Plutonium is the most radiotoxic radioactive element in spent fuel
The figure shows the relative contribution of the various constituents of spent fuel to overall radiotoxicity and the natural radioactive decay over time. With the exception of a short period of fifty years, which is still ongoing, plutonium is the most toxic. It will take 200,000 years for a spent fuel assembly to return to the same level of toxicity as the natural uranium from which it was made.