Which exact element makes Spent Nuclear Fuel dangerous?

Question

I understand that beta and gamma emissions are what makes the decay of a radioisotope dangerous. However, U-238, which is what SNF is mostly made of, doesn't emit gamma or beta particles frequently enough to be considered dangerous on its own. I also know that SNF contains many different elements, but which of these elements and which decays are actually responsible for SNF's radioactivity and danger?

Answer 1

This is a challenging question to answer, because it is difficult to precisely define a metric for ``dangerous''. Dangerous to whom, and under which scenarios and conditions? There are too many options to address them all. Even attempting to calculate equivalent dose estimates would be challenging because equivalent dose depends not only on the type of radiation, but also on the energy, type of tissue being exposed, bioaccumulation effects, etc. Instead of attempting to address all of that, I think it is much simpler, and practically just as interesting, to simply calculate which isotopes are responsible for the activity of the waste.

I was able to find a nice source here which provides estimates for the isotopic composition of fuel unloaded from an irradiated pressurized water reactor. The source mentions that 1000 kg of fuel pulled from the reactor will include roughly 30 kg of short-lived fission products, which "stabilise within around ten years, essentially leaving only approximately 0.1 kg of strontium-90 and 0.1 kg of caesium-137". Unfortunately, without any information about precisely which isotopes these products are, it is not possible to determine their activity exactly. In principle, we could estimate that "stabilise" probably means "reduce to non-significant levels" (relative to the activity of the other isotopes), and that such a thing probably happens in some number of half-lives, and use that along with the beginning mass to very roughly estimate a specific activity. If we estimate that their average half-life will be in the neighborhood of 2 years or so (allowing ~5 half-lives to pass over ten years), then the activity of the SNF will be completely dominated by these products (whatever they may be) for those first few years. It is unfortunate that this still does not answer your question over that time period, because the exact composition of these fission products is not clear (although we could make some educated guesses).

If we are willing to restrict ourselves to the period beginning 10 years after the fuel is removed from the reactor, then we are able to give much better answers. Note that this analysis ignores the effects of additional activity contributed by the radioactive daughters of these isotopes. Of the list provided by the source above, there are about six isotopes that have meaningful contributions to the SNF activity over ~1500 years: Pu-241, Pu-238, Am-241, Pu-240, Pu-239, and Cm-244.

I created the above chart by starting with the masses listed by the source above, and using the corresponding half-lives to track how much remained each year for ~1500 years. This was multiplied by that isotope's specific activity to determine the total activity it contributed, and that total was divided by the total activity of the SNF for that year (found by adding all isotopes' contributions) to determine the fractional activity contributed by each isotope to the total activity of the SNF at a given moment in time. You can see that the isotopes trade off between which is the dominant contributor as the years pass by. All the other isotopes mentioned in the above source have essentially negligible contributions to activity over the plotted time period. From this chart, we can see that the element responsible for most of the activity in every covered period of time is plutonium (there is a brief period where Am-241 is the dominant isotope, but even then plutonium is the dominant element).

Remember that this is a rough approximation which makes a great number of assumptions, and that although in this estimation plutonium is responsible for the majority of the activity, I would not attempt to make a claim about what "fraction of the danger" it represents.