A look inside highly radioactive facilities: Researchers have made the inside of a nuclear reactor visible for the first time using 3D muon scans. Because these fast elementary particles, which hit the earth’s surface everywhere, are absorbed and deflected by the reactor material to varying degrees, they can image the inside of a reactor core that cannot be entered in a similar way to tomography, as a test at a decommissioned French nuclear power plant showed.
Whether Fukushima , Chernobyl or nuclear power plants planned for dismantling in our country: nuclear reactors are still highly radioactive for decades, even after they have been shut down or after a nuclear accident. Their condition can therefore not be determined by simple inspections – in Fukushima even reconnaissance robots failed in the most heavily contaminated areas. It is all the more important to find other methods that can be used to get a picture of the inside of the reactor.
Muons as mapping aids
One of these methods is fluoroscopy using muons. These elementary particles are around 200 times heavier than an electron and are formed when high-energy cosmic rays hit the Earth’s upper atmosphere. On average, around 10,000 muons, traveling at almost the speed of light, hit every square meter of the earth’s surface every minute. Thanks to their small size and high energy, they can even penetrate solid concrete, steel or rock, but are partially absorbed and deflected in the process.
This makes it possible to use muons as passive mapping aids: If several muon detectors are placed around a building, the pattern of the incoming particles reveals whether there are cavities and other structures inside. Archaeologists have already used such muon scans to find hidden chambers and passages in the Cheops pyramid . In Fukushima, a 2D version of such scans provided the first indications of the condition of the damaged reactors.
Core of a decommissioned nuclear reactor as a test
Now, for the first time, Sébastien Procureur from the University of Paris-Saclay and his colleagues have examined in more detail whether muon scans can also provide a three-dimensional image of the inside of a reactor core. Similar to X-ray tomography, the two-dimensional particle patterns captured by the detectors must be assembled into a 3D image using a special algorithm.
The team chose the G2 nuclear reactor in Marcoule, France, as the test reactor. This nuclear reactor was shut down in 1980 and is now awaiting dismantling. In the reactor core, which is 34 meters long and 20 meters thick, uranium was used as the nuclear fuel and graphite as the moderator. “The graphite moderator forms an almost cubic structure with an edge length of around nine meters, which is interspersed with 1,200 horizontal shafts for the fuel rods,” the scientists explain.
Tomography reveals interior
For their muon scan, Procureur and his team set up four 50 x 50 centimeter muon detectors in alternating positions below and next to the reactor block. “From March 2021, we captured a total of around 370 million muons in 1,100 detector days and from 27 different perspectives,” they report. An algorithm they developed generated a 3D image of the inside of the reactor core without first knowing the construction plans or the basic structure of the plant.
The result: “Despite its complexity and large dimensions, the interior of the reactor could be reconstructed in a relatively short time and with good quality,” the researchers report. The muon scans revealed the shape and rough structure of the large block of graphite that served as the moderator, the concrete base, the piping for the cooling system, and even some distribution caps for cables outside the reactor core.
Resolution can still be increased significantly
“This is probably the most complex and largest object that has ever been mapped three-dimensionally using muons,” state Procureur and his colleagues. Although each muon detector was only at a measuring position for around three days, the particle density was still sufficient for a resolution of a few dozen centimetres. If more and larger particle detectors were used, this could be increased significantly, as the team explains.
According to the scientists, computer-aided muon tomography opens up new possibilities for mapping and monitoring nuclear reactors – both during their lifetime and after they have been shut down or after a nuclear accident. (Science Advances, 2023; doi:10.1126/sciadv.abq8431 )
Source: American Association for the Advancement of Science (AAAS)