Piping in nuclear power plants is exposed to severe environmental conditions so that it is very susceptible to failure caused by the growth of defects. Thus, it is necessary to have thorough inspection of piping in order to detect defects before failure. Unfortunately, however, inspection of piping in nuclear power plants is not easy in practice because of its long length as well as the radioactive environment. To take care of this difficulty, a research endeavor to develop techniques to monitor piping in nuclear power plants continuously and actively using ultrasonic guided wave is currently undertaken. This paper reports initial results of our endeavor including design of an ultrasonic array system for active monitoring of piping. © 2004 American Institute of Physics