Multi-compartment THAI+ test facility, Becker Technologies GmbH.

In the wake of the NEA THAI project, conducted in three phases from 2007 until 2019, a new project, THAI Experiments on Mitigation measures, and source term issues to support analysis and further Improvement of Severe accident management measures (THEMIS) has been launched to follow up on outstanding questions in the area of combustion risks and iodine-aerosol behaviour in containment during a severe accident. Such outstanding questions, considering insights from the Fukushima-Daiichi accident analyses performed in the NEA BSAF project, have been identified through international efforts conducted under CSNI WGAMA, e.g. the status report on hydrogen management and related computer codes and the workshop on source term. THEMIS addressed issues related to combustion risk in ex-vessel phases of a severe accident with H₂/CO mix, the presence of fine aerosols, the evolution of airborne iodine oxides, the retention of fission products under pool scrubbing conditions and radionucleides remobilisation phenomena. They have been identified to require additional research. With regards to the radionucleides remobilisation phenomena, THEMIS investigations were complementary to the NEA ESTER project.

The project was operated by Becker Technologies GmbH in the THAI+ facility, which is owned by the German Federal government, and was supported by 25 partners from 13 countries and was completed by the end of 2024. 

All tests related to PARs performance in H₂/CO containment atmospheres, combustion and flame propagation in a H₂/CO mix, fission products retention by pool scrubbing and iodine oxide aerosols behaviour in containment (thermal stability and interactions with background aerosols) were completed in 2023. The last test of the programme, the most experimentally challenging due to its integral nature, was performed in 2024 and investigated PAR interaction with iodine oxide aerosols and fine nuclear aerosol in H₂/CO containing atmosphere.

The results of the tests have undergone extensive analysis and validation work in the frame of the analytical working group of the project. The code benchmarks conducted based on THAI experiments, on one hydrogen deflagration test, on one hydrogen recombiner test and on one pool scrubbing test supported the enhancement of the predictive capabilities of advanced lumped parameters codes and CFD codes and also contributed to improving the severe accident modelling and reactor safety assessments, particularly for late-phase accident conditions.

Becker Technologies has proposed a phase 2 of the project to address from mid-2025 key safety issues in nuclear reactors by investigating phenomena relevant to both large water-cooled reactors and generic water-cooled SMRs. The overall objective of the THEMIS-2 project is to reduce uncertainties in accident analysis and enhance accident management measures by addressing key phenomena across passive safety systems, hydrogen risk, source term, and by considering integral experiments involving safety system interactions.

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