en.Wedoany.com Reported - Researchers at the Paul Scherrer Institute (PSI) in Switzerland have, for the first time, conducted experimental studies on the passive cooling system of a small modular reactor (SMR) under realistic conditions at the PANDA test facility and collected high-resolution measurement data. The research findings have been published in the journal *Nuclear Engineering and Design*, and a related international benchmark test program has been officially launched.

Unlike traditional reactors that rely on active safety measures such as pumps and valves, many SMRs adopt passive cooling concepts, relying on physical effects like condensation, gravity, and density differences to ensure safety in emergency situations. However, PSI points out that experimental data currently available for validating simulation results of such systems remains limited.

The PANDA test facility has five floors, stands 25 meters high, consists of multiple vessels with a total volume of approximately 500 cubic meters, and contains no radioactive materials. The facility is equipped with over 80 valves, allowing gas mixtures to be extracted at different locations and analyzed using a mass spectrometer, and is fitted with around 1,450 sensors. Steam is generated by a 1.5-megawatt electric heater, reaching temperatures up to 200 degrees Celsius and pressures up to 10 bar.

In this experiment, the PSI project team tested a closed cooling loop consisting of a vertical pipe approximately six meters high. If steam leaks into the containment during an accident, it strikes the cold surface of the pipe, condenses into water droplets, and drips back into the reactor as liquid water. The released heat is transferred to the water inside the pipe; the heated water, being less dense, naturally rises and releases the heat into a storage tank, while the cooled water flows back down, forming a natural circulation based entirely on density differences, requiring no pumps or electricity.

Using high-speed cameras, the researchers recorded in detail the tiny water droplets condensing on the pipe surface and observed, for the first time, the separation of gases inside the containment: more air accumulates in the lower part, while more steam remains in the upper part. PSI notes that if this effect is ignored, the system's heat dissipation efficiency will be reduced. Furthermore, the researchers tracked tiny particles in the gas, confirming that they move very slowly near the pipe. The cooling process is not determined by larger air currents but is primarily driven by diffusion, meaning the cooling process is highly dependent on local conditions.

Yago Rivera Durán from the Center for Nuclear Engineering and Science at the Polish Institute of Scientific Research stated: "Previously, when developing simulation programs, researchers could not be sure if the calculation results matched the actual situation. We are closing this gap through PANDA."

PSI indicates that the newly released report marks the official launch of an international benchmark test program based on PANDA data, with 25 institutions already participating, using the experimental results to validate and improve their simulation methods. The subsequent project, PANDA-2, will build on this foundation to further investigate complex scenarios and the long-term autonomous operation of passive safety systems. This international project is expected to continue until 2030, with related projects by various countries and the EU also planned into the 2030s.

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