In the framework of its missions on assessment of the criticality hazard, IRSN decided in 2005 to perform a new critical experimental program dealing with various structural materials to address the need of criticality calculation codes and nuclear data validation. In 2007, the program evolved into an international collaboration involving IRSN, the French industrial group AREVA, the French National Radioactive Waste Management Agency ANDRA (Agence Nationale pour la gestion des Déchets Radioactifs) and the United States Department of Energy (DOE). Phase-1 of the MIRTE Program, which is now finished, consisted of 43 sub-critical approaches extrapolated to critical conditions using the neutron amplification method. The 43 experiments included reproducibility and “reference”, also called replacement experiments. These experiments are being used to evaluate the eventual bias due to the tested structural materials. A second phase, called MIRTE 2, was planned to test other kinds of materials in the same configurations or with very small adaptations.