Half life

Atoms will always have a set number of protons that will never change, this is known as the atomic number. However, the neutron number can change. When it does the atom becomes an  isotope. Every element has isotopes but only a couple are stable. The unstable ones ae called radoiactive isotopes. They emit radioactive sorces in an attempt to become stable. This process is natural but completely random. The activity can be measured using a Geiger counter, which will tell you how many atoms it loses per second. 

The radioactive decay is random and cannot be predicted. But you can find out the half life by counting the amount of time it takes for half of a nuclei to decay. Activity is measured in becquerals (Bq). One Bq is one decay per second. Half life can last milliseconds to millions of years. Some are more unstable so decay faster. 

If we look at the ratio of undecayed nuclei to the decayed nuclei we can get an idea of how long a radioactive substance has been decaying. This is called the net decline. E.g If a pure sample of uranium-238 starts decaying, it'll become thorium-234. Over 1 half life it'll decay by 50%. Then on the second half life it'll increase to 75% and so on. The older the sample, the more decayed it is. So if we look at the ratio we can get a good idea of how old the sample is.

Nuclear waste can be bad for the enviroment. Waste is disposed depending on how radioactive it is and how long the half life is. Low radioactive levels on nuclear substances can be can be encased in concrete until it gets to a safe radioation level. But high radioation levels can stay radioactive for thousands of years. So they can't just be cased in concrete until they're safe. So some idiot that has no respect decided to shove it underwater for 20 years before storing it underground in concrete.