At the end of the 80ies it clearly appeared that the development of a thermodynamic database devoted to the nuclear field was a necessity, and more precisely for the applications in security and severe accident calculation codes in which the data used were simplistic. At the same time the emergence of the CALPHAD method allowed to hope an important progress in the quality and reliability of thermodynamic data for complex thermochemical systems. More than 10 years were required to develop such a database because it was a largescale task. Today NUCLEA allows the thermodynamic properties and equilibria determination in a chemical system with 18 elements : Ag-Al-B-Ba-C-Ca-Cr-Fe-In-La-Mg-Ni-O-Ru-Si-Sr-U-Zr.