Nuclear Power plant applications.

[MSimon]

How good are your eyes? This graph displays the ratiotoxic inventory of PWR spent fuel over time. Most of the radiotoxicity comes from plutonium except in the first three hundred years. More comments on this page.

I don't get why the uranium inventory rises after 1,000 years.

How does all this compare to moving the population from coastal areas to Denver?

[olivier]

Radiotoxicity, on the y-axis, is expressed in Sieverts per metric ton of initial heavy metal. Thus the rising uranium curve does not reflect the proportion of uranium by mass but by activity. I do not have an evolution code at home to do a simulation but, at first sight, the increasing uranium activity could be the result of plutonium alpha-decay (several Pu isotopes have a half-life in the 5,000-25,000 yr range). A nuclear chemist could correct me or be more precise (anyone around?).

[JohnP]

I think the most important idea to get across with nukes is that there is no such thing as "waste".

It's all the same "green recycling" mantra. The main trick is economics.

I've read for a long time about IFR's http://en.wikipedia.org/wiki/Integral_Fast_Reactor and such, but isn't there still a threshold below which the radioactivity of the material just isn't enough to be useful for fuel?

[drmike]

A radioisotope thermoelectric generator (RTG) can be used instead of nuclear fuel with neutrons. Once the heat gets below boiling of water, then yes, it's probably too low to use as fuel.

After that it can be used as tracers. Lots of industrial and medical applications require the ability to find out where things go, and radioactive tracers of very low level are very useful. It's a problem of chemistry to extract the different elements, so energy sources are important.

Once fission occurs, the remaining elements can not be used as fuel within the reactor. So the fuel has to be reprocessed. The nice thing about the IFR is that the fuel is very easy to reprocess compared to the ceramic rods used in LWR's and BWR's. I got to work with some of the guys who originally designed the IFR at Argonne, so I'm kinda biased in its favor.

[rnebel]

Olivier:
I believe that Pu239 decays into U235. I don't know this, but I suspect that's why the uranium content rises.

[olivier]

Spent fuel contains a significant amount of Pu238, 239, 240 and 241 but I do not know the exact proportion.
I did some wikipedia search:
Pu239: 24,000 yr => U235 :700,000,000 yr
Pu238: 88 yr => U234: 246,000 yr
Pu240: 6500 yr => U236: 20,000,000 yr
Pu241: 14 yr => U237: 6 days
Now, because the uranium curve decreases after 200,000 years and because U234 is more active than other isotopes, my intuition says U234 is predominant. But that is intuition, not science. That's not so important but it is a good exercise!

[Brent]

I'm a little late to this discussion, but perhaps a chart of the nuclides would help predict decay products. At least it is an idea. I found an Interactive Chart of the Nuclides from the National Nuclear Data Center at BNL. There is a decay radiation search that may be performed from this page that confirms that Pu239 decays to U235. I suppose it could be done for all the other isotopes as well.
see:http://www.nndc.bnl.gov/chart/

Here is a quote from the website description:
"Using the Chart of Nuclides, it is possible to search for nuclear level properties (energy, half-life, spin-parity), gamma-ray information (energy, intensity, multipolarity, coincidences), radiation information following nuclear decay (energy, intensity, dose), and neutron-induced reaction data from the BNL-325 book (thermal cross section and resonance integral)."