I'm trying to understand the nuclear waste problem, and I've read enough to know that even the 10,000 years that the Waste Isolation Pilot Plant is licensed for is far too short to eliminate the risk posed by spent nuclear fuel.

This table on Wikipedia gives some good numbers on the biggest problem isotopes. It shows that up to 7% of the fission products are Caesium-135, which has a has a half life of 2.3 million years and high bioavailability.

It gives numbers on the decay energy of the isotopes (269 KeV for 135Cs), but I'm not sure how to convert that to something familiar like Sieverts.

Here's a specific question: How radioactive is spent nuclear fuel from a PWR after 10,000 years, as a percentage of the initial (let's say 30 year old) radioactivity? To resolve any ambiguity, let's use Sieverts to compare. How about in 1 million years? How long until it only emits, say, twice the normal background radiation?

I also know that there are a lot of other factors influencing how dangerous the waste actually is, like how mobile the elements are in the environment and their bioavailability. I'm interested in any mitigating factors like that for the major contributors to the radioactivity at different points in the future.

  • $\begingroup$ It is a very complex time dependent radiation behavior, depending on many factors including initial enrichment and burn-up when removed from the reactor. From a safety perspective, while WIPP may not be ideal (and was NOT intended for spent fuel - that is Yucca Mountain), it would be more ideal than leaving all that fuel lying around near all the reactors. $\endgroup$ – Jon Custer Jul 2 at 21:21
  • $\begingroup$ @JonCuster Oh, I guess since Wikipedia gives numbers, I was hoping that the output of, say, most US power reactors was consistent enough to say something about its makeup. Also, I thought WIPP was for all waste from weapons programs, which would include a lot of spent fuel from plutonium production reactors. $\endgroup$ – Nick S Jul 2 at 21:29
  • $\begingroup$ WIPP is not certified for fuel of any kind. It is intended for low/mid level waste (gloves, equipment, etc.). There currently are no licenses high-level waste disposal sites in the US. As it says on WIPP's website: "WIPP was constructed for disposal of defense-generated TRU waste from DOE sites around the country. TRU waste consists of clothing, tools, rags, residues, debris, soil and other items contaminated with small amounts of plutonium and other man-made radioactive elements" $\endgroup$ – Jon Custer Jul 2 at 21:32
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    $\begingroup$ You might look through inis.iaea.org/collection/NCLCollectionStore/_Public/29/015/… and see if that helps. $\endgroup$ – Jon Custer Jul 2 at 21:46

The time evolution of fuel radioactivity is complicated:

enter image description here (Source of plot)

Cs137 is an intense source, but short lived. Cs135 lasts longer, but because of that it’s dose rate is smaller: there are fewer decays per second. Some isotopes, most notably Th229, actually increase as other decays produce them.

It takes a few thousand years for a ton of fuel to be no more radioactive than a ton of uranium ore. Of course, because there are now different elements involved the comparison isn’t exact: the waste migrates in ground water differently, etc.

  • $\begingroup$ This is just the sort of diagram I was looking for, complete with the uranium ore equivalent! If I'm reading this right, it seems by 10,000 years it's no more radioactive than the ore it came from. Can that be true? Are there any gotchas like that it's a more dangerous 4 TBq than uranium ore's 4 TBq? $\endgroup$ – Nick S Jul 3 at 3:23
  • $\begingroup$ Btw could you include a link to where you found that diagram? I looked up that source, but all I could find is this, which didn't have it: iaea.org/inis/collection/NCLCollectionStore/_Public/25/067/… $\endgroup$ – Nick S Jul 3 at 3:41
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    $\begingroup$ Added a link. There are small differences in the energy of different decays, hence in the conversion from Bq to Gy (or rads or REM). But those are small multiplicative factors. On a log plot that covers eight orders of magnitude, they make no difference. $\endgroup$ – Bob Jacobsen Jul 3 at 3:47

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