How much more radioactive is waste produced by a nuclear power plant than the initially mined material

Question

Often people say that the radioactive material we use in nuclear plants just comes from the ground so we can just put the waste back in the ground and there's no change. This isn't fully true since nuclear plants produce radioisotopes that weren't in the original material we mined (that also wouldn't be produced if we just left the ore in the ground)... But how close is it it to true?

Say you have a nuclear plant that runs for 50 years before being decommissioned, if you put all the waste generated in the plant over its lifetime in one place and measured the sV at a specified distance, how much more would this be than if you just put all the ore that had been mined for this plant in one place and measured the sV at the same distance? How would this vary over time, i.e. would the waste from the plant decay faster/slower? How would the distribution of types of radiation vary? Does it depend largely on the type of plant? Is there anything else important that would change?

Answer 1

235U has a half-life of almost a billion years, so its decay rate is very low. This makes it relatively safe to be around. When it does decay, there is then a decay chain in which alphas, betas, and gammas are emitted. The alphas can't hurt you unless you're exposed internally. The betas are more penetrating but still usually not very harmful unless you're internally exposed. E.g., low-energy betas won't get through your clothing. The gammas a very penetrating.

Waste from a reactor contains a witch's brew of various neutron-rich isotopes, which tend to undergo beta and gamma decay. The half-lives vary a lot. So if you bury concentrated nuclear waste in one spot, the intensity of the radiation will be very high at first, because the short-lived isotopes have high rates of decay. Then as time goes on, those isotopes disappear, and all you're left with is the ones with longer half-lives and therefore lower decay rates.

So as an example, say you store the waste for 30 years behind the walls of the power plant where it was produced, which is what has been happening with a lot of US waste because Harry Reid got congress to renege on its promises to the nuclear industry about Yucca Mountain. After 30 years, you have nothing left of the isotopes with half-lives of minutes, hours, or days. The most intense radiation will be coming from isotopes whose half-lives are on the order of 30 years. The intensity will be up relative to the intensity of the original 235U by a factor of about a billion divided by 30, which is a big factor.

Making this even worse are the facts that (1) the 235U wasn't separated from the less radioactive 238U in the environment, and (2) the waste is putting out more of the more penetrating forms of radiation.

What would actually be pretty sane would be to build the Yucca Mountain facility.

Answer 2

This is something that can be calculated knowing the composition of the spent fuel, the decay constant for each isotope, and the "radiotoxicity" of each isotope. A typical graph looks something like this: Note that both axis are log scales.

In rough numbers, it takes about a million years before the radioactive waste is the same toxicity as the natural uranium ore. Note that initially, most of the radiotoxicity is due to the fission products because they are more radioactive, but also decay faster. At longer time scales, the radiotoxicity comes from the actinides.

If you can separate out the actinides, the radioactive waste is only more dangerous than natural uranium ore for about 270 years.