The fourth meeting of the NEA COPS (COrium Properties for reactor Simulation and uncertainties) project took place in Paris on 24–25 November 2025, bringing together 35 experts from 16 organisations across 8 countries (Canada, Czechia, France, Germany, Hungary, Japan, Sweden, and the United States). Conducted under the NEA’s co-operative projects framework and operated by the French Commissariat à l’Énergie Atomique et aux Énergies Alternatives (CEA), COPS is advancing the global understanding of corium behaviour in a severe accident through the generation of high-quality experimental data.
COPS is producing rigorous measurements of key thermo-physical properties of corium — including density, surface tension, and viscosity — for 40 melt compositions representative of both in-vessel and ex-vessel conditions across a wide spectrum of existing and advanced reactor designs. The small-scale experiments continue to progress, with validated datasets and associated uncertainty evaluations now available for approximately ten compositions. Initial results have already prompted substantive technical discussion, in particular on the occurrence of multiple liquid phases in certain compositions and on the liquidus and solidus of complex mixtures.
In parallel, preparations are underway for two large-scale integral tests aimed at strengthening the understanding of how thermo-physical properties influence key phenomena relevant to corium melt stabilisation strategies, both inside and outside the reactor vessel. Current discussions are centred on identifying suitable experimental facilities, refining the test objectives, and defining the analytical work needed to support the development of the test matrix. Final decisions on the configuration and scope of these integral tests are expected at the fifth project meeting, scheduled for late May 2026.