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Radioactive stuff and what is meant by it having a "half life"
If you're wondering what a HALF LIFE is, and why scientists talk about atomic elements having a half life rather than a whole life, here are some answers.
Stuff is made of atoms. Different stuff is made of different atoms. Most types of stuff (gold, oxygen, iron, helium, etc) have atoms which are stable and will continue to remain the same even if they are kept a very long time. In contrast, radioactive elements (such as plutonium, uranium, radium, etc) have atoms which are unstable.
SPLITTING THE ATOM
Radioactive elements' unstable atoms will SPLIT. They will do this at random, on their own, without any persuasion. It's as if the radioactive atoms are too big for comfort and just have to split. Splitting of the atom occurs spontaneously, and randomly. In general a few small bits come off. The small bits are the radioactivity and what's remaining is an atom which is a different element. So, what you see is one kind of stuff changing into a different kind of stuff, and giving off particles at the same time. The particles of radioactivity coming off can be detected using a Geiger-counter which makes that familiar clicking sound.
LEAD TURNS TO GOLD?
It would be handy if lead turned to gold, but it doesn't usually go that way, as lead isn't unstable. What usually happens is that a heavy radioactive element turns into another radioactive element which then turns into lead.
Each different radioactive element has its own characteristic and will turn into another material at a particular rate. The timescale at which this happens is known as "the half life".
Why is it HALF LIFE?
Supposing there is a radioactive element Frankensteinium which has a characteristic of changing into lead over a period of years and it is described by scientists as "having a half life of one year". That would mean that after one year, half of the Frankensteinium has turned into lead. So, if there was one kilogramme of Frankensteinium to start with, then a year later there would be only 500 grammes of Frankensteinium, together with 500 grammes of lead.
The misconception which is commonly believed by folk is that after two years it would all have turned to lead and there would be no radioactive material remaining. It's not so. What happens in the second year is very similar to what happened in the first year. Half of the Frankensteinium turns to lead. So after two years there is 750 grammes of lead and 250 grammes of Frankensteinium yet to turn. After another year, half of the 250 grammes remaining will turn into lead leaving only 125 grammes. This goes on indefinitely, and for each year (each half-life), half of the remaining material will turn into lead.
The amount of radioactivity given off reduces, as 125g of Frankensteinium gives off less split off material than the 1000g which was there to start with.
After ten half lives (ten years in the case of the hypothetical element Frankensteinium), only one gramme of the radioactive element remains, the other 999g having turned to lead.
After twenty half lives, only one millionth of the original material still remains.
Real atomic elements
For actual atomic elements the situation is more complicated. Materials such as plutonium, uranium, and thorium change into other elements which then turn into other elements, in a sequence. Also, there's more than one kind of each element. These different types (isotopes) have different radioactive characteristics. Also, the different materials have widely different half-lives. Some are several minutes, some several years, and some have a half life of millions of years, or a tiny fraction of a second. See extended periodic table. Carbon14 is a naturally occurring radioactive isotope of carbon whose half-life is 5,700 years. This makes it very handy for carbon dating, ie telling how old things are.
How long before it's all gone?
So, if you are wondering how long a heap of radioactive waste is going to be around, the answer is generally "far too long", as the stuff will be a continuously changing mixture of stuff which will go on atomically splitting and changing (transmuting) for a very long time before it eventually turns to lead. It's an ecological problem.
Although in theory the radioactive material will never have "all gone", there is a practical way of thinking about it based on what an "empty bottle" is. If you have a bottle of drink, and you pour it all out into glasses, so no more comes out, then is that an "empty" bottle? If you think it is practically empty, even if there's a drop left in it, then it's possible to consider some point at which radioactive material has ALMOST all gone.
However, if the half life of the radioactive material is "70,000 years" (a guess placed on modern radioactive waste being produced from nuclear power stations), then it's going to be around for a very long time whichever way you think about it. Certainly much longer than the lifespan of a nuclear industry that can't be sure of balancing its accounts in 50 years time.
Other references: ecology, science, electricity companies, alternative fuel, Elements - how many different types of atom are there?, Periodic Table, Extended Periodic Table, misconceptions
Don't get me wrong; I'm not saying "don't have a nuclear industry". In the 1980s I'd have said that, but now the world politics has changed there's more of a situation that all small countries need to have a nuclear industry so they can have atomic bombs as a deterrent against being invaded by aggressive powers. This will be the case until the silly warmongering stops. Update 2009: It is looking as if there will be an improvement, as one of the previous worst offenders in belligerence has elected a more reasonable leader.