Langbein said:
It's one basic question about nuclear disposal that I have never understood.
Nuclear "fluel" like Uran, etc comes from nature - right ?
The way nuclear fuel is produced is by increasing the consentration of nuclear material - right ?
As the nuclear fuel is used and energy is taken out should'nt the amount of redioactive energy be less than it was at the starting point ?
While burning radioactive fuel, and producing energy from this, should'nt the amount of radioactivity in the world be less and not greater ?
Why is is not possible in some way to reverse the radiactive fuel back to the original materiel after it has been used ?
Why is it like that radioactive stuff that has been used for energy production is very dangerous, while beeing in the nature without having been used for energy production, it is not dangerous.
When producing nuclear energy, you are taking something out of the nature, radioactive fuel that is concidered to be not so dangerous, and when you "burn it" and the energy and the radioactive content is less, then it is more dangerous.
How can this be ?
Many good questions. One could also ask - why are some isotopes radioactive and others not?
Firstly, why are radioactive materials bad/dangerous? Well, it has to do with the ionizing radiation emitted when a nucleus decays. Alpha (nucleus of He), beta (high energy electron, and similar positron, which is a positive electron), and gamma (high energy photon) can damage cells by destroying molecules by disrupting atomic bonds. That is why radioactive materials are controlled, i.e. isolated from the environment at an appropriate distance and/or behind appropriate shielding. I should point out that alpha decay is restricted to the heavy elements, with Bi effectively being the lightest element to have an isotope undergoing alpha-decay (although Pb-210 may on very rare occasions undergo alpha decay). http://www.nndc.bnl.gov/chart/ (use Zoom 1 to see details of individual nuclei)
It is true that uranium is mined from nature and processed and concentrated to use in nuclear reactors for energy generation. By mining and processing, we put uranium in a more chemically reactive form that would make it easier for the U to get into the environment without additional barriers such as metal cladding and various storage or containment systems.
As to why we create more reactivity with fission, one must realize that the fission process creates two nuclei from each fission, and we call these two new nuclei
fission products. Most fission products are radioactive and will emit beta particles (or positrons) and gamma rays, by which the nucleus
decays to a lower energy state. When beta (or positron) particles are emitted, the nuclear charge (Z) changes, and so therefore does the nature of the nucleus (element). Gamma decay simply reduces the nuclear energy, without changing the nuclear species.
Radioactivity, by that I mean rate of decay, depends on the half-life of the nuclide. The longer the half-live, the lesser the rate of decay, and therefore for a given number of atoms, the lower the level of radioactivity. The two prinicipal isotopes of U have long half-lives: U-235 (700 million yrs) and U-238 (4.47 billion yrs), so they have very low activity. Fission products have half-lives on the order of fractions of seconds to millions of years, so some are highly radioactive while others are much less so. The good news is that the shorter the half-life, the faster the decay rate, so the radioactivity decreases with time.
Let's say a given isotope has a half-life of 1 second. In 10 half-lives, the number of nuclei is reduced by a factor of 2
10 or about 1000. In 20 half-lives, the number of nuclei is reduced by a factor of 2
20 or about 1 million, and in 30 half-lives it is about 1 billion, and so on. That's good for short half-lived nuclei. However, some nuclei have half-lives on the order of 100's or 1000's of years, so in order to keep them out of the environment, they must be isolated in an inert containment systems for 10's or 100's thousands of years - and that is a key issue at the moment for the Yucca mountain repository. How do we guarantee the isolation for thousands of years - since man made structures are only hundreds or a few thousand years old. Well - we put the isolation system in a geologic formation that has been stable for the last million years, and surround by a man-made system that itself is relatively stable/inert.
