Characterization of SH Wave Electromagnetic Acoustic Transducer (EMAT) at Elevated Temperature

The in service inspection (ISI) of MONJU fast breeder reactor vessel is performed in a severe environment at elevated temperature (about 200 °C ) and high-level radiation field (about 10Sv/h). Liquid sodium is used as the coolant of MONJU reactor. A horizontally polarized shear (SH) wave electromagnetic acoustic transducer (EMAT) is used to perform the volumetric inspection of the MONJU reactor vessel. In this paper, the magnetic flux density and the sound pressure distributions for the SH wave EMAT with a so-called Halbach magnet structure are analyzed from room temperature up to 240°C. The magnetic flux density and the sound pressure of the EMAT decrease with the increase of temperature. At 200°C, the relative reduction of the peak magnetic flux density and the relative thermal demagnetization at liftoff distance 0.5mm are less than 9% and 2%, respectively. This thermal demagnetization can be understood by analyzing the distribution of the permeance coefficient. As a result, the Halbach magnet structure can be used at elevated temperatures without large performance reductions as compared to its performance at room temperature. The Halbach EMAT will demonstrate good performance not only at room temperature but also at high temperature up to 240°C. This new structure EMAT is suitable for MONJU reactor vessel ISI.