h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

There's a serious misconception at work whenever people say something will be "radioactive for XX thousands of years." They generally assume that means that the waste that is dangerous now will be at the same level of danger for that XX thousand of years.

What people think of as nuclear waste is usually spent fuel rods. Inside those fuel rods are a number of different isotopes, all of which have different levels of radioactivity and different half-lives.

When I worked at a nuclear power plant, the rad waste guys had a rule of thumb: the hotter an isotope is, the shorter its half life. It makes logical sense, if you think about it.

For example, if you had 1000 atoms of an element that had a half life of a minute, then the first minute 500 atoms would decay and emit 500 units of radiation (alpha particles, beta particles, what have you).

If you had 1000 atoms of an element that had a half life of a day, it would take a day to emit 500 radiation units. If its half life was 24000 years, then the rate of radiation emission would be very slow, indeed. In fact, plutonium 239, which is what all the fuss about, has a relatively low rate of radiation. Being exposed to Pl-239 would be more dangerous due to heavy metal poisoning than from its radioactivity. That scene in "James Bond: The World Is Not Enough," where the villianess holds a sphere of plutonium in her hand, is pretty accurate. A person can hold pure plutonium without immediate danger to him/herself.

There's a type of nuclear waste called "transuranics." Those are elements, like plutonium, which are not fission products. They are not daughter products of fission reactions; rather, they are elements which are transformed when they incidentally catch neutrons. Pl-239 is generated when Uranium 238 catches a neutron, becomes U-239, and then decays into Pl-239.

Transuranics are the longest-lived waste, but they comprise a small fraction of what makes up nuclear waste. The easiest way to get rid of them is to turn them into more reactor fuel. When they undergo fission, they generate fission products, which are much less long lived.

The fission product with the longest half-life is Strontium-90, with a half-life of 29 years. It would take about 10 half-lives for it to be considered safe, or about 290 years.

The information I present here came from my own experience and from a search for "nuclear isotope half life" on Yahoo!

Wampus