Thursday, December 6, 2007

Nuclear waste II

There are no lack of solutions for the nuclear waste issue. For example, U proposed yesterday in a comment in "Energy from Thorium":

"I started writing today about the issue of "spent" LWR fuel, which of course contains enough fissionable materials to very power a CANDU reactor. It has long been known that MSRs are excellent burners plutonium (Pu) and other actinides. There is no reason why a MSR could not be used to dispose of actinides from LWR "spent" fuel. At the end of the burn any remaining actinides can be seperated out of the salt mixure, and dumped back into the reactor at the beginning of the next cycle. "

"If a HWMSR design is used, the U235 can also be burned until the % of U235 remaining in the uranium salt is at depleted uranium levels. But the U235 does not have to be completely burned. In the mean time thorium can be breed in a blanket. The heat from the reactor can be used to generate electricity, and the U233 produced in the breeding cycle can be used to enrich the depleted uranium, which goes back into the LWR, with perhaps a little extra added U235 or Pu239 . Fission byproducts from both the LWR fuel cycle and the MSR operation would of course be removed from the molten salts, and disposed of. "

"The reactor would be more complex than a one fluid MSR, but the MSR and reprocessing system would not be more expensive than a LWR and a chemical reprocessing system, and many of the problems related to used LWR fuel would be solved."

We are thus lef
t with some thing like 6% of the uranium used in the two reactors would end up as radioactive "daughter products" of nuclear fission. When an atom of U233, U235, or U238 splits, it leaves behind two atoms of different elements. These atoms contain too much energy to remain stable, so over time rgey release radiation in a process that also changes them into an another element. Often the form of tat element or isotope is also unstable, so it in time releases more radiation and in tern changes into something else. Thus you have a chain of changes, each with radiation. Eventually you reach the point that this chain of changes leads to stable atoms.

A sinple example of nuclear decay involves the isotope carbon 14. The earth's atmosphere is constantly being struck by radiation from outer space, called cosmic radiation. Through interaction with the cosmic rays, nitrogen 14 is transformed into carbon 14. Carbon 14 is an unstable isotope. It emits an energetic electron, called a beta particle. The beta decay process does something strange to the carbon 14 atom, it changes it back into nitrogen 14. Nitrogen 14 is very stable unless it runs into cosmic radiation or a black hole.

Because carbon 14 emits radiation in the form of beta particles it is radioactive. And it is radioactive at a known and constant rate. In a period of 5730±40 years, half of all carbon 14 atoms will under go beta decay, and will change into nitrogen 14 atoms. So if you have 16 Carbon 14 atoms, in 5730±40 years, 8 will be transformed into nitrogen 14 atomes and 8 will still be carbon 14. In another 5730±40 years 4 more carbon 14 atoms will be transform into nitrogen 14, and so on until there is none left. The 5730±40 years is called the Half life of carbon 14. All radioactive isotopes have half lives. The more radioactive the isotope, the quicker the half life.

When U235 fissions in a reactor, it splits into two atoms,

The two new atoms are called daughters. One typical daughter product is Xenon 135. Xenon is not the sister of a mythical warrior princess. Xenon is a colorless, heavy gas, that is chemically inactive. Xenon 135 is produced as a byproduct of nuclear fission in reactors and is notorious among nuclear scientist because it poisons nuclear chain reactions. Xenon 135 is very radioactive, but this also means that it has a short half life. In fact the half life of Xenon 135 is a little over 9 hours, and decays into Caesium 135. Caesium 135 is weakly radioactive. It has a half life, of 2.3 million years, and then it decays by releasing a weak beta particle, and is transformed into barium 135. Barium 135 is stable. Because other, more dangerous, isotopes of Caesium are produced in chain reactions, Caesium 135 comming out of nuclear fuel is a candidate for long term disposal.

The problem of of nuclear waste then is not simple. Most of the marerial found in "spent" nuclear fuel os useful. Part of it is fissionable, and can and should be disposed of in chain reactions. Most of the fuel is U238, which can be reused in reactors. A small percentage of the materials coming out of reactors, are fission daughter products. Some fission daughter products have medical or industrial uses, others have short half lives and will eventually decay into a useful and stable isotope. Finally, some radio isotopes born directly or indirectly as the result of chain reactions, should be disposaed of on a long term basis.

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