Morphology Change of CRUD Formed on Fuel Cladding by Deposition Time in Pressurized Water Reactor Environment

Abstract

In this study, to investigate morphology and thermal property change of fuel CRUD, simulated CRUD deposition experiment was conducted for three different period (24 hours, 72 hours and 168 hours) in PWR primary water condition. The specimens were examined by scanning electron microscopy with focused ion beam instrument and energy dispersive X-ray spectroscopy. To investigate the effect of CRUD morphology on its thermal properties, thermal conductivity of CRUD was calculated from difference of cladding wall temperature which measured before and after the CRUD deposition experiment. In 24 hours exposed specimen, CRUD layer was usually composed of equiaxed nickel ferrite particle and few needle like nickel oxide particles. Whereas in 72 hours and 168 hours exposed specimen, composition of needle-like nickel oxide particles was increased. It can be reasonably explained by particle formation energy of nickel ferrite and nickel oxide particle. Because nickel ferrite formation is spontaneous reaction in PWR primary water, it forms prior to nickel oxide, which is not preferred reaction in the environment. Since nickel oxide formation energy decrease with temperature increase, when thickness of CRUD layer increased enough so that temperature of CRUD layer increased, nickel oxide particles are formed.