Dial μ for Assistance
This past March, a 23-foot-high tsunami struck the Tohoku region of Japan and destroyed the Fukushima Daiichi nuclear power plant. It is still uncertain as to the status of the reactor core—the large, tower-like assembly of fuel rods that contains not only the uranium and plutonium fuel, but also the extremely radioactive waste products of nuclear fission. If the core remains intact and standing, it should be possible to remove the assembly as a unit, and execute some strategy to mitigate its hazards. But if the core melted and nuclear fuel spilled from the fuel rods, suffice it to say there are no easy solutions.
Los Alamos might be able to assess the status of the core by scaling up a demonstration muon tomography system so that it's capable of imaging the inside of the reactor. The uranium and small amount of plutonium in the fuel assembly is heavier and contains more protons per nucleus than any of the steel, concrete, or other materials making up the reactor; hence, the fuel will scatter more muons. By placing detectors on either side of the reactor, one set to measure the incoming muon flux and the other to measure the angular distribution of the transmitted muons, one should be able to locate the heavy materials and deduce the state of the fuel assembly.
The measurement should be made with (mostly) horizontally-moving muons that cross through the vertically-standing fuel assembly, but few muons travel in a horizontal direction, and the low flux makes imaging problematic. So a team of Los Alamos researchers, led by physicist Cas Milner, built a mock reactor core (upper photo)— a tower of lead bricks surrounded by empty space and blocks of concrete. By looking at the horizontally-moving muons, and using newly developed algorithms, the team was able to reconstruct an image of the core (lower image).
Should the Japanese government request assistance in diagnosing the Daiichi power plant, Los Alamos will be able to respond quickly.