Particles From The Edge Of Space Shine A Light On Fukushima
It's one of the greatest, and most disturbing, questions of the Fukushima disaster: What happened to the nuclear fuel inside the plant? Now physicists are trying to shed some light on the problem using particles from the edge of space.
The Fukushima accident was broadcast around the world. On March 11, 2011, an earthquake and tsunami struck the plant, knocking out cooling in three working reactors. The uranium fuel inside melted down.
But nobody's quite sure where it went.
"Right now we don't know where the fuel is," says Christopher Morris, a fellow at Los Alamos National Laboratory in New Mexico.
A lot of the fuel is probably still in the bottom of the reactors. But some of it may have fallen down into the concrete buildings that house them. Finding the fuel and disposing of it is a critical part of the cleanup, but there's just no way to look inside and see.
"The radiation levels are so extraordinarily high that cameras break rather quickly after they get near the reactor," Morris says. Debris is also blocking the view.
Morris is a physicist, and he and some other researchers think a subatomic particle called the muon can help. Muons are closely related to electrons, except they're 200 times heavier. They're made when particles from deep space collide with the upper atmosphere. From there, the muons shoot down to Earth and become part of the natural radiation we're exposed to every day.
To learn why muons might be useful at Fukushima, NPR conducted an experiment at the Forest Glen subway station outside Washington, D.C. We invited Kara Hoffman, a physicist at the University of Maryland, to bring a portable muon detector, borrowed from the Lawrence Berkeley National Laboratory in California.
Even at 20 stories below the surface, the detector picked up muons. Hoffman wasn't surprised. She works on an experiment carried out 2 miles below ground in Antarctica that regularly detects muons.
"One-hundred ninety-six feet is not very much for a muon," she says.
If muons can penetrate a subway tunnel, they can certainly pass through a nuclear reactor. That's why Morris thinks they can help at Fukushima.
"You can make something that looks like an X-ray, so you can take a picture of what's inside the reactor," he says.
Uranium is extremely dense, so it shows up as a shadow in a muon photograph, just as bones cast shadows that are white in an X-ray.
Two Japanese laboratories have already taken some fuzzy muon pictures of two of Fukushima's reactors.
"They don't see much of a shadow," Morris says. "That means a lot of the core must be missing from the region where the core was."
The fuel is gone. To figure out where it went, Morris is working with the Japanese firm Toshiba on more powerful muon detectors.
The detectors are already built. The next step is to put them near the reactors, though that's proving more difficult to do than the team thought. The detectors are large and bulky, and it would require a huge team to install them in the hazardous environment around the reactor. Morris says Toshiba is investigating whether it can be done more cheaply with less labor.
If they can get it to work, it may finally be possible to learn what's happened inside Fukushima.
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Transcript
AUDIE CORNISH, HOST:
Four years have passed since the meltdowns at the Fukushima nuclear power plant in Japan, but engineers still can't look inside the damaged reactors. The radiation is too harsh. NPR's Geoff Brumfiel reports that physicists are trying to help, using particles from the edge of space.
GEOFF BRUMFIEL, BYLINE: When a massive tsunami struck the Fukushima Daiichi Nuclear Plant in Japan, three reactors overheated and the uranium fuel inside melted. But what happened to the fuel after that is anyone's guess.
CHRISTOPHER MORRIS: Right now we don't know where the fuel is.
BRUMFIEL: Christopher Morris is a fellow at Los Alamos National Laboratory in New Mexico. A lot of the fuel is probably still in the bottom of the reactors, but some of it may have fallen down into the concrete buildings that house them. It's important to know exactly what's happened, but there's just no way to look inside and see.
MORRIS: The radiation levels are so extraordinarily high that cameras break rather quickly after they get near the reactor.
BRUMFIEL: Morris is a physicist and he and some other researchers think they have a solution. It's called...
MORRIS: Muons.
BRUMFIEL: Muons are subatomic particles. They're actually related to electrons except they're 200 times heavier. Muons come from the sky. They're made when particles from deep space collide with the upper atmosphere. From there, the muons shoot down to Earth. They're part of the natural radiation we're exposed to every day. To learn why muons might be useful at Fukushima, I met another physicist at a Metro station outside Washington, D.C.
I'm guessing by that large case that you're Kara.
KARA HOFFMAN: You must be Geoff.
BRUMFIEL: I am - perfect timing.
HOFFMAN: Yeah, excellent, OK.
BRUMFIEL: Kara Hoffman works at the University of Maryland. The large case she's carrying is a portable muon detector. She had it shipped out from the Lawrence Berkeley National Lab in California just for this story. It's about the size of a carry-on. Inside are painted green circuit boards and a display with big red numbers on the front. It's totally harmless, but it does look a little suspicious.
Whatever you do (laughter) when we pass the metro guys, don't say the word radiation, given that we're holding something that looks kind of...
HOFFMAN: Like a bomb.
BRUMFIEL: A little bit like a bomb.
HOFFMAN: (Laughter) Yeah.
BRUMFIEL: We get into the elevator and travel down 196 feet, about 20 stories beneath the ground.
UNIDENTIFIED WOMAN: Train level - thank you for using the Metro Rail.
BRUMFIEL: This depth is nothing for Hoffman. Her day job is to work on a particle detector in Antarctica that's buried two miles beneath the surface.
HOFFMAN: One of the backgrounds we deal with are muons hitting the atmosphere.
BRUMFIEL: So they get through two miles of Antarctic ice?
HOFFMAN: They do, yes.
BRUMFIEL: Muons are the freight trains of the subatomic world. Almost nothing can stop them. Hold out your hand and a muon will fly through it every second like it's not even there. Even here on the platform of the deepest Metro station in the D.C. area...
(SOUNDBITE OF BEEPING)
BRUMFIEL: Well, that didn't take very long.
HOFFMAN: (Laughter) No, 196 feet is not very much for a muon.
BRUMFIEL: If muons can penetrate a subway tunnel, they can certainly pass through a nuclear reactor, which is why Christopher Morris thinks they can help at Fukushima.
MORRIS: You can make something that looks like an X-ray, so you can make a picture of what's inside the reactor.
BRUMFIEL: Uranium is super-dense, so it shows up as a shadow in a muon photograph, just like bones show up as shadows in an X-ray. Some Japanese laboratories have already taken fuzzy muon pictures of two of Fukushima's reactors.
MORRIS: They don't see much of a shadow. That means that a lot of the core must be missing from the region where the core was.
BRUMFIEL: The fuel is definitely gone. To figure out where it went, Morris is working with the Japanese firm Toshiba on more powerful muon detectors. They're already built. The next step is to put them near the reactors, though that's proving more difficult to do than the team thought. If they can get it to work, though, it might finally be possible to learn what's happened inside Fukushima. Geoff Brumfiel, NPR News. Transcript provided by NPR, Copyright NPR.
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