What acid can dissolve thorium dioxide?
Thorium dioxide can be dissolved in the mixture of hydrofluoric acid and hydrochloric acid, as described by Kluger and Lieser in 1981.1

Proceed with caution since hot condensed acid are known to be highly corrosive and hydrofluoric acid is also highly toxic and penetrating.




Kluge, E.; Lieser, K. H. Dissolution of ThO2 in HCl/HF-mixtures. Radiochim. Acta 1981, 29 (2–3), 167–168. DOI: 10.1524/ract.1981.29.23.167.