MOX is considered orders of magnitudes worse than the isotopes used in most reactors for a number of reasons. Here’s one reason: it contains plutonium, which has a half-life of 24,000 years. Plutonium is a by-product of nuclear energy production: it is found in the spent fuel rods of all nuclear reactors. It is sometimes added to new fuel rods is to save money and reduce the amount of waste they have to store. Learn some other nasty facts about DOX fuel in this article published by Time magazine.