In the criticality-safety practice, density laws are a key point for determining the composition of fissile solution media. Various fissile materials are encountered in the fuel cycle from manufacturing of fuel pellets to reprocessing of fresh or used fuel. During fuel dissolution, uranium/plutonium oxide is assumed to coexist with uranyl/plutonium nitrate under various physical forms: uranium/plutonium oxide powder, uranium/plutonium pellets’ dissolution residues, undissolved uranium/plutonium oxide particles in acidic nitrate solutions. To assess these configurations a dedicated density law (called “OXNIT”) is therefore necessary to characterize the composition of the oxide-nitrate mixture in criticality studies. A first density law was programmed in a criticality-safety GUI. However, due to many constraints limiting its use by practitioners, this former OXNIT density law did not answer the needs for criticality-safety assessment. Consequently, in 2017, IRSN and ORANO decided to propose a new law enlarging its use. This was done in the LATEC workbench [1], which is part of the CRISTAL V2.0 criticality safety package [2]. This law is based on the volume addition of a crystal of oxide in a solution of nitrate, whose density is determined using the “isopiestic” nitrate density law. Thanks to the LATEC framework, this law now meets the needs of users regarding the various parameters that can be taken into account in the criticality safety assessment. Thus, the OXNIT mixture is defined by the following parameters:  Oxide physical form (the form of a crystal or a powder),  Isotopic vector of uranium/plutonium oxide and uranyl/plutonium nitrate,  U/Pu content in oxide and in nitrate; thus it is possible to simulate a faster dissolution of U in nitrate,  Moderation expressed in H/X or in C(X),  Accounting for acidity and poison, such as Gd. By comparison with the previous version of CRISTAL V1 [3], a large flexibility is now offered to the criticality safety practitioners in the definition of the fissile media. The object of the paper is to present the breakthroughs that were detailed previously and to show the validation that was done to guarantee the correctness of the obtained compositions.