THAI experimental facility, which is located in Eschborn (Germany), is essentially a container of cylindrical shape with the volume of 60 m³, the height of 9.2 m and a diameter of 3.2 m. Owner and operator of the device is the Becker Technologies GmbH company. The device is in use from the early 2000s to carry out experiments in the field of phenomena in the containment of a nuclear power plant during severe accidents: distribution and combustion of hydrogen, operation of a autocatalytic hydrogen and behaviour of fission products. In the Reactor Engineering Division they had so far, within the International standard problem OECD No. 47 (2002-2007), simulated the mixing of the atmosphere in the containment and, in the context of the European project CESAM (2013-2017) and our own independent research, the experiments of hydrogen combustion. Most experiments in the THAI facility, which belong to the very top of the research in this area, takes place within the OECD projects in which JSI is unable to participate due to the lack of funds for participation financing.

In 2014, the THAI facility has been expanded with parallel cylindrical container with similar height and with the volume of 17.5 m³. Both containers are interconnected with two horizontal pipelines, one at the bottom and one at the upper part of the two vessels. To test the updated facility (now called THAI+) the TH-27 experiment was conducted. During the experiment the circulation of the atmosphere was generated at first by the injection of the steam into the main vessel and subsequently by injecting the helium into secondary smaller container. The objective of the experiment was to observe how the inhomogeneous atmosphere (in terms of the composition) within the entire experimental facility will evolve. The entire experiment lasted 120 hours.

Even before the experiment the comparative calculation ( "benchmark") was initiated, the purpose of which was the simulation of the first 60 hours of an experiment with computer applications. The calculation was opened as for the Computational Fluid Dynamics programs as for the system programs based on zero-dimensional description of the atmosphere in the container. Potential participants were given specifications, in which the intended experiment was described together with the planned initial and boundary conditions for the realization of the "double-blind" simulations. After submitting the required results, participants received the actual initial and boundary conditions of the experiment (which was at that time already conducted), but not the results of the experiment itself. Newly received data was used to carry out "blind" simulations. The contributions of all participants were united in a single report which contains also the experimental results. JSI has attended the comparative calculation with the ASTEC system software to simulate severe accidents.

For the conclusion of the comparative calculation, workshop was organized at which participants could present their experiences and eventual "open" simulations carried out after receiving the experimental results. Workshop THAI-27, which was organized by the owner and manager of the THAI+ experimental facility Becker Technologies GmbH company in Eschborn (Germany) and was held on 24.11.2016, was attended by dr. Ivo Kljenak from JSI Reactor Engineering Division who presented the contribution: »From lumped-parameter simulations of THAI ISP-47 to lumped-parameter simulations of THAI TH-27: what has been achieved in 11 years ?«