Under sodium sensor has been developed to inspect heat exchanger pipes in FBR plants. Several methods have been applied to manufacture the volumetric inspection sensor. Low temperature diffusion bonding method was selected to bond piezoelectric element and thin stainless steel plate together for the boundary between piezoelectric element and sodium. Under sodium test has been made to verify the detection ability of the newly developed volumetric inspection sensor. Under sodium tests obtained that the sensor could detect 20% depth EDM (Electro discharge method) slits in outer and inner surface of 1mm thickness pipe wall.