Effect of solid impurities on crystal purification performance in uranium crystallization system

Uranium crystallization system has been developed as innovative technology for advanced aqueous reprocessing of FBR spent fuel in Japan Atomic Energy Agency (JAEA). In the crystallization system, a great part of uranium is separated from dissolver liquor with high heavy metal concentration by cooling as uranyl nitrate hexahydrate (UNH) crystal. However, the purity of UNH crystal is not high, so we have discussed the application of crystal purification technology to improve the purity of UNH crystal and selected Kureha crystal purifier (KCP) as one of the desirable purifiers for UNH crystal. In the previous study, we have found that the purity of UNH crystal depends on the behavior of solid impurities in the crystallization system. In this study, the effects of grain size and density of solid impurities on the purification performance were evaluated in the UNH crystal purification tests to discuss the purification mechanism of KCP. From the results, a decontamination factor (DF) of UNH crystal with the smaller grain size of solid impurities was larger than that of the bigger one, on the other hand, the density effect was not significant. It is explained that the smaller size of solid impurities is easy to pass through between UNH crystal grains and the decontamination of the solid impurities in the purifier is promoted by pushing down them along the flow of melt. (author)