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| Fig. 1: Types of Muon Radiography, as described in Fujii et al. and Miyadera et al. [6,7] (Source: R. Margraf) |
A muon is a fundamental particle much like an electron, but with greater than 200 times its mass. Cosmic muons, which are produced naturally by primary cosmic rays which hit Earth's upper atmosphere, have been used in multiple imaging applications including imaging the interiors of pyramids and volcanoes. [1-3] Muons are ideal for imaging large static structures with inaccessible interiors because they interact much less readily with matter than many other particles, and thus are highly penetrating. [4] Another recent application of muon imaging, also known as "muon radiography", is the imaging of the damaged nuclear reactors at nuclear accident sites, such as Fukushima Daiichi.
The Fukushima Daiichi Nuclear Power Station in Japan was badly damaged by a tsunami in March 2011. [5] The four nuclear reactors at the site were irreparably damaged. To decommission these reactors without exposing workers to high levels of radiation, a map of the radioactive areas of the reactor is needed. Muon radiography was proposed as a method to assess the damage and determine if the radioactive core had melted though the floor of the reaction chamber.
Following the Fukushima Daiichi accident in 2011, in 2013 two types of muon radiography measurements were proposed. [6,7] These two types of measurements are diagrammed in Fig. 1.
Both techniques utilize muon detectors, composed of several scintillators or drift tubes. [6,7] Muons passing through a scintillator produce light which is amplified and recorded by the detector, and muons passing through a drift tube ionize electrons in the tube which then drift to a charge collector to record the event. As muons hit multiple scintillators or drift tubes in the detector, the detector reconstructs the trajectory of the muon. Some detectors additionally incorporate magnets which bend the trajectory of the muon and enable a finer measurement of the muon's energy. [6] Detectors also incorporate shielding to prevent other cosmic rays or particles from nuclear decays from interacting in the detector.
The first technique proposed, absorption-based muon radiography, is the most conventional. [6] As a muon travels through matter, it has a probability of being absorbed and stopped by the material which depends on the material density. [4] A muon detector records the trajectory of each muon that passes through the reactor being studied. By analyzing these trajectories in combination with a knowledge of the flux of cosmic muons hitting a given area of the earth's surface in a given time - which is generally constant for a given detection angle and muon energy - one can uncover areas of the detector where muons are being absorbed. This enables the muon detector to image the density distribution within a nuclear reactor.
The second technique proposed was scattering-based muon radiography. [7] Unlike absorption-based muon radiography, this technique requires two detectors, and utilizes muon scattering. Muons can scatter off atomic nuclei, and statistically will scatter off heavy elements (such as the uranium in the core of a nuclear reactor) at higher angles than lighter elements (such as calcium and silicon in the concrete of the building). By using two detectors, this technique can capture both the incoming and outgoing trajectory of a muon and use these tracks to calculate the location inside the reactor where the muon scattering occurred. This technique has high contrast between light and heavy atoms within the structure because heavy atoms scatter muons at high angles, making it ideal for locating the nuclear fuel within a reactor.
Approximately one muon hits any given centimeter of the Earth's surface every minute. [5] Thus both techniques rely on capturing millions of muons over a period of several months and have a resolution limited by the number of muons collected.
Set-ups for both types of muon radiography were developed to image the reactors at Fukushima Daiichi. [5] Between 2014 and 2015 several measurements were taken using an absorption-based muon radiography set-up of the Unit-1 nuclear reactor. [8] From 2015 to 2017, additional measurements using the same technology were taken to include reactors Unit-2 and Unit-3. However these results have not yet been published. [5]
The absorption-based muon radiography of the Unit-1 nuclear reactor used three muon detectors placed next to two adjacent walls of the nuclear reactor building. Adding a second detector enhanced the ability of the measurement to capture the three-dimensionality of the structure while preserving some simplicity in the data processing. The measurement lasted 90 days, and produced images which reveal several structures within the reactor complex. These images were used to estimate the amount of uranium core still present in the reactor loading zone. The amount of dense material, inferred to be uranium, in the reactor loading zone was much less than had been present in the reactor before the accident, suggesting that the core had indeed melted through the bottom of the reactor containment vessel. However, one limitation of muon radiography is that cosmic muons generally only come from the skyward direction, making it difficult to image a sample below the detector. Thus, if the core has melted through the floor of the reactor building, it would be very difficult to locate with muon radiography. [8]
The more complex scattering-based muon radiography has not been deployed at Fukushima Daiichi, despite the likelihood that it would have provided images with higher elemental contrast to locate the uranium core. By 2017, when the absorption-based muon radiography measurements were completed, robots had been developed which were able to withstand the radiation inside the reactor building in order to perform radiation mapping, making the measurements less necessary. [5]
Should another nuclear accident occur in the future, muon radiography may be useful to image damaged reactors inaccessible to robots - either because of the danger of releasing radioactive materials or in cases where very high radiation levels would prevent robot operation. While the technique has limitations, such as a long measurement time, resolution limits due to low muon flux and difficulty with measuring structures underground, it is still an interesting imaging utility to explore. The measurement at Fukushima Daiichi demonstrates the applicability of muon radiography for imaging nuclear reactors at nuclear accident sites, thus roviding useful information to the crews decommissioning the reactor. It is one additional tool in helping to heal the scar left by 2011 Fukushima Daiichi accident.
© Rachekl Margraf. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] T. K. Gaisser, "Spectrum of Cosmic-Ray Nucleons, Kaon Production, and the Atmospheric Muon Charge Ratio," Astropart. Phys. 35, 801 (2012).
[2] K. Morishima et al., "Discovery of a Big Void in Khufu's Pyramid by Observation of Cosmic-Ray Muons," Nature 552, 386 (2017).
[3] L. Olah et al., "High-Definition and Low-Noise Muography of the Sakurajima Volcano with Gaseous Tracking Detectors," Sci. Rep. 8, 3207 (2018).
[4] L. Bonechi, R. D'Alessandro, and A. Giammanco, "Atmospheric Muons as an Imaging Tool," Rev. Phys. 5, 100038 (2020).
[5] K. Vetter, "The Nuclear Legacy Today of Fukushima," Annu. Rev. Nucl. Part. Sci. 70, 257 (2020).
[6] H. Fujii et al., "Performance of a Remotely Located Muon Radiography System to Identify the Inner Structure of a Nuclear Plant," Prog. Theor. Exp. Phys. 2013, 073C01.
[7] H. Miyadera et al., "Imaging Fukushima Daiichi Reactors With Muons," AIP Adv. 3, 052133 (2013).
[8] H. Fujii et al., "Investigation of the Unit-1 Nuclear Reactor of Fukushima Daiichi by Cosmic Muon Radiography," Prog. Theor. Exp. Phys. 2020, 043C02 (2020).