Reality → Energy → Nuclear energy → Fission
The splitting of a nucleus by bombarding it with neutrons is an artificial process [1] . The uranium isotope U-235 is the most fissile naturally occurring substance. When bombarded with a slow neutron, the nucleus splits and releases on average 3 fast neutrons and an energy of 200 MeV [2] . If a critical mass of fissile uranium is present, the released neutrons trigger a chain reaction that can be controlled at a constant level in nuclear reactors [3] . Usable energy is recovered principally from the kinetic energy of the recoiling fission products [4] . Safe long-term disposal of high level radioactive waste from nuclear power plants is a major unresolved problem [5] . Increased reprocessing of the nuclear waste could eventually reduce the amount for long-term disposal but is not being implemented due to controversy [6] .
Nuclear fission was discovered in 1938 by the chemist Hahn as an unforeseen result of his experiments to create transuranium isotopes by bombarding uranium with slow neutrons. Natural fission no longer occurs on Earth (the exhausted uranium mines of Oklo, Gabon, are the only known sites where natural fission may have occurred over a period of a few hundred thousand years between 1 and 2 billion years ago).
The energy released by the complete fission of 1 kg U-235 is equivalent to 3,000 t of coal (based on 0.1% mass deficit of fission products converted to energy according to the formula E = m c2). In a nuclear reactor only about half of the fissile fuel is burned and therefore the effective equivalent of 1 kg U-235 (contained in about 25 kg of reactor-grade uranium) is 1,500 t of coal.
Today's existing nuclear power plants are using thermal reactors that slow down (moderate) the fast neutrons produced by fission to achieve a stable chain reaction of the fissile U-235. The most common and effective moderator is water (the hydrogen protons of water molecules have practically the same mass and size of neutrons, thereby effectively reflecting (not absorbing) and slowing down ('cooling') fast neutrons. About 25 % of reactors are graphite-moderated and 5 % are heavy water moderated (the latter type is known as CANDU reactor). The chain reaction is controlled by neutron-absorbing control rods (commonly made of cadmium or high-boron steel).
The kinetic energy of the fission products amounts to more than 80% of the total energy released by a fission event. In a pressurized water reactor (the most common reactor), the nuclear energy is transferred to the moderating (cooling) water of the primary closed cycle and then through heat exchangers to the second water/steam cycle which drives turbines and generators in the same way as a conventional thermal power plant.
As the fuel rods ‘burn‘ their relatively small content of fissile U-235, they accumulate radioactive fission products and transuranium isotopes as well as ‘surplus‘ neutrons that hinder further fission. The highly radioactive and incompletely spent fuel is replaced after 3-6 years of operation. Half-lives of the various waste products range from hours to millions of years. The spent fuel rods are cooled and stored 10-20 years underwater in spent fuel pools at the reactor site, and are then sent for reprocessing or dry cask storage (CASTOR). There is no consensus on the selection of deep geological sites for final storage (even experts doubt whether it is possible to store the waste safely underground over thousands of years).
Reprocessing (performed in France, the UK, Russia, and Japan) is tarnished by radioactive pollution, health and safety issues, and a history of plutonium production for weapons (fast neutrons generated by the fission of U-235 are absorbed by U-238, which thereby transmutes by beta decay into Pu-239, the primary fissile isotope of nuclear weapons). Fissile uranium and plutonium can be recovered in a messy and expensive chemical process for recycling as components of MOX fuel. Fast reactors run as breeders would be able to burn most of the nuclear waste, but related reprocessing issues and poor results of the Superphoenix would have to be overcome.