MOX: The Fukushima Word of the Day and Why it’s Bad News

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With four busted reactors at the Fukushima Daiichi site, engineers and rescue workers have plenty to do just to keep all their plates spinning. But over the past few days, there always seems to be one reactor causing them more headaches than others. Yesterday it was reactor 4, with its coolant pool  empty of water and the spent fuel rods stored there emitting massive waves of gamma radiation.

Today it’s rector 3. The day began—at least in the West—with images of helicopters flying over the reactor building, dumping seven-ton loads of water in an attempt both to cool the containment vessel and prevent that storage pool from drying up as well. But what makes reactor 3 so special? In one acronymic word: MOX.

All of the fuel rods in all of the other reactors are made essentially of uranium with a zirconium cladding to seal in radioactive emissions. Reactor 4 uses something different. Its fuel rod are only 94% uranium, with 6% plutonium stirred in and then the same zirconium shell. This mixed oxide (hence the MOX moniker) formulation has one advantage—and a number of disadvantages.

The advantage—no surprise—is money. Plutonium is a natural byproduct of radioactive decay and spent fuel rods are thus full of the stuff. You can always put them into long term storage for a few dozen millennia—which is where most spent rods have to go–but you can also reprocess some of the waste and combine it with pricier uranium for a cheaper and still energy-intensive rod. With nuclear power still more expensive than fossil fuels like coal, manufacturers need to save where they can to remain competitive, and MOX is a good budget cutter.

But MOX is also temperamental. Physicist Arjun Makhijani, president of the Institute for Energy and Environmental Research in Takma Park, MD., spoke to TIME earlier in the week and heaped scorn on the Mark 1 reactors used at the Daiichi site. His criticism in that conversation was the comparatively flimsy (by nuclear reactor standards at least) containment vessels used in the Mark 1s. But he’s no fan of the use of MOX either.

“This sort of fuel is more difficult to control than uranium fuel,” he told the Augusta Chronicle. “The risk of accidental criticality are different. You have the same kinds of problems, they are just more intense with plutonium.”

What Makhijani means by “accidental criticality,” of course, is that the stuff just combusts more easily. That’s particularly dangerous in a Mark 1, according to some studies. A report by the Sandia National Laboratories in Albuquerque, for example, found that in the event of a core meltdown, a Mark 1’s containment vessel has a 42% chance of failing—a whole lot closer to a coin flip than you want with something like a nuclear reactor.

And when plutonium is dispersed into the wind you want to be pretty much anywhere else. As I reported last week, there are four kidns of carcinogenic isotopes released when a nuke plant blows: iodine-131, cesium-137, strontium-90 and  plutonium-239. Plutonium is not only the most lethal of the four (“extrordinarily toxic” is how Dr. Ira Helfand, a board member for Physicians for Social Responsibility, describes it), it also hangs around the longest. It’s half life is a whopping 24,000 years, and since radioactive contamination is dangerous for 10 to 20 times the length of the isotope’s, that means plutonium emitted in Fukushima today will still be around in close to half a million years.

That, more than anything, explains why the day began with flyovers by water helicopters. And that explains why we’re likely to see a lot more of the same—at least until another Daiichi reactor starts to look even deadlier.