Reactors primarily use uranium, but plutonium is routinely used in the form of mixed-oxide (MOX) fuel (about thirty reactors in Europe, at least). One reason is

Reactors primarily use uranium, but plutonium is routinely used in the form of mixed-oxide (MOX) fuel (about thirty reactors in Europe, at least). One reason is to spend the plutonium from decommissioned nuclear weapons. Using MOX in conjunction with thorium is also being tested to improve the void coefficient, making reactor operation more stable and safe.