The increasing interest of thorium in the recent years is due to its promising application in several fields including nuclear fuel, catalyst, nuclear medicine, optical coatings, etc., but also because of the existence of large stockpiles of thorium nitrate, generated as a side product of increased rare earth metal mining that will have to be considered as costly radioactive waste without proper valorisation.
The most stable form of thorium, thorium(IV)oxide or thoria (ThO2), has been considered worldwide as a potential nuclear fuel since many decades due to its high abundance in the earth crust and a lower generation of transuranium elements in the thorium fuel cycle compared to its uranium analogue. Thoria is also used in the fabrication of thorium-based spallation targets for accelerator-production of isotopes, and as ThO2-based catalysts for the oxidative coupling of methane with molecular oxygen to produce higher hydrocarbons.
Recent work, both internationally and at SCK•CEN, focused on ThO2 powder production routes to improve its sintering potential. Oxalate conversion is the most well-known manufacturing method for ThO2 powder production. This route converts aqueous thorium nitrate solutions to solid ThO2 powders by means of precipitation-filtration and calcination. Research at SCK•CEN on the impact of process variables on powder properties successfully resulted in optimized oxalate production processes. A fundamental drawback of the oxalate process, however, remains the 228Ra in the liquid process waste. We recently developed a new two-steps precipitation process in which 228Ra is removed from the liquid waste. Process variation effects and scalability of the novel precipitation route remain to be studied.
A comprehensive, systematic study about the influence of the variables in each step of the new process is required, as well as the identification of the final properties of thoria powder for the intended applications. Additionally, the scrap recycling route for out-of-spec thoria is compulsory to make a larger scale production process practical.
The PhD thesis will focus on the development of a flexible process in which it will be possible to tune the parameters towards end product characteristics, but also concerning minimisation of nuclear liquid waste generation. The PhD student will modify the conditions of the precipitation and calcination steps in the new two-steps precipitation conversion approach and study the effect on the ThO2 powder obtained. The characteristics of the powders will be analysed by means of advanced characterization techniques available at SCK•CEN such as high-resolution thermal analysis equipment coupled with a mass spectrometer (TGA-MS), dilatometer, X-ray diffraction (XRD), electron microscopy (SEM), physisorption analysis, etc. Additionally, he/she will perform experiments to improve the re-processing of production residues. He/she will be involved in the scale-up study of the process and contribute to the application-specific parameter optimization.