Researchers at France’s Alternative Energies and Atomic Energy Commission (CEA) have created a 3D image of the G2 reactor at Marcoule using muons. Such imaging will aid in dismantling the reactor, which is now undergoing decommissioning. The research has been published in the journal Science Advances.
Muons are subatomic particles created when cosmic rays enter Earth's upper atmosphere. They hit the Earth's surface at a rate of around 10,000 muons per square metre per minute. Muon telescopes designed at CEA can detect and track these high-energy particles as they pass through objects.
Muons, like the runners in a marathon, are not going to arrive with the same energy. Depending on the difficulty and length of the race, a number will stop along the way, and only a fraction of them will cross the finish line,” said CEA researcher Sébastien Procureur. “This fraction which crosses the line informs us about the difficulty of the race.” It depends on the material they pass through. “Concretely, if we notice in certain places of an object an excess of muons, we can deduce a lower density, and therefore possibly a cavity, a hollow, etc,” he explained.
CEA’s muon telescopes are equipped with Micromegas detectors, invented in 1994 by CEA's Institute of Research into the Fundamental Laws of the Universe (IRFU). In 2016, CEA muon telescopes were associated with the ScanPyramids project, aimed at probing several Egyptian pyramids including that of Kheops. The aim was to discover hidden cavities in the thickness of the walls.
The muon telescopes then went from the pyramids to the G2 reactor dismantling site. Marcoule, established in the 1950s, is the CEA's reference site for nuclear fuel cycle research including used fuel processing and recycling, and radioactive waste management. It was home to three prototype gas-cooled reactors - the 2 MWe G1 reactor, which was commissioned in 1956 and operated until 1968; and two 40 MWe reactors, G2 and G3, (1958-1980 and 1959-1984).
The new application for the muon telescopes was initiated by Laurent Gallego, head of the G2 and G3 reactors dismantling project. After the first phase of dismantling of the G2 reactor, which ended in 1996, teams entered the reactor to produce videos and radiological contamination measurements, and metal samples to check corrosion status. But it was not enough. « It was then that I discovered via an article the results of the muon telescope as part of the ScanPyramids project,” said Gallego. “We asked the teams if it was possible to adapt this type of acquisition to structures such as our reactors, which are very massive and not very accessible.”
Muography made it possible to avoid unpleasant surprises during dismantling and guarantee site safety. “Thanks to the muon telescope and the images acquired, we obtained elements of appreciation on the real state of the reactor compared to the expected state,” Gallego added. Next step for the muon telescopes is the G3 reactor.
During their contribution to the G2 reactor dismantling project, the CEA-Irfu teams were asked to verify, using muography, how well the existing reactor plans, dating from the late 1950s, corresponded to the actual structure of the reactor. For this, Hector Gomez-Maluenda first carried out a simulation of the reactor and a 3D model from the documents he had at his disposal.
This high precision simulation allowed him to obtain a digital twin reactor and make a direct comparison between the experimental measurements and the simulation. He found that there were two elements appearing in the data and not defined in the plans. Once this anomaly was discovered, the plans were updated and then again simulated and tested to see if any differences remained.
Once this research finished, the rest of the measurements, around 30 in all, were used to carry out a 3D reactor reconstruction. The team combined 27 projections from four telescopes to derive the density map without any prior information on the reactor internal structure. The 3D reconstruction of the reactor was completed in March 2022.
"Despite its complexity and large dimensions, the reactor could be imaged in a relatively short time, with reasonably good-quality reconstruction being achieved with only a few days in each projection," the researchers said. "Although anticipated from the simulation, the analysis confirmed that only a limited number of projections are enough to visualise and localise the main elements of the reactor."
They added: "These conclusions open up new perspectives for the inspection and monitoring of nuclear sites over their entire operating lifetime as in their decommissioning phase, thus contributing to nuclear safety."
Image: Muon telescope installed under the center of the G2 reactor for the first muography measurement (courtesy of CEA)