volkswagen or a Cooper-Mini.

How the ARC 100 Differs from the IFR

Standardised factory built units, less expensive to produce.
Modular, smaller than the IFR was intended to be.
Pyroprocessing facility is external.
Fuel assemblies are packaged as total replacements.

Uranium Enrichment

is the process which separates natural uranium into depleted uranium and enriched uranium.
Enriched uranium is uranium with enough of the fissile isotope that it can sustain a nuclear chain reaction. The fissile isotope ²³⁵U differs from the non-fissile ²³⁸U only by three neutrons in its nucleus, and the mass difference is all that the enrichment operation can use. Two exceedingly tedious methods can work. Both work with uranium hexafluoride, UF₆, whose molecules have molecular 'weights' of 349 and 352, depending upon the uranium isotope.
A centrifuge makes use of this mass difference. At the huge pseudo-gravitational force of the centrifuge, the heavier gas collects on the outside, and the lighter enriched gas in the center.
OR
A gas diffusion apparatus can force the gases through porous membranes (brick-like) through which the gases diffuse at speeds proportional to the square root of the molecular weight.
Gradually, the isotopes become somewhat segregated.
As I said, the process is tedious. It is far more energy-intensive than mere chemical refining. Before this, the fluorine is combined with, and after is dissociated from, the uranium. Reactor-grade enriched uranium for Pressurised Water Reactors (PWRs) is 3.6% ²³⁵U. At uranium.org, there is an example of the enrichment results which starts with 8.05 tonnes natural uranium, producing 1.0 tonne of reactor grade (3.6% U-235) uranium and 7.05 tonnes uranium depleted to 0.3% fissile U-235. In 1999, the USA had stocks of depleted uranium amounting to about 480,000 tonnes.

Fission Products

When a nucleus responds to collision with a neutron by splitting, this is called nuclear fission, and there are usually two fragments of about half the atomic weight of the nucleus that was split, and some more neutrons. These are called fission products. Many of them are so unstable that they decay in under a second to become another atomic nucleus and a small particle such as a helium nucleus or an electron, these being also called alpha and beta radiation. The fission products and further decay products are correctly called nuclear waste. Neutron capture products, being usable as fuel, should not be so classified.