Both the Washington Post and the Wall Street Journal reported in their articles today that the danger of a nuclear accident at the stricken Fukushima Daiichi unit 3 could be heightened by that reactor's use of mixed-oxide fuel (MOX). The Journal went into some detail about how MOX tends to burn hotter than conventional nuclear fuel, so there would be more residual heat in the reactors, which might make it more dangerous to try and cool down.
This is all correct, as far as I know, but both papers omitted something about the making of MOX. The Post and the Journal cited a program called "Megatons to Megawatts", an ongoing effort between countries with large nuclear stockpiles to draw down their stockpiles and convert highly enriched uranum (HEU) to low-enriched uranium (LEU), the latter of which is suitable for commercial nuclear reactors.
1. The Journal cites M2M as part of an international effort to reduce nuclear stockpiles, and says that the MOX in the Fukushima reactor was produced in that program, while the Post is vague about the nature and scope of the program. As far as I know, the M2M program is solely between the U.S. and Russia. It's entirely possible that the fuel was resold to Japan by the U.S., but the articles imply that this is a global effort. As far as I know (again), Russia is the only one of the eight nuclear powers that's having its stockpile reduced in this manner.
2. Also according to the link up above from the United States Enrichment Corporation (which was started by the U.S. government but is now privately owned), the M2M process primarily involves turning HEU into LEU. The UNEC website's main page, FAQ and FAQ on the actual process don't mention plutonium. However, the Journal says that MOX is produced in part by "mixing low-enriched uranium with plutonium that has been recycled from a global stockpile of defunct nuclear weapons", while the Post says that MOX is a "mixture of plutonium oxide and uranium oxide, produced from recycled material from nuclear weapons"*. These do not appear to be supported by the facts of the process, although again, it's quite possible that I'm wrong here.
3. Here's the big omission: MOX can be produced commercially, through the use of a spent nuclear fuel (SNF) reprocessing plant. It doesn't have to come from old weapons. As I wrote in my term paper last semester, upon being removed from the reactor, nuclear waste is made up of around 0.8% U-235, 1% P-238, 93.2% U-238 and 5% other actinides, which aren't readily usable. If the fuel is reprocessed, however, the U-235, P-239 and U-238 can be recycled right back into the reactor. That mixture, now called MOX, will give you around a quarter of the original fuel's energy-not bad for recycled nuclear waste. Reprocessing isn't done in the US for a combination of political and economic reasons, but it is popular in several other countries, including Japan.
4. MOX is more common than one might think, if you consider an informal definition of the fuel. When I met with Nuclear Regulatory Commission (NRC) spokesman Scott Burnell in the course of my research last year, he told me the following:
"In a certain way, every reactor towards the end of a given fuel cycle, when it gets close to having to refuel, in a sense it’s running on MOX. Because plutonium builds up in the fuel during operation. And at the very end, I mean very end, the last few weeks of a given reactor’s operational run, it’s plutonium that in large part is powering the core. But again, it’s doing so in keeping with our regulations. So with MOX you’d simply be starting with plutonium from the get-go.”
In other words, plutonium is transmuted from U-238 during the normal operation of a nuclear power plant, and builds up in the fuel on its own. It doesn't have to come from old weapons.
That's all I've got. Hopefully, both papers will make these corrections in their future reporting, and otherwise continue their exemplary reporting on the rest of the topic.
*The Post is technically correct in this statement--they could be referring to just the uranium oxide as having come from nuclear weapons--but they're being extremely vague about it, and could just as easily be referring to both the plutonium and the uranium.