Cosmic-ray muon radiography for reactor core observation

Highlights: • Cosmic-ray muon radiography was evaluated by the HTTR. • Cosmic-ray muon radiography could be performed from the outside of the RPV. • Cosmic-ray muon radiography could be performed from the outside of the CV. • The major structures of the HTTR could be detected by Cosmic-ray muon radiography. • The detectable minimum volume is estimated at approximately 1 (m³) in the HTTR. - Abstract: One of the critical problems that have arisen from the accident at TEPCO's Fukushima Daiichi nuclear power plant is the removal of fuel debris. For solving this problem, an examination of the internal reactors has been planned to identify the fuel debris. However, the high radiation dose around the reactors has necessitated the development of a remote sensing method that would enable observation of the internal reactors from the outside. In our study, we focused on a nondestructive inspection method by which cosmic-ray muons could be used to observe the internal reactor from outside the reactor pressure vessel (RPV) and containment vessel (CV). We conducted an observation test on the high-temperature engineering test reactor (HTTR) at the Japan Atomic Energy Agency to evaluate the applicability of the method to the internal visualization of a reactor. We also analytically evaluated the resolution of existing muon telescopes to assess their suitability for the HTTR observation, and were able to detect the major structures of the HTTR based on the distribution of the surface densities calculated from the coincidences measured by the telescopes. Our findings suggested that existing muon telescopes could be used for muon observation of the internal reactor from outside the RPV and CV