Radioactivity is an imperfect descriptor of radioactive risks. It makes no allowance for the major difference in harmfulness between radioactive atoms, or for the nature of the emitted radiation and the exposure conditions.
For example, alpha rays from plutonium contain approximately 1,000 times more energy than beta rays from tritium. Alpha rays are highly toxic to living matter. However, alpha rays are blocked by the thickness of a sheet of paper, and are totally harmless in case of external exposure.
How to realistically assess risk? In principle, all reasonable precautions are taken to ensure that radioactive atoms in spent fuel assemblies and radioactive waste packages remains trapped inside them forever, or at least until they become harmless. They should almost never come into contact with living tissue and the most highly-penetrative rays will not reach us after waste is buried.
The danger relates to the tiny minority of radioactive atoms that may migrate from the waste into the biosphere, contaminating it and presenting the risk of ingestion or inhalation by our distant descendants. The risk posed by a radioelement depends on the atom's mobility and toxicity. Plutonium and minor actinides, which are generally alpha emitters, are by far the most toxic; fortunately, these heavy nuclei are very immobile. Fission products are more mobile, in particular in species such as iodine and caesium, and to a lesser extent technetium.
As it is not possible to predict the quantity of radioactive atoms ingested in the distant future, physicists and engineers adopt the worst-case assumption that all such atoms are ingested. They work with a potential radiotoxicity indicator.
The decrease in potential radiotoxicity over time mirrors the decrease in activity. It reflects the change in risk in case of ingestion of the most radiotoxic elements: plutonium and minor actinides. However, this risk should not be confused with the actual risk, as it does not take the mobility of the relevant radioelements into account.
Vitrified waste from which the plutonium has been removed will consequently be less radiotoxic than untreated fuel. This is one of the arguments in favour of reprocessing, subject to reusing the plutonium elsewhere. Fuels may also be compared from a radiotoxicity perspective. For example, spent MOX fuel will be more toxic than spent uranium fuel, as it contains more plutonium and minor actinides.
Access to page in French