” A particle that barely ranks as a footnote in a physics text may be about to lift the cleanup of the stricken Fukushima Daiichi nuclear complex in Japan over a crucial obstacle.
Inside the complex, there are three wrecked reactor cores, twisted masses of hundreds of tons of highly radioactive uranium, plutonium, cesium and strontium. After the meltdown, which followed a tsunami and earthquake in 2011, most of the material in the plant’s reactors resolidified, in difficult shapes and in confined spaces, wrapped around and through the structural parts of the reactors and the buildings.
Or at least, that is what the engineers think. Nobody really knows, because nobody has yet examined many of the most important parts of the wreckage. Though three and a half years have passed, it is still too dangerous to climb inside for a look, and sending in a camera would risk more leaks. Engineers do not have enough data to even run a computer model that could tell them how much of the reactor cores are intact and how much of them melted, because the measurement systems inside the buildings were out of commission for days after the accident.
And though the buildings may be leaking, they were built of concrete and steel so thick that there is no hope of using X-rays or other conventional imaging technology to scan the wreckage from a safe distance.
To clean up the reactors, special tools must be custom-made, according to Duncan W. McBranch, the chief technology officer at Los Alamos National Laboratory, and the tools “can be much better designed if you had a good idea of what’s inside.” But “nobody knows what happened inside,” he said. “Nobody wants to go in to find out.”
That is where muons come in.
In the next few days, Toshiba, the contractor in charge of the initial cleanup work, and the laboratory expect to sign a formal agreement to deploy a new technology that experts believe will yield three-dimensional images of the wrecked reactor cores, and will be able to differentiate the uranium and plutonium from other materials, even when 10 feet of concrete and steel are in the way.
The Energy Department has been working on the technology for years, and already licenses it in a less advanced form for a more limited job: A Virginia company is using it in a device that screens shipping containers for smuggled uranium or plutonium that could be used in a nuclear bomb. The lab’s new version will be much more ambitious and will focus on mapping rather than just detection.
The technique takes advantage of the fact that everything on earth is constantly being bombarded by muons, subatomic particles that are somewhat like electrons, though about 200 times as heavy. Muons are shaken loose from molecules in the atmosphere by cosmic radiation. Traveling near the speed of light, they rain down on the earth and can penetrate hundreds of feet into it.
But occasionally, one of the muons will happen to hit an atomic nucleus, and when it does, it will change direction in a way that gives a clue about the shape of the target and the target’s density. The technique of detecting those scattered particles and inferring what it was that they bounced off is called muon tomography.
“There is a similarity to X-ray, but the details of the physics are different,” Dr. McBranch said.
Decision Sciences International, a Virginia company, says it can use muon tomography to screen a 40-foot shipping container in 45 seconds and sense whether there is uranium or plutonium in it, though not in great detail. As altered by the Los Alamos scientists for use at Fukushima, the process requires a much longer exposure — it could take weeks. But the result will be a three-dimensional image; concrete, steel and water will all be distinguishable from uranium, plutonium and other very heavy materials.
“You don’t need a quick image, you just need a good image, and you have plenty of time,” said Stanton D. Sloane, the chief executive of Decision Sciences. Testing will begin later this year, officials say, and final images will be produced next year.
“I would expect to be able to distinguish fairly readily between what would be described as random results from the meltdown, versus engineered structural components,” Mr. Sloane said.
The Department of Energy, which runs the Los Alamos lab, does not yet have a formal agreement with Decision Sciences to produce the necessary hardware, but the company is likely to do so.
Mr. Sloane would not say how much the equipment would cost, but the project is small by nuclear standards. Toshiba will reimburse Los Alamos for its costs, which officials said would come to less than half a million dollars. Los Alamos has spent about $4 million developing the technology. Decision Sciences spent additional money to commercialize it, but has not said how much.
The Los Alamos contribution to the Fukushima project is mostly software. The accompanying apparatus, which has already been tried out on a small, intact reactor, consists of two billboard-size detectors, set up on opposite sides of the building. Each detector is like an array of pipes in a church organ, with each pipe filled with inert gases, including argon, that give an indication when a muon hits. The detectors keep track of which pipes were hit on the way in and on the way out, and at what angle. (It is not possible to “tag” a muon, but by timing the detections, the engineers can tell that they spotted the same muon coming and going.)
The detectors do not have to go inside the reactor building. In fact, they would work less well inside, because gamma radiation coming off the melted fuel would make it harder to spot the muons. Instead, the detectors will be set up a few feet away from the reactor buildings’ outer walls, and will be shielded with four inches of steel, which will stop the gamma rays but makes no difference to the muons.
At sea level, about 10,000 muons will pass through each square meter of the detectors every minute. Only a few of them will be deflected and yield useful data, so the detectors will need to run for weeks to gather enough for a clear picture.
Muon tomography is not completely new; it was used in the 1960s to peer inside the Great Pyramid at Giza. But the current version produces images of much higher resolution, according to Dr. McBranch.
Japan is increasingly turning to other countries for the technology needed to clean up Fukushima. This month, Tepco, the utility that operated the power plant, announced a deal with Kurion, a waste-handling company based in Irvine, Calif., for a mobile system to scrub radioactive strontium from 340,000 tons of contaminated water at the site.
Lake H. Barrett, an engineer who is not directly involved in the muon project, said the technique was certainly worth trying. Mr. Barrett was the director of the Nuclear Regulatory Commission office on site at the cleanup of the Three Mile Island nuclear plant near Harrisburg, Pa.; he is now an adviser to the president of Tepco.
Referring to the technology’s use in detecting smuggled weapons fuel, he said, “It’s nice to see the synergy of nonproliferation technologies, on which we in the U.S. have spent hundreds of millions of dollars, applied to another area.”
“How effective it is, we’ll have to wait and see,” he continued. “But we’re all optimistic.” ”