DIP-COATING PROCESS OF ZIRCALOY-2 FUEL CLADDING WITH COLLOIDAL GRAPHITE

DIP-COATING PROCESS OF ZIRCALOY-2 FUEL CLADDING WITH COLLOIDAL GRAPHITE. The intensive researchs on high discharge burn-up of Light Water Reactor (LWR) fuel element have been continously performed due to the extension of fuel element's utility life. One of these researches was allowing for alteration of the existing zirconium-based clad system through coating. A coating technique with the coating layer thickness of 10 - 30 μm will improve the corrosion resistance of cladding without changing the dimension of cladding. The scope of this current research is to obtain the zircaloy-2 cladding coated with ZrC layer by dipping process of zircaloy-2 specimens in colloidal graphite at room temperature. The dip-coated specimens undergo heating process at 700 oC, 900 oC and 1100 oC respectively in Argon gas atmosphere for 1 hour and are subsequently characterized by optical microscope and XRD. The optical microscope images show that the coating layers thickness are increased as the heating temperature increased. The coating layers thickness are 10 μm, 20 - 40 μm and 100 μm for the specimens heated at 700 oC, 900 oC and 1100 oC respectively. The calculated diffusivity of carbon into zircaloy-2 cladding for the coated specimens at 700 oC, 900 oC and 1100 oC are 3,10216E-11 cm2s -1 ; 3,60479E-11 cm2s -1 and 4,00613E-11 cm2s -1 respectively. From XRD examination analysis reveals that the ZrC phase appears in the specimens heated at 1100 oC but it is not the case for specimens heated at both 700 oC and 900 oC. The coating layer of specimens heated at both 700 oC and 900 oC mostly consists of carbon. At these heating temperatures, carbon atoms have diffused into zircaloy-2 and substituted the zirconium atoms with a limited occupation to form C-Zr solid solution. At the temperature of 1100 oC, due to the increase in vibration energy, the carbon atoms have enough energy to diffuse to form the carbide phase. Heating process at higher than 700 oC, however, will degrade the zircaloy-2 cladding. It is concluded that the dip-coating process of zircaloy-2 cladding in graphite colloid with subsequent high temperature heating is not the proper method to gain the ZrC-coated zircaloy-2 cladding. Therefore, as for the future works of this research, the others method should be searched and investigated to obtain the proper ZrC coating process on LWR zircaloy cladding which fulfills the dimension and quality requirements.