Some nuclei are unstable as a result of an imbalance in the numbers of neutrons and protons in the nucleus. The radioactive decays end when stability is reached
Radioactivity can be natural or artificial: one important use of nuclear reactors is in making radioisotopes which have important medical applications.
Radioactive nuclei all have a half-life, the time in which half the nuclei in a sample decay. The half-life of any unstable nucleus can be determined.
More unstable nuclei have shorter half-lives: the half-life depends on the probability that a given nucleus will undergo fission within a given time.
Nuclear fusion involves two light nuclei being combined into a heavier nucleus with less mass than the original nuclei and releasing energy as a result.
Nuclear fission involves a heavy nucleus forming two nuclei lighter (in total) than the original nucleus and releasing energy equivalent to the lost mass.
As a general rule, the nuclei in the centre of the periodic table have less energy available because energy was released during their formation.
The mass deficiency at the end of a nuclear reaction is linked to the energy released in accordance with the much-misquoted "e equals mc squared".
A critical mass is an amount of fissile material formed so that each fission generates products (usually neutrons) that trigger, on average, one more fission.
The amount of fissile material needed to make a critical mass is least when the fissile material is in the shape of a sphere, as fewer neutrons escape.
A nuclear chain reaction requires a critical mass of fissile material in a small space, and control systems which need to be highly reliable, except in a bomb.
Beta particles are energetic electrons ejected from the nucleus during nuclear decay, and they indicate that a neutron has become a proton in the nucleus.
Alpha particles are the most massive form of radiation. Each alpha particle is made up of two neutrons and two protons, ejected from a fissioning nucleus.
Radioactivity involves the release of energy, and the release comes in three forms, originally simply called alpha, beta and gamma radiation.
Gamma radiation is a form of electromagnetic radiation, rather like X-rays, which is emitted as a way of losing energy during some forms of nuclear decay.
Nuclear energy produces no greenhouse emissions: the damage from continuing greenhouse emissions can be predicted, unlike the damage from nuclear reactors.
The energetic radiation coming from radioactive material can be harmful to living cells, depending on the radiation produced, and how close the source gets.
Most nuclear accidents have been caused by poor training and careless operation of facilities and operations by people who feel over-confident with their tasks.
Nuclear waste can be classified as high, medium or low-grade waste, depending on its half-life, the products of decay and how much of it there is.
Some nuclear waste will need to be stored safely for many thousands of years, while the radioactive products break down. It might harm our descendants, one day.
Burning fossil fuels to obtain the energy we all demand is now considered to cause global warming, and that will, without a doubt, harm our descendants, soon.
Some spent nuclear fuel rods can be recycled to produce new fuel rods. The recycling processes need to be managed and supervised with very great care.
Nuclear weapons bring a variety of technical problems in maintenance and storage as the fissile materials in them slowly decay, and need to be refurbished.
Burning one gram of hydrogen gas in the normal way with oxygen provides the energy that is needed to light a 100 watt bulb for about 40 minutes only.
If the same gram of hydrogen could be converted completely to energy by some form of nuclear reaction, it would power a 100 watt bulb for 56,000 years.
In 1939, Otto Hahn and Fritz Strassman bombarded uranium salts with thermal neutrons and found barium among the reaction products, indicating fission.
In 1939, Rudolf Peierls and Otto Frisch worked out the critical mass and theory of the uranium-235 fission bomb, with a critical mass of about 10 kilograms.
In 1976, Shlyakhter used samarium ratios from a 2 billion-year-old natural fission reactor in Gabon to show the laws of physics have not changed in that time.
In 1939, Teller, Szilard and Einstein, sent a warning letter to President Roosevelt about the possibilities of the atomic bomb, starting the Manhattan Project.
In 1932, Leo Szilard realized that nuclear chain reactions may be possible, and by 1934, he had filed a patent on the principles, and gave it to the War Office.
Early on, Frederick Soddy calculated that the energy liberated in the complete change of 28 grams of radium would be equal to that from burning 10 tons of coal.
This file is http://members.ozemail.com.au/~macinnis/scifun/splatsnuke.htm, first created on February 16, 2008. Last recorded revision (well I get lazy and forget sometimes!) was on February 16, 2008.