DISQUS

enantiomer2000 · 1 year ago

I like how it is safe after a few hundred years of disposal.

nextbigfuture · 1 year ago

If you burn all of the uranium and plutonium (those are the things with long half lives.) Everything left has a half life of 30 years or less. So after thirty years all of the material is either half or less (half life means half is gone after so much time) After three hundred years you are left with 0.1% or less of the original radioactive. Ten halvings or more.And the remainder is at about the level of background radiation. Thus what is left is mostly inert and just as safe as everything around.

Bill_Woods · 1 year ago

"If you burn all of the uranium and plutonium (those are the things with long half lives.) Everything left has a half life of 30 years or less."

That isn't quite true, though the long-lived fission products are much less important than the actinides.

"Only seven fission products have long half-lives, and these are much longer than 30 years, in the range of 200,000 to 16 million years. These are known as long-lived fission products (LLFP). Two or three have relatively high yields of about 6%, while the rest appear at much lower yields."
http://en.wikipedia.org/wiki/Long-lived_fission...

That's why the graph of the radioactivity of fission products over time has an L-shape.

http://www.world-nuclear.org/images/info/hlw.gif
(from http://www.world-nuclear.org/info/inf60.html)

nextbigfuture · 1 year ago

The first three have comparable halflives, between 200 thousand and 300 thousand years; the last four have longer halflives, in the low millions of years.

1. Technetium-99 produces the largest amount of LLFP radioactivity. It emits beta particles of low to medium energy but no gamma rays, so has little hazard on external exposure, but only if ingested. However, technetium's chemistry allows it to form anions (pertechnate, TcO4-) that are relatively mobile in the environment.

So the Technium and the Iodine have some risks but not from the radiation so much as actually getting it into your body and tissues. Just like mercury and arsenic from coal which have no halflife. Some things are just poisons.

In total, the other six LLFPs, in thermal reactor spent fuel, initially release only a bit more than 10% as much energy per unit time as Tc-99 for U-235 fission, or 25% as much for 65% U-235+35% Pu-239.

====So if do not eat the Technium you are OK. and the other six release 10% of the radiation of the Technium.

2. Tin-126 has a large decay energy (due to a following short-halflife decay) and is the only LLFP that emits energetic gamma radiation, which is an external exposure hazard. However, this isotope is produced in very small quantities in fission by thermal neutrons, so the energy per unit time from 126Sn is only about 5% as much as from 99Tc for U-235 fission, or 20% as much for 65% U-235+35% Pu-239. Fast fission may produce higher yields. Tin is an inert metal with little mobility in the environment, helping limit health risks from its radiation.
3. Selenium-79 is produced at low yields and has weak radiation. Its decay energy per unit time should be only about 0.2% that of Tc-99.
4. Zirconium-93 is produced at a relatively high yield of about 6%, but its decay is 7.5 times slower than Tc-99, and its decay energy is only 30% as great; therefore its energy production is initially only 4% as great as Tc-99, though this fraction will increase as the Tc-99 decays. 93Zr does produce gamma radiation, but of a very low energy, and zirconium is relatively inert in the environment.
5. Caesium-135's predecessor xenon-135 is produced at a high rate of over 6% of fissions, but is an extremely potent absorber of thermal neutrons (neutron poison), so that most of it is transmuted to nonradioactive xenon-136 before it can decay to caesium-135. If 90% of 135Xe is destroyed, then the remaining 135Cs's decay energy per unit time is initially only about 1% as great as that of the 99Tc. In a fast reactor, less of the Xe-135 may be destroyed.
135Cs is the only alkaline or electropositive LLFP; in contrast, the main medium-lived fission products and the minor actinides other than neptunium are all alkaline and tend to stay together during reprocessing; with many reprocessing techniques such as salt solution or salt volatilization, 135Cs will also stay with this group, although some techniques such as high-temperature volatilization can separate it. Often the alkaline wastes are vitrified to form high level waste, which will include the 135Cs.
Fission caesium contains not only 135Cs but also stable but neutron-absorbing 133Cs (which wastes neutrons and forms 134Cs which is radioactive with a halflife of 2 years) as well as the common fission product 137Cs which does not absorb neutrons but is highly radioactive making handling more hazardous and complicated; for all these reasons, transmutation disposal of 135Cs would be more difficult.
6. Palladium-107 has a very long halflife, a low yield (though the yield for plutonium fission is higher than the yield from uranium-235 fission), and very weak radiation. Its initial contribution to LLFP radiation should be only about one part in 10000 for U-235 fission, or 2000 for 65% U-235+35% Pu-239. Palladium is a noble metal and extremely inert.
7. Iodine-129 has the longest halflife, 15.7 million years. Initially it has only about 1% as intense radioactivity as Tc-99. However, radioactive iodine is a disproportionate biohazard because the thyroid gland concentrates iodine. I-129 has a halflife nearly a billion times as long as its sister isotope iodine-131 which is a hazard from nuclear explosions, and a smaller decay energy, so is only about a billionth as radioactive per unit mass.

nextbigfuture · 1 year ago

Details on long life fission products and the liquid flouride thorium reactor

http://thoriumenergy.blogspot.com/2008/04/long-...

Long-lived fission product radioisotopes are not a threat to the survival of the human species, but the risk involved is their disposal should be taken seriously. Some long-lived radioisotopes can be altered into non-radioactive isotopes by neutron bombardment. Other long lived radioisotopes can be disposed of by vitrification and deep burial.