A refueling apparatus for charging nuclear fuel in a reactor vessel, the refueling apparatus including: a refueling unit loading new nuclear fuel to a core or extracting spent nuclear fuel from the core; and a waveguide sensor unit including an ultrasonic wedge to form a Lamb wave, a waveguide with an end connected to the ultrasonic wedge and with another end transmitting the Lamb wave into the reactor vessel, and an ultrasonic sensor connected to the ultrasonic wedge and sensing a reflection signal reflected from an inside of the reactor vessel, the waveguide being formed in a plate shape and mounted in an end of the refueling unit. The waveguide integrally moves with the refueling unit, and the waveguide sensor unit detects a condition of the inside of the reactor vessel, while the refueling unit refuels the fuel assembly in the reactor vessel.