In the frame of the renewal of the different nuclear plans, the molten salt reactor is one of the six concepts of reactors of IVth generation. This reactor has the particularity to use a liquid fuel based on LiF-ThF4-UF4-BeF2 mixtures. Particular importance for ThF4 and UF4 as a nuclear fuel is an understanding of the phonon contribution to the thermal properties namely the heat capacity. This latter shows an anomaly at low temperature (~ 6 °K)for UF4, that was tentatively attributed to a purely magnetic transition. However, our present knowledge of the lattice dynamics and phonon contribution at low temperatures remains weak. Here, we propose to measure the phonon DOS of four samples: diamagnetic ThF4 and ThCl4 and magnetic UF4 and UF3 at temperatures in the range of 2 - 300 °K. Results will provide a comprehensive understanding of the observed Cp (T) anomaly in these nuclear fuels and shed light on the phonon contribution to the heat transfer .