Bulletin of Taras Shevchenko National University of Kyiv. Astronomy, no. 69, p. 59-66 (2024)
MUON TOMOGRAPHY: LOOKING INSIDE NUCLEAR REACTORS
Alona MOZGOVA, PhD (Phys. & Math.)
Taras Shevchenko National University of Kyiv, Kyiv, Ukraine
Bohdan HNATYK, DSc (Phys. & Math.), Prof.
Taras Shevchenko National University of Kyiv, Kyiv, Ukraine
Elizaveta ZHYHANIUK,
Taras Shevchenko National University of Kyiv, Kyiv, Ukraine
Sergey GABIELKOV, DSc (Phys. & Math.)
Institute for Safety Problems of Nuclear Power Plants, NAS of Ukraine, Chornobyl, Ukraine
Abstract
Introduction. Cosmic rays – high-energy charged particles (electrons, protons, heavier nuclei) – constantly bombard the Earth’s atmosphere and generate showers of secondary cosmic rays, in particular, high-energy muons. Muons have high mean range even in materials with high density, therefore they are an effective source of signals for tomographic studies of large-scale objects up to hundreds of meters and even up to kilometers. In particular, muon tomography is now the only method for remotely studying the spatial distribution of various components of nuclear reactors. In this paper a scheme for studying the structure of a nuclear-dangerous accumulation in the destroyed fourth reactor of the Chornobyl NPP with the help of muon tomography is proposed.
Methods. Primary cosmic rays reach the Earth’s atmosphere, interact with atmospheric nuclei (N, O, etc.) and, as a result of nuclear cascades, generate showers of secondary particles. These include the muon flux. Since our atmosphere is constantly bombarded by cosmic rays, the flux of muons is constantly coming from the atmosphere to the Earth’s surface and due to the high energy of muons (from 1 GeV to tens of TeV), they have a high penetration power and can penetrate underground to depths of hundreds of meters and up to several kilometers into solid rocks. At the same time, due to energy losses and scattering, the integral intensity of muons decreases depending on the passed column density X as the product of the density of the medium ρ by the passed distance L: X(L)=ρ∙L. Position-sensitive muon detectors, in particular, hodoscopes, record the integral intensity of muons at a certain solid angle and, using the integral intensity map, allow to reproduce the value of X – the distribution of the absorbing substance along the line of sight. Based on observations of an object from several locations with different zenith and azimuth angles, it is possible to reproduce a 3D distribution of absorbers in the object.
Results. A method for muon tomography using to determine the internal structure of the melt of fuel-containing materials, in particular, a nuclear-dangerous accumulation in the destroyed fourth reactor of the Chornobyl nuclear power plant, is proposed. The integral intensity of muons with momentum p>1.12 GeV/c at the zenith angle of 75° (the observation direction of the hodoscope) is I(>p=1.12 GeV/c)=6.90·10-4 cm-2∙s-1∙sr-1. The number of muons recorded in the solid angle (pixels in the sky) δΩ=1.0·10−3 sr with an effective area of Σ=5.76 cm2∙sr and an observation time of 100 days (8.64·106 s) would be Nμ =3.43·104. If there is an absorbing object with a density ρ, length L and the corresponding column density X(L)=ρ∙L on the line of sight of the telescope, then when a layer of concrete 10 m thick, muons with an initial momentum of p>5 GeV/c will fall on the detector. If the density of the absorbing object – a nuclear-dangerous cluster – is equal to 5 g/cm3, muons with an initial momentum of p>10.4 GeV/c, integral intensity I(>p=10.4 GeV/c)=2.65·10-4 cm-2∙s -1∙sr-1, and the number of registered muons – 1.32·104. That is, the sensitivity of the proposed method is sufficient to confidently determine the internal structure of the melt of fuel-containing materials.
Conclusions. Muon tomography is currently the only effective method for remote study of the spatial distribution of nuclear reactor components. In this paper a scheme for studying the structure of a nuclear-dangerous accumulation in the destroyed fourth reactor of the Chornobyl NPP with the help of muon tomography is proposed. It is shown that for the specified parameters of the hodoscope, it is possible to perform muon tomography of the reactor with an observation time from one location of about 100 days.
Key words
muons, cosmic rays, muon tomography, nuclear safety, nuclear reactors, ChNPP.
References
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