A thing’s half-life is how long until only half of it is left. It’s used
in radioactive decay, medicine, and many other things. Remember:
Hazards in radiation are proportional the rate at which energy comes out.
Short half-lives imply energy is coming out quickly, and are the primary hazard.
Long half-lives imply energy is coming out very slowly, and are less
of a hazard. For example, you can hold long half-life isotopes of natural
uranium in your hand without accumulating a hazardous dose.
In the extreme, an infinite half-life would mean energy never comes out, representing zero radiological risk.
This graph depicts a single nuclide with its single half-life decaying to stability. Nuclear
waste is typically composed of thousands of different nuclides that
each have their own unique half-life, and often decay to other radioactive daughter
nuclides before making it to stability.