Scientists Are Using Cosmic Radiation To Peer Inside Fukushima’s Mangled Reactor

Not even our best imaging tools can penetrate the plant's radiation shielding. Thanks, space!

​Scientists designed the reactor walls at the Fukushima Daiichi nuclear power station to prevent some of the most powerful radiation in the universe from getting out—but they're lucky that at least one kind of radiation can still get in.

In the wake of 2011's disaster, ​scientists realized they might be able to harness the impressive penetrating power of natural, high-energy cosmic rays to paint a much-needed picture of the ruined Japanese reactor's cores.

Now a team from the Los Alamos National Laboratories (LANL) is attempting to actually image the innermost reaches of Fukushima by flanking its reactors with two enormous cosmic ray detectors. If successful, the team will be able to tell cleanup crews where each of the melted down nuclear samples has ended up, and how to best approach one of the worst environmental disasters in recent memory.

The cosmic rays in question are called muons—elementary particles so powerful that not even the fusion reaction at the heart of a star nor the explosion of a nuclear bomb can create them. On Earth, they are created mostly when high-energy particles from distant supernovae impact the planet's upper atmosphere.

Thousands of these cosmically-generated muons hit every square meter of the Earth each minute. They belong to a class of particle known as leptons, which only partially interact with regular matter, and often travel deep into the Earth before stopping. This makes them the perfect interrogating particle for a nuclear cleanup effort. Muons are part of the natural background radiation on Earth, and often reach Fukushima with enough raw energy to get into, through, and out the other side of a shielded nuclear core.

Nuclear physicist and LANL Fellow Christopher Morris told Motherboard that using muons for imaging is actually quite simple. "Basically, it's two big cosmic ray detectors, [one on] either side of the core," he said. By comparing the muon's exact direction of motion upon entering and then upon exiting the core, the team can make inferences about the placement of dense objects in the dark space between those two readings.

This means that only muons which hit the first detector and make it all the way through the core and hit the other detector on their way out will provide useable data; muons entering with too little energy to make all the way through, or at too steep an angle to hit both sensors, are useless. "We'll only get a few thousand useful muons per day... [so] making a good, high quality image of the core will take a couple of months," Morris said. "It's not like going to your doctor's office."

The technique makes up for its pickiness by delivering unprecedented detail per data-point. Called muon scattering tomography, the technique provides high enough resolution images that Morris has previously used it to search for small quantities of nuclear material hidden in shipping containers.

A similar technique called muon stop tomography, which looks only at whether muons do or do not pass all the way through a particular object, has also been used to image the interior of super-dense objects like volcanoes and the Great Pyramid of Giza. Other teams have attempted to look into the Fukushima cores with this older form of muon tech, but LANL believes that with the power plant's small size and low mass (compared to a Wonder of the World), the extra detail provided by a muon's change in trajectory is the only way to produce the detailed readings needed to safely decommission the reactors.

The Fukushima muon detectors are still under construction—but if they work properly, could start reporting cosmic rays in preliminary test readings as early as this week. The 21-foot by 21-foot sensors must be encased in about three inches of steel for shielding against the dangerous radiation levels immediately outside the core walls. Construction workers can only stay in the area for one hour per shift, in which they'll be hit with roughly 100 millirems of radiation—a fifth of the average American's annual dose.

Installation of the muon measurement device at Unit 1. Image: ​Tokyo Electric Power Company

Since the detectors are both so heavy and so delicate, the team can't configure them in the most efficient way, which would be to suspend one directly above the core and bury the other directly beneath it. This would minimize the volume of air through which the muons must travel to reach the power station, and thus result in a greater number of muon strikes per day. As it is, the roughly horizontal configuration will only be able to see those muons that approach from a shallow angle, roughly along Earth's horizon.

Interestingly, muons have a half-life of just two millionths of a second, meaning that few of them ought be surviving to reach the reactor along such a long, shallow path. Since muons travel near the speed of light, however, relativity dictates that time must move more slowly for them. Two microseconds, from the highly time-dilated perspective of a speeding muon, is more than enough time to reach the Earth's surface along the longest possible straight line from the upper atmosphere.

Though it will take months to resolve, the eventual picture formed by these muon detectors will be crucial to ending the threat at Fukushima Daiichi. Investigators are unsure if the radioactive material in each core may have remained in its pressure vessel, or drained down into the containment area below, or moved even further down into a so-called dry well. Each of these possibilities will require a very different approach to cleanup, so a lot rests on getting accurate readings about the state of the core interiors.

"All this basic research to go do crazy stuff, to go look at energy levels in nuclei with pions, it sounds like a useless waste of government funding," said Morris. "But this sort of work has spin-offs that end up being incredibly important."