Temperature and irradiation effects on Am(III)/Eu(III) separation in an i-SANEX system with a tetrasulphonated BTP masking agent for heterogeneous spent nuclear fuel reprocessing

The efficient and radiation-resistant separation of minor actinoids from spent nuclear fuel is essential for advanced reprocessing strategies such as the i-SANEX process. In this study, the extraction behaviour of Am(III) and Eu(III) was investigated with emphasis on the thermodynamic driving forces governing actinoid/lanthanoid selectivity in an aqueous phase containing 0.018 M tetrasulphonated bis-triazinylpyridine (SO3H)2-BTP in 0.35 M HNO3 contacted with 0.2 M TODGA in 5% octan-1-ol/kerosene. The influence of temperature (15–45 °C) and absorbed dose (up to 300 kGy) on extraction equilibria and masking efficiency was examined, including irradiation of the aqueous phase alone or in contact with the organic phase.Extraction equilibrium for both metal ions was reached within 2 min at 45 °C, and the overall mass-transfer coefficient increased with temperature. Thermodynamic analysis revealed a pronounced preference for Eu(III) over Am(III), reflected in a negative free energy change for Eu(III) extraction, whereas Am(III) extraction remained thermodynamically unfavourable due to its lower affinity for the O-donor TODGA ligand and the enthalpic cost of aqueous-phase complexation with BTP ligand. Irradiation led to progressive degradation of (SO3H)2-BTP, particularly at elevated temperatures, resulting in increased Am(III) distribution ratios and reduced separation factors. The presence of the organic phase during irradiation significantly mitigated ligand degradation, most likely due to radical scavenging. Mass spectrometric analysis identified hydroxylated and alcohol-adduct degradation products of (SO3H)2-BTP. Our results demonstrate that increasing temperature accelerates both extraction kinetics and radiolytic degradation, ultimately decreasing system selectivity. These findings highlight the importance of carefully balancing operational parameters in i-SANEX systems.