Cosmic Ray Particles Will Reveal the Molten Hearts of Fukushima Daiichi's Reactors

Photo: TEPCO
Workers at Fukushima Daiichi installed a muon detector for testing this week.

In the radioactive ruins of the Fukushima Daiichi nuclear power plant, engineers are testing a new sensor technology. The goal is to see through layers of steel and concrete to determine the location of nuclear fuel at the hearts of three melted-down reactors

The sensor technology makes use of muons, subatomic particles generated when cosmic rays collide with molecules in Earth’s upper atmosphere. About 10,000 muons reach every square meter of our planet each minute, and they whiz through most substances largely unimpeded. However, their progress can be blocked by heavy elements like uranium and plutonium.

Based on this discrepancy, several research teams around the world are developing systems that use muons the same way your dentist uses x-rays. By placing muon detectors near a Fukushima reactor building and determining where the particles’ progress is being blocked, researchers can produce a map of the globs of melted uranium fuel inside the reactor.

There’s a critical need for such maps. The 40-year decommissioning of the Fukushima Daiichi power plant is well underway: Robots are busily surveying and decontaminating the shattered reactor buildings, and workers are removing spent fuel rods from pools. But the hardest step is yet to come. Someday, TEPCO workers will have to remove the melted nuclear fuel that glooped at the bottom of the three reactors’ pressure vessels, leaked through fissures and weak spots, and pooled in unknown nooks and crannies.

Before TEPCO can remove this highly radioactive fuel, the company must first figure out its exact location inside the melted-down reactors. That’s a big challenge, as it will be many years before robots or heavily protected humans are able to remove the tops of the reactor vessels to drop down radiation-shielded cameras. What’s more, those cameras still won’t be able to locate the fuel that seeped out through the bottoms of the presure vessels. 

That’s where the muons come in. TEPCO is first testing a system developed by Japan’s High Energy Accelerator Research Organization, putting the device near the heavily damaged Reactor 1. This system uses a “muon permeation” method; essentially just determining where muons are blocked in their progress by uranium. According to an email from TEPCO, this first test is just to serve as a proof of principle, and won’t produce detailed maps of the melted fuel’s location. 

Another system is under development by the U.S. company Decision Sciences, using a “muon scattering” method invented at Los Alamos National Lab in the early 2000s. This method places muon detectors on two sides of an object of interest, and tracks the trajectory of muons as they enter and leave the object. Because some muons interact with uranium nuclei and ping away in new directions, mapping this scattering can create a more precise map of a uranium blob’s location and contours.  Toshiba, a contractor for TEPCO, has enlisted Decision Sciences to develop its system for Fukushima Daiichi. That device will be tested later this year at Reactor 2. 


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