Deep space exploration

NASA’s Plutonium Problem Could End Deep Space Exploration

At Oak Ridge National Laboratory in Tennessee, nuclear scientists used the High Flux Isotope Reactor produce a few micrograms of plutonium-238. A fully reconstituted plutonium program described in the The latest DOE planreleased this week, would also use a second reactor west of Idaho Falls, called the Advanced Test Reactor.

This facility is located on the 890 square mile nuclear ranch at the Idaho National Laboratory. High desert scrub passes visitors in the early morning as the sun sets over the Teton Range. Armed guards stop and inspect vehicles at a roadside outpost, waving those with the proper credentials to a reactor complex lined with barbed wire and electrified fencing.

Idaho National Laboratory


Beyond the final security checkpoint is a concrete-floored room the size of a warehouse. Yellow lines painted on the ground delineate what looks like an above-ground swimming pool topped with a metal cover. A bird’s eye view reveals four huge retractable metal slabs; jump through one and you’d dive into 36 feet of radiation-absorbing water. Halfway down is the 4-foot-tall reactor core, its four-leaf clover shape dictated by thin, wedge-shaped uranium rods. “That’s where you would stick your neptunium,” said nuclear chemist Steve Johnson, showing a diagram of the radioactive trefoil.

Neptunium, a direct neighbor of plutonium on the periodic table and a stable byproduct of Cold War-era nuclear reactors, is the material from which plutonium-238 is most easily made. In Johnson’s arrangement, engineers pack tubes with neptunium-237 and slip them into the reactor core. Occasionally, a neptunium-237 atom absorbs a neutron emitted by decaying uranium in the nucleus, later losing an electron to become plutonium-238. A year or two later, after the harmful isotopes had disappeared, technicians could dissolve the tubes in acid, remove the plutonium, and recycle the neptunium into new targets.

The inescapable rate of radioactive decay and limited reactor space means it can take five to seven years to create 3.3 pounds of battery-ready plutonium. Even if full production reaches this rate, NASA must squeeze every last watt out of what will inevitably always be a rather small stockpile.

The standard power source, called a multi-mission thermoelectric generator — the type that now powers the Curiosity rover — won’t be enough for the future of space exploration. “They’re trustworthy, but they use a lot of plutonium,” Johnson said.

In other words, NASA doesn’t just need new plutonium. It needs a new battery.

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Is it safe to launch nuclear batteries?

Anti-nuclear activists often claim that a single microscopic particle of plutonium-238 inhaled into the lungs can lead to deadly cancer. There is something to that claim, because pure plutonium-238 – ounce for ounce – is 270 times more radioactive than the plutonium-239 inside nuclear warheads. But the real risks to anyone launching a nuclear battery are often misrepresented or misunderstood.
Statisticians compare apples to apples by looking at the severity, likelihood, and affected population of a threat. An asteroid capable of wiping out 1.5 billion people, for example, hits Earth about once every 500,000 years or so – so the risk is very serious, but unlikely. Nuclear battery disasters, on the other hand, exist as low-severity, low-probability events, even close to the launch pad.