In-service inspection (ISI) is carried out to confirm the integrity of the main components of a Fast Breeder Reactor (FBR) “MONJU”. The ambient temperature during the inspection is 200 degree C and the irradiation field is about 10 Gy/hr. A Periodic Permanent Magnet (PPM) structure has been used for electromagnetic acoustic transducer (EMAT). The PPM structure is the structure of putting in order alternately the S pole and N pole laterally magnetized magnets. A Halbach magnet structure is the structure which sandwiched the longitudinally magnetized magnets between each magnet of PPM structure. Thereby, magnetic flux density is improved to 1.4 times. Moreover, it is also shown that the detection sensitivity increased to 4 times by using a new signal processing method. The magnetic flux density of a SH wave EMAT with a Halbach magnet structure is calculated from room temperature up to 240 degree C. The characteristics and durability at elevated temperature for Halbach type EMAT in 200 hours under the 200 degree C are reported. Twenty to thirty percents of sensitivity reduction can be observed but this is enough for MONJU ISI.

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