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Part 3 of 5 Parts (Please read Parts 1 and 2 first)
   For three months in 2016, the team recorded the tracks of almost a half a million muons. The tracks were used to determine which of the twenty-four fuel assembly slots in the cask actually contained fuel assemblies and which of the slots were empty. They successfully identified full and empty slots in four out of six slot groupings.
     The team at Los Alamos’ ultimate goal is to use muons to determine not only whether a slot in a cask is empty but also whether a fuel assembly has been removed and replaced by a dummy assembly made of a dense material such as lead. This process would require combining muon scattering to determine atomic numbers and muon absorption to determine the density of the material.
     The team has run computer simulations that show that this combined process should be possible, but they have not verified it experimentally yet. Their tests at INL were not completely successful because strong winds moved the detectors out of alignment. They have not yet received sufficient funds from Department of Energy’s National Nuclear Security Administration to carry out the necessary experiments.
    The Los Alamos team had intentions to utilize their new technology to image the damaged reactor cores at Fukushima. The Tokyo Electric Power Company (TEPCO) wanted to use the muon detectors to find the missing nuclear fuel in the reactors that melted down at Fukushima. Between 2015 and 2017, TEPCO determined that the missing nuclear fuel had melted down through the bottom of the containment vessels. Morris said that absorption radiography is not the best solution for these tasks because nuclear fuel and water have similar density and there is water in the ground below the reactor vessels at Fukushima.
     Morris and his team worked with Toshiba physicists to develop muon scattering detectors. They have successfully tested their new technology on a small Toshiba reactor in Yokohama. Unfortunately, this particular technology is too expensive and disruptive to utilize at Fukushima. Morris said, “We were very disappointed not to make measurements.”
    Interest in muon monitoring has been rising in Europe, especially in Germany. Germany decided to close all of their nuclear power plants by 2022 following the Fukushima nuclear disaster. When all the reactors in Germany have been permanently shut down, it will not be possible to open up casks in cooling pools to run tests. It is unlikely that any of the other nuclear nations in Europe such as France will be willing to allow their cooling pools to be used for tests. Morris said, “Everybody has enough problems dealing with their own nuclear waste.”
    German scientists are researching ways to look inside dry casks such as muography which do not require opening the casks. Katharina Aymanns works at the Jülich Research Centre near Cologne, Germany. She thinks that that this research is very important because Germany will not have a permanent spent nuclear fuel repository until 2050 at the earliest. She says it is very important, “to make sure that all the fuel assemblies that you think are in a cask are actually in a cask”.
Please read Part 4