“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.