As of the moment, Japan’s two nuclear power plants in Fukushima prefecture are in very different states. According to the International Atomic Energy Agency, the Daini plant is in “cold shutdown,” the point at which the reactors are “considered to be safely under control.”

The Daiichi plant, meanwhile, seems to be headed toward a worse fate. There are conflicting reports about whether or not workers have abandoned it, and whether or not its reactors are going into “melt down,” a non-technical term that doesn’t really mean anything beyond the cores themselves being so damaged that it no longer makes sense to do anything other than let them destroy themselves until they reach a new steady state and containment and cleanup can begin.

Neither reactor appears to be in any danger of what’s known as a runaway nuclear reaction. To grossly oversimplify, a mass of nuclear fuel is “critical” in an operational nuclear power plant or “supercritical” in an atomic bomb. Control rods have been lowered into the reactors at both plants, and these absorb the neutrons being spat out by the nuclear fuel, preventing them from knocking loose more neutrons in other atoms of nuclear fuel and thus creating a critical nuclear reaction.

The World’s Largest Atomic Batteries

Instead, both plants are now effectively giant atomic batteries – and this is where the Voyager Spacecraft comes in. Venturing too far from the sun to make photovoltaic solar cells useful, Voyager 1 and 2, as well as 22 other spacecraft that preceded them used the heat produced by a block of plutonium-238 to generate power.

Atomic batteries work because even though the nuclear material they contain is too diffuse or too gummed up by neutron absorbers to go critical, they still contain radioactive elements that are slowly decaying, their atoms shooting out radiation until they no longer can.

Atomic batteries are great for spacecraft on decades-long missions because they just keep going and going. Their giant terrestrial counterparts, “melted-down” nuclear reactors, share this trait. Chernobyl, after all, is still highly radioactive inside the concrete “sarcophagus” that has entombed its nuclear material.

Plutonium in Space – And in Daiichi

Normally nuclear power plants are fueled by uranium, but last September the #3 reactor at Daiichi started using what’s known as MOX fuel, which contains plutonium–the same stuff that’s in the Voyager Spacecrafts’ nuclear batteries.

Unfortunately for Japan, physics dictates that the Daiichi nuclear power plant has this in common with the Voyager Spacecraft: it has embarked on a long journey, and there is no turning back.

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