Phase separation in sodium borosilicate glasses containing molybdenum oxide has been studied in a compositional range of interest for nuclear glasses. Two series of quaternary compositions (63.5SiO$_{2-}$19.6Na$_2$O-16.9B$_2$O$_3$)$_{1-x}$ (MoO$_3$)x with 0 $\le$ x $\le$ 3 mol.% MoO$_3$ have been synthesized and characterized, by a range of complementary experimental techniques both in situ in temperature, by viscometry and hot stage SEM, and ex-situ, by HRTEM, PXRD, EPMA and dissolution calorimetry. The phase separation temperatures between silicate and molybdenum oxide rich liquids were determined for each composition. Molybdenum oxide solubility limits were established for quenched and annealed glasses.For molybdenum oxide contents exceeding these limits, crystalline sodium mono-molybdates precipitate in the glasses as shown by XRD results and the fractions of precipitates were indirectly quantified by MoO$_3$ mole balance and enthalpy balance relying on solution calorimetry experiments. The experimental results were compared with thermodynamic calculations performed with Thermo-Calc software and a CALPHAD database of the SiO$_2$-Na$_2$O-MoO$_3$ ternary system. The ternary model allows the calculation of the nature and quantity of the precipitated phases in reasonable agreement with the experiments, even if, in the current state of development of the thermodynamic database, the absence of boron restricts the relevance of the calculations. Thus, for the construction of a predictive thermodynamic calculation tool, the addition of boron to the database is the most promising prospect.