Carbon source assessment for ISOL ThCx target materials

The production of high-purity radionuclides for medical applications, particularly 225Ac for targeted alpha therapy (TAT), requires advanced target materials capable of resisting extreme conditions in Isotope Separation On-Line (ISOL) facilities. Thorium-based materials are good candidates to produce 225Ac according to FLUKA simulations, especially, open-porous thorium carbide with excess carbon (ThCx) with small grain size. However, under the extreme operating ISOL conditions, the target materials tend to undergo sintering, leading to densification and coarsening. ThCx is typically synthetised from thorium oxide (ThO2) and graphite as carbon source via carbothermal reduction process (CTR). However, the use of other novel carbon sources could improve the performance of the ISOL target material within the irradiation conditions. Since ThCx is a pyrophoric material, it must be handled in glovebox environment under inert atmosphere. Additionally, thorium waste is a radioactive waste which is expensive to dispose of. In the early stages of optimization studies where various novel carbon sources are tested, it is more practical to conduct experiments in a fume hood environment using a surrogate material to efficiently identify promising carbon sources and determine optimal processing conditions, while minimizing the generation of radioactive waste. Titanium oxide (TiO2) was selected as surrogate material for ThO2 because it is a non-radioactive material which forms non-pyrophoric titanium carbide (TiC) via CTR. Also, the selected TiO2 has agglomerates with a size similar to the ThO2 produced by the most common synthesis process, i.e., oxalate conversion, hence, it is reasonable to expect a comparable microstructural behaviour. Four carbon sources: graphite, expanded graphite (EG), multi-walled carbon nanotubes (MWCNT), and graphitized carbon aerogels, were mixed with TiO2 and processed through CTR and sintering. The resulting pellets were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), helium pycnometry, and geometrical density measurements. Two molar ratios (C/TiO2 = 4 and 5) were tested to assess the influence of carbon excess on porosity, density, shrinkage, and grain growth. Results show that MWCNT pellets provided the most favourable combination of high open porosity (up to 88%), low shrinkage, and stable TiC crystallite size, indicating effective sintering inhibition. Graphite-based pellets exhibited the highest density but lower porosity, while EG and aerogels pellets showed high porosity but were mechanically fragile. XRD confirmed successful TiC formation in most samples, with incomplete reactions observed in EG-based pellets due to the further expansion of the EG during the CTR. This work concludes that MWCNT pellets are the most suitable carbon source for producing porous, thermally stable TiC structures, making them a strong candidate for future ThCx target development in ISOL facilities.