Dissolution of (U,Th)O2 heterogeneous mixed oxides

•Polished heterogeneous U1-xThxO2 pellets have been prepared, characterized then submitted to dissolution tests in nitric acid solutions.•Different behaviours during dissolution have been observed between embedding U-enriched phase and Th-enriched heterogeneities.•Increasing the quantity of heterogeneities in the pellets led to the decrease of the dissolution rates.•U1-xThxO2 sintered powders with x > 0.58 dissolved slowly in the absence of HNO2 catalytic species.