Efficient prevention of cracking of external gelation-made thorium oxide microspheres by hydrothermal treatment

In this study, a hydrothermal treatment (HT) at 200 degrees C was used to deal with the cracking of the external gelation-made thorium oxide (ThO2) microspheres and the effects of HT on the composition, microstructure, and cracking fraction of the microspheres were investigated. The results showed that HT removed the light weight impurities such as ammonium nitrate, degraded polyvinyl alcohol with light weight as well as absorbed and bonded water, thus resulting in a reduction of the weight loss of the gel microsphere by 26%. Owing to the removal of the impurities, HT led to the improvement of the crystallinity of the dried microspheres and the increase of specific surface area and pore size of the dried ones. Further, HT had distinct effect on the heat treatment of the microspheres: (1) For the drying process (240 degrees C) where an atmosphere of humidified air and wet microspheres were essential, the cracking fraction of the treated microspheres was only one-tenth of that of the original ones due to the removal of the bonded water by the HT. (2) In the case of elevated temperature treatment (650 and 1350 degrees C), the removal of residual polyvinyl alcohol by thermal oxidation was easy for the treated microspheres without obvious occurrence of cracking, however, it was not the case for the original counterparts. With the aid of HT, high quality crack-free ThO2 microspheres were available after the heat treatment and would be potentially used as nuclear fuel for a solid-fueled thorium molten salt reactor.