Skip to Content

Voyager Spacecraft Sheds Light On Japan’s Nuclear Reactors

Nothing can stop radioactive decay–that’s why it’s so useful in spacecraft batteries yet so troublesome in shut-down nuclear reactors.

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.

Follow Mims on Twitter or contact him via email.

Keep Reading

Most Popular

DeepMind’s cofounder: Generative AI is just a phase. What’s next is interactive AI.

“This is a profound moment in the history of technology,” says Mustafa Suleyman.

What to know about this autumn’s covid vaccines

New variants will pose a challenge, but early signs suggest the shots will still boost antibody responses.

Human-plus-AI solutions mitigate security threats

With the right human oversight, emerging technologies like artificial intelligence can help keep business and customer data secure

Next slide, please: A brief history of the corporate presentation

From million-dollar slide shows to Steve Jobs’s introduction of the iPhone, a bit of show business never hurt plain old business.

Stay connected

Illustration by Rose Wong

Get the latest updates from
MIT Technology Review

Discover special offers, top stories, upcoming events, and more.

Thank you for submitting your email!

Explore more newsletters

It looks like something went wrong.

We’re having trouble saving your preferences. Try refreshing this page and updating them one more time. If you continue to get this message, reach out to us at with a list of newsletters you’d like to receive.