SEDIMENTATION BEHAVIOR OF NOBLE METAL PARTICLES IN SIMULATED HIGH-LEVEL WASTE BOROSILICATE GLASSES

摘要

Solubility of noble metal elements (NME) in the melted borosilicate glass is much smaller than its normal concentration of the high level liquid waste (HLLW). Thus most of NME show small particles in the melted glass and tend to sediment in the bottom region of the vitrification melter due to their higher density than that of glass. The melted glass containing higher concentration of the NME particle indicates high viscosity and high electric conductivity, which cause negative effects to steady operation of the melter. Studies of formation processes and characteristics of the sedimentation are strongly required for considering countermeasures of avoiding and/or removing it. Experiments of the sedimentation of NME particles in the melted glass were carried out under static condition. Three conditions of initial NME concentration (1.1, 3.0, 6.1 wt% with an equivalent for each oxide) in the simulated glass were set and held at 1100°C up to 2880 hours. The specimen with 1.1wt% initial NME concentration indicated zone settling, and the settling rate of the interface is constant: 2.4 mm/h. This sedimentation behavior is the type of rapid settling. Following the rapid settling, the settling rate goes gradually slower; this is the type of compressive settling. The specimens with 3.0wt% and 6.1wt% initial NME concentration showed compression settling from the beginning. From the settling curve of the interface, the maximum concentration of NME in sediment was estimated to be around 23-26wt%. Growth of NME particles was observed by holding at 1100°C for up to 2880 hours.