PURPOSE: To prevent a corrosion product from sticking on a fuel cladding tube and to reduce a radioactive corrosion product by using the fuel cladding tube of which the zeta potential of the corrosion product is changed or regulated.
CONSTITUTION: A zeta potential regulating device 21 is provided on the outlet side of a recirculation pump 15 of a BWR plant. Zeta potential varies according to differences in pH and the form of a crystal and the zeta potential of an iron oxide is so conditioned that it shows a plus when the pH is on the acid side and a minus when the pH is on the alkali side and is inverted (becomes zero) in a neutral area. When the regulating device 21 is provided and thereby the zeta potential of a clad is regulated, a repulsive force acts on the surface of a fuel cladding tube and the clad. The form of the clad changes according to temperature and the concentration of dissolved oxygen and the particle size is made minute by an ultrasonic vibration bath. Thereby the zeta potential is regulated.