Up to now, criticality safety experts used density laws fitted on experimental data and applied them outside the measurement range. Depending on the case, such an approach could be wrong for nitrate solutions. Seven components are concerned: UO2(NO3)2, U(NO3)4, Pu(NO3)4,Pu(NO3)3, Th(NO3)4, Am(NO3)3, and HNO3. To obviate this problem, a new methodology based on the thermodynamic concept of mixtures of binary electrolytes solutions (one electrolyte +water) at constant water activity, a so-called “isopiestic” solution, has been developed by the Institute de Radioprotection et de Sûreté Nucléaire (IRSN) to calculate the nitrate solutions density. This paper presents its qualification, and, as an application example, the minimum criticality values of water-reflected uranyl and plutonium nitrate solutions. The theory and the implementation are also given. Qualification results of the uranyl and plutonium nitrate solutions show that the new density law (also called the isopiestic law) is in good agreement with the benchmarks. Thus, no bias is put into evidence for the uranium solutions, and a small negative bias equal to 0.2% is found for the plutonium solutions. Moreover, the isopiestic law corrects the observed 1% overestimation of keff due to the empirical IRSN Leroy and Jouan density law for uranium solutions and the observed 3.4% underestimation of keff due to the ARH- 600 density law for plutonium solutions. The isopiestic density law has been implemented in CIGALES V2.0, the graphical user interface of the French criticality safety package CRISTAL that calculates the atom densities of nuclides (and writes the input file for CRISTAL computations).