A radioactive nuclide or radionuclide is an unstable nuclide and therefore degenerates emitting ionizing radiation. Although some physicists sometimes use the word radioisotope vulgarly to designate it, it should be noted that the strict or formal language of the physics and technology of nuclear energy is wrong, since a nuclide and an isotope are not the same.
When a radionuclide emits radioactivity it reaches a more stable state, which requires less energy than before and, in general, transforms into another different nuclide (or in the same one, but less excited, if it has emitted gamma radioactivity), which can also be radioactive or not be radioactive.
This radioactive process occurs in principle spontaneously, but the human being has learned to provoke it artificially. In either case, the resulting radioactivity has exactly the same characteristics.
Description of a radionuclide
Radionuclides are characterized by having a finite half-life, which can range from small fractions of a second to thousands of years. In fact, some of them have such a long half-life that it has not yet been possible to quantify them experimentally and there are even those that had been considered, and for certain practical, stable applications. Of the currently known nuclides there are ninety theoretically stable and two hundred and fifty-five that have not been observed to disintegrate.
On the other hand, there are almost double, about six hundred and fifty, which if they have been observed radioactivity and have a half-life of at least one hour. On Earth about three thousand radionuclides with a half-life of more than an hour are known, most of which (about 90%) are anthropogenic (produced by humans), about two thousand four hundred of half-life less than one hour and still others so unstable that their average life is very short.
Use of radionuclides in nuclear technology
Radionuclides are applied to the technology of nuclear energy to obtain electrical energy, in the industry (quality controls, etc.), medicine (radiotherapy, etc.) and nuclear weapons (basically to the propulsion of vehicles and tools for kill).
It should be remembered that radionuclides of natural origin, such as uranium or plutonium, exist in finite quantities on Earth, so they must be used in a sustainable manner. On the other hand, its use generates radioactive waste, which can be very dangerous and for which the only treatment that is usually done is to cover them until their radioactivity is close to the natural one.
The possible treatments for which it would take more than thirty years to do it (all spent nuclear fuel from nuclear power plants and for military purposes, for example) are still in the theory, research or experimentation phase.