Background: A varity of rare-earth fluorides can be produced with thorium uranium fuel circulating post-processing. These fluorides need to be dissolubled in solvent for seperation and contents measurement. Purpose: This paper aims to develop a fast and efficient pretreatment method to dissolve slightly soluble rare earth fluorides in nitric acid. Methods: Rare earth fluorides related to the thorium molten salt reactor (TMSR) fuel circulating post-processing including NdF3

EuF3

GdF3 and CeF3 were studied by using Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES). After 2-h microwave digestion at temperature of 190 oC

the maximum solubility of these fluorides in 10 mL nitric acid (16 mol?L?1) is determined by ICP-AES. Results: The saturated concentration varies from 2.5 g?L?1 to 3.9g?L?1

which are two or three orders of magnitude higher than their solubility reported in previous literature. The standard deviation (n=3) of measurement by ICP-AES is under 4.2% and the linear correlation coefficient (r) is in the range of 0.999 47?0.999 96. Conclusion: An efficient chemical procedure for dissolving some rare earth fluorides has been developed. It provides a reference for pretreatment of insoluble rare earth fluorides in the solid molten salt samples of thorium uranium fuel circulating post-processing. Abstract