How long are old nuclear fuel rods dangerous?

Answer 1

Radioactive waste is actually the fumes that come off of uranium when we use it. It is placed deep in the earth's crust, so as not to harm humans. It lasts forever.

Update: u-238's half-life is 4.5 billion years. That means it takes 4.5 billion years for 1 lbs of u-238 to become .5 lbs and so on. But it never really goes away : ) hence "Matter cannot be created nor destroyed"-first law of thermodynamics

Answer 2

By 'waste' I mean high level waste which is contained in the spent fuel. The behavior of the fission products in reactor fuel is dependent on a number of factors;

1. The yield from fission

2. The effect of neutron capture whilst in operation

3. The half life of the radioactivity

4. The type and energy of the radioactivity

Clearly the amount of radiation per unit mass of any radioactive emitter is higher the shorter the half life, but then it decays more quickly than for long lived emitters. Since in practical terms the spent fuel is stored on site for several years at least, half lives of less than 1 year are not so important as those of say tens of years, so these medium life emitters are more important. Two of these are Strontium 90 at 29 years and Caesium 137 at 30 years.

In the much longer term, there are seven isotopes identified which will still be active after millions of years: Technetium 99, Tin 126, Selenium 79, Zirconium 93, Caesium 135, Palladium 107, Iodine 129. Caesium 135 for example has a half life of 2.3 million years.

Thus effectively you can say it stays dangerous for ever, or as long as humans are likely to exist, but the most dangerous parts will have decayed to only a small proportion of their original activity after a few thousand years. See Wikipedia article 'Long-lived fission products'

Answer 3

6,000,000 years is probably the clearest answer, in terms of how it is measured and what is meant by safe.

There are different standards to measure this. The best I have seen is a European criterion saying that radioactive waste is to be considered safe when it is no more dangerous, in terms of radiation, than naturally occurring uranium ore. (Of course, this begs the question of whether uranium ore is safe.)

Unfortunately we cannot look at that half life and use it, because that just means half is left. If we start with a ton of plutonium, at the end of the half life, we have half a ton.

The current view is of the US government seems to be that nuclear waste will be safe in a million years. I have not seen how this was determined

Answer 4

It depends on how much has been wasted but I'd say it would stay in the ground for the next 10 years for minimum and for a maximum would be for the next century (100 years)