MST Melt Spreading Test. Photo: ANL.
Two major NEA joint projects in the severe accident research domain, Reduction of Severe Accident Uncertainties (ROSAU) and Experiments on Source Term for Delayed Releases (ESTER), have been concluded recently. These projects addressed key technical challenges highlighted by the Fukushima Daiichi Nuclear Power Plant accident and have delivered critical insights into melt and debris coolability, fission product behaviour and radioactive release, significantly advancing severe accident modelling capabilities.
ROSAU Project
The ROSAU project conducted 5 DCAM and 5 MST tests at Argonne National Laboratory (ANL), utilising depleted UO₂ to simulate in-vessel and ex-vessel scenarios. DCAM experiments targeted debris coolability, while MST tests investigated melt spreading and cooling dynamics. These tests led to the development of unique experimental capabilities and supported the enhancement of the open-source codes CORQUENCH and MELTSPREAD.
ROSAU also contributed to the post-Fukushima Daiichi Nuclear Power Plant accident understanding of ex-vessel phases and informed the development of a new corium stabilisation strategy by EDF in French pressurised water reactors (PWRs). This strategy, drawing on insights from the NEA’s MCCI (Molten Corium Concrete Interaction) and ROSAU projects, involves dry spreading of corium into the cavity and instrumentation room, followed by top-flooding with water to achieve stabilisation.
Building on these achievements, ROSAU-2 (2025–2029) is being launched to address remaining modelling uncertainties — particularly for cooling of high-metal-content melts and accident-tolerant fuel (ATF) debris.
ESTER Project
The ESTER project leveraged the CHROMIA platform operated by ASNR (Autorité de sûreté nucléaire et de radioprotection), which includes the CHIP facility (fission product transport in the Reactor Coolant System), EPICUR (radiochemistry in containment) and complementary testing at LECA-STAR (CEA). The latter involved remobilisation tests on VERDON-generated deposits from re-irradiated UO₂ samples.
ESTER focused on two key mechanisms: remobilisation of fission products from RCS deposits and organic iodide formation in containment. The project results significantly reduced uncertainties related to radioactive releases during severe accidents, particularly concerning the delayed release of volatile species such as Cs isotopes — phenomena observed during the Fukushima Daiichi Nuclear Power Plant accident.
The upcoming FORESEEN (2026–2029) project will expand this research into ATF-related scenarios. It will incorporate the AMIX facility at Idaho National Laboratory, enabling high-precision experiments on fission product release from irradiated chromium-doped UO₂ fuel.
Together, ROSAU and ESTER projects supported NEA member countries in advancing severe accident research in alignment with the evolution of nuclear fuel technologies. They mark a pivotal step toward refining predictive capabilities, improving safety assessments and informing robust accident management strategies for current and future reactor systems.
CHIP facility for remobilisation tests. Photo: ASNR
