Nuclear waste is a term used to describe radioactive waste. Therefore, to understand what constitutes nuclear waste, we must first describe what 'radioactive' means. To be 'radioactive' means to emit ionizing radiation and/or particles. Radioactive substances are therefore substances that emit ionizing radiation and/or particles, and radioactivity is the process of emitting ionizing radiation and/or particles. We must also understand what 'ionize' means. To 'ionize' means to convert an atom, molecule, or substance into an ion or ions. Consequently, nuclear waste refers to the unwanted/unused/discarded substances/materials that contain elements that emit radiation/particles that convert an atom, molecule, or substance into an ion or ions. 

Radioactivity is a spontaneous process, but it can also be induced. The processes involved in radioactivity include nuclear fusion and nuclear fission. 

Nuclear fusion refers to a reaction in which two or more atomic nuclei come close enough to form one or more different atomic nuclei and subatomic particles - neutrons or protons - and the difference in mass between the products (the new substances formed) and the reactants (the substances that reacted) is manifested as the release of large amounts of energy. Nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts i.e. lighter nuclei. The fission process often produces free neutrons and gamma protons, and very large amounts of energy. 

In both nuclear fusion and nuclear fission, nuclear radiation is emitted. The 4 most common forms of nuclear radiation emitted are alpha particles, beta particles, gamma rays, and neutrons. Alpha particles are made up of 2 protons and 2 neutrons, i.e. the helium nucleus. Beta particles are electrons i.e. 1 beta particle = 1 electron. Gamma rays are photons emitted from the nucleus. Gamma rays are electromagnetic - i.e. they have electric and magnetic properties - and they have the smallest wavelength and the most energy of any other wave in the electromagnetic spectrum. 

Substances undergoing radioactive decay reduce in amounts/value. This is to mean that a substance that continually undergoes radioactive decay will decrease and decrease until it's all gone. Half-lives are a measure used to determine how long it will take for the substance to reduce to half its original value. For example, oxygen-14 has a half-life of 1.177 minutes. This is to mean that if you have 10g of oxygen-14 that is undergoing radioactive decay, after 1.177 minutes, the oxygen-14 will have reduced to 5g. After another 1.177 minutes, it will have reduced to 2.5g. This goes on and on until the oxygen-14 is completely depleted. Half-lives, therefore, are very important when it comes to nuclear waste. Half-lives tell us how long it will take for the waste to stop producing harmful radiation, and when the waste will be completely depleted. Some substances have very short half-lives, e.g. hydrogen-7 has a half-life of 23 yoctoseconds - yocto means one-septillionth in the short scale and one-quadrillionth in the long scale. Others have very long half-lives, e.g. tellurium-128 has a half-life of 2.2 yottayears - yotta means a septillion in the short scale and a quadrillion in the long scale. Therefore, depending on the substances present in the nuclear waste, it may take seconds, minutes, days, weeks, months, a couple of years or even millions and millions of years for the waste to stop producing harmful radiation and for it to be completely depleted.

Nuclear waste is gotten from numerous artificial sources, all of which apply nuclear technology i.e technology that applies nuclear reactions.These include nuclear weapons, nuclear power (electricity production from nuclear energy), nuclear medicine, industrial radiology, nuclear well logging, nuclear gauges, radioactive tracers, radioluminescence, betavoltaics, tritium illumination, and food irradiation. Nuclear waste also occurs naturally from naturally-occurring radioactive substances. Naturally-occurring radioactive materials (NORMs) can also be produced as a by-product from some petroleum wells.

The danger of nuclear power lies in the emission of ionizing radiation. This is why nuclear technology, and nuclear waste, must be handled with utmost care. Everything, living or nonliving, is made up of matter. Once ionizing radiation, which is also matter, comes into contact with other forms of matter, the radiation will ionize said form of matter thus altering its natural state. In nonliving things, this could mean that the ionized matter will now become radioactive, leading to a nonending chain reaction in which more and more things get ionized and therefore get radioactive. It could also lead to explosions, e.g. in power plants exposed to high radiation. Magnetic things exposed to radiation become magnetized, and magnetic fields have numerous bad effects as well. For example, magnetic fields cause haywire in electric systems, and this means anything from watches running faster, your computer's hard drives, as well as your memory cards, getting erased, planes falling from the sky, and total systems failure in irradiated institutions. In living things, high magnetic fields cause biological failure. Ionization in living things leads to imbalances such as electrolyte imbalance and disruption of cell division processes. This leads to numerous problems, one of them being cancer. Cancer is basically the uncontrolled growth of cells which comes as a result of disruption of the cell division process, as well as bodily imbalances. Nuclear power is still in use, however, because very form of energy has its dangers, and with proper handling, these dangers can be avoided. Furthermore, as you have seen above, nuclear power has very important uses. Nuclear plants adhere to very strict regulations to prevent any nuclear accidents - nuclear accidents are usually as a result of ignoring guidelines - and to minimize outside exposure to ionizing radiation. They are also subject to strict waste disposal guidelines. Nuclear energy is perhaps the only form of energy that is directly regulated by the government. The International Atomic Energy Agency, IAEA, was created in 1957 to internationally oversee the peaceful use of and to inhibit the militarization of nuclear energy.

1. Conserve Energy.

2. What Is Nuclear

3. World Nuclear Organisation.

4. Wikipedia