Getting out from between “MOX” and a “Hard Place”
In 2003, an article in the Bulletin of the Atomic Scientists, Between MOX and a Hard Place, described the difficulty of disposing of the vast amounts of weapons-grade plutonium and uranium left over from the Cold War. “Weapons-grade” material generally means plutonium that consists predominantly of the isotope plutonium-239, and uranium that consists predominantly of the isotope uranium-235.
The plan is to “blend” these materials with standard uranium, and form what is called in the nuclear industry “MOX”, for “mixed oxides of uranium”. Then the MOX fuel would be introduced into typical light-water nuclear reactors as a fuel. In the course of power production, the Pu-239 and U-235 would fission and release neutrons, but due to the poor performance of Pu-239 at thermal (slowed-down) neutron energies, in about a third of the instances when Pu-239 absorbed a neutron, it would not fission and rather become Pu-240, whose characteristics make it much less desirable for nuclear weapons.
This contamination of the original weapons-grade plutonium with other isotopes of plutonium is considered attractive in today’s strange nuclear world, where the goal of sustainable and expanding energy from fission is considered a distant memory. Rather, the prime goal of those executing the MOX program seems to be to eliminate the weapons-value of this material as quickly as possible, even if that means foregoing far more valuable uses of the material.
A Better Choice
While I am no fan of the uranium-plutonium fuel cycle, I recognize that there are some far better, and ironically, easier options to dispose of the plutonium fuel. If a liquid-chloride reactor, with its very fast neutron spectrum, were constructed, the plutonium could be completely consumed and the neutrons generated could be captured in a blanket of thorium tetrafluoride, leading to the production of large amounts of uranium-233, which would easily be removed from the blanket by fluorination to uranium hexafluoride. This 233UF6 could then be reduced to 233UF4, which could be used as the “start charge” for many, many liquid-fluoride reactors. Once started, these reactors would not require additional U233, but would be able to sustain themselves solely on thorium, which is abundant and inexpensive.
Much like seeds that could either be eaten and provide a moment’s nutrition, or planted and yield so much more, this Cold War plutonium, purchased at such great cost from the reactors at Hanford and Savannah River, could be used to create the U233 to start many, many liquid-fluoride reactors.