Nuclear waste is mostly even-numbered Uranium isotopes. Uranium 238 can be converted to plutonium for fuel (Fig. \ref{407549}).
0.7% of natural uranium is ‘fissile’. This is odd numbered uranium isotopes.
It is mainly the ratio of Uranium 235 versus Uranium 238.
A thermal neutron is a free neutron with a kinetic energy of about 0.025 electron volts with a speed of 2.2 km per second.
A fast neutron is a free neutron with and 1 Megaelectron volt with a speed over 14,000 km per second.
Hit Uranium 238 with a fast neutron and you have a good chance can get Plutonium 239.
Enriched uranium gets the uranium 235 from 0.7% to 5% or more. Highly enriched goes higher. 90% enrichment is bomb grade. There are probabilities and nuclear physics.
Although 235U and 239Pu are less sensitive to higher-energy neutrons, they still remain somewhat reactive well into the MeV range. If the fuel is enriched, eventually a threshold will be reached where there are enough fissile atoms in the fuel to maintain a chain reaction even with fast neutrons.
The primary advantage is that by removing the moderator, the size of the reactor can be greatly reduced, and to some extent the complexity. This was commonly used for many early submarine reactor systems, where size and weight are major concerns. The downside to the fast reaction is that fuel enrichment is an expensive process, so this is generally not suitable for electrical generation or other roles where cost is more important than size.
Another advantage to the fast reaction has led to considerable development for civilian use. Fast reactors lack a moderator, and thus lack one of the systems that remove neutrons from the system. Those running on 239Pu further increase the number of neutrons, because its most common fission cycle gives off three neutrons rather than the mix of two and three neutrons released from 235U. By surrounding the reactor core with a moderator and then a layer (blanket) of 238U, those neutrons can be captured and used to breed more 239Pu. This is the same reaction that occurs internally in conventional designs, but in this case the blanket does not have to sustain a reaction and thus can be made of natural uranium or depleted uranium.
Due to the surplus of neutrons from 239Pu fission, the reactor produces more 239Pu than it consumes. The blanket material can then be processed to extract the 239Pu to replace losses in the reactor, and the surplus is then mixed with other fuel to produce
MOX fuel that can be fed into conventional slow-neutron reactors. A single fast reactor can thereby feed several slow ones, greatly increasing the amount of energy extracted from the natural uranium, from less than 1% in a normal
once-through cycle, to as much as 60% in the best fast reactor cycles.
Given the limited reserves of uranium ore, and the rate that nuclear power was expected to take over
baseload generation, through the 1960s and 1970s fast breeder reactors were considered to be the solution to the world's energy needs. Using twice-through processing, a fast breeder increases the energy capacity of known ore deposits by as much as 100 times, meaning that existing ore sources would last hundreds of years. The disadvantage to this approach is that the breeder reactor has to be fed expensive, highly-enriched fuel. It was widely expected that this would still be below the price of enriched uranium as demand increased and known resources dwindled.