Effect of coolant chemistry on thermal property of CRUD in PWR fuel cladding - T025-3

To investigate CRUD deposition on fuel cladding under various water chemistry condition in simulated PWR primary water, the experimental facility has been set up. In previous study, effect of heat flux on CRUD structure and chemistry was examined for high metal ion concentration condition. In this study, to investigate effect of metal ion concentration condition on CRUD formation, CRUD deposition experiment was conducted for three different metal ion concentration. The specimens were observed by optical microscopy and scanning electron microscopy with focused ion beam instrument. The chemistry and microstructure of CRUD specimens are examined by scanning electron microscopy and energy dispersive X-ray spectroscopy. Thickness of CRUD increased depend on metal ion concentration. In lower metal ion concentration condition primitive structure of CRUD chimneys were found while developed chimney structure were formed in higher metal ion concentration condition. From chemical analysis, it is found that more Ni oxide species deposited in the surface of cladding in lower metal ion concentration condition. CRUD layer formed in high metal ion concentration condition shows low Ni/Fe ratio near cladding surface while Ni/Fe ratio increased in outer layer. Although higher metal ion concentration specimen has thicker CRUD layer, thermal conductivity of CRUD was higher. This can be explained by growth rate of chimney structure, which can promote heat transfer in CRUD layer by enhancing Wick boiling behavior. (authors)