In this work the LiF-BeF2, NaF-BeF2, and BeF2-PuF3 binary phase diagrams have been thermodynamically assessed. The first two systems have been optimized based on the known experimental data, whereas the last one has been treated ideally. To describe the excess Gibbs parameters of the liquid solution the modified quasi chemical model based on the quadruplet approximation has been used. The results obtained together with the data of the (LiF + PuF3), (NaF + PuF3), and (LiF + NaF) systems, which have been assessed in previous studies, were used to extrapolate the (LiF + NaF + BeF2 + PuF3) quaternary system. The calculated (LiF + NaF + BeF2) ternary subsystem has been compared with the experimental results published in literature. The nuclear fuel properties such as the melting behaviour, the vapour pressure, or the solubility of PuF3 in the matrix of LiF-NaF-BeF2 have been derived based on our assessment and compared with measurements in literature. (C) 2009 Elsevier Ltd. All rights reserved.