Researchers at the Paul Scherrer Institute (PSI) in Switzerland have generated the first high-resolution experimental data validating passive cooling systems for small modular reactors (SMRs). The original study, published in the journal Nuclear Engineering and Design, bridges a long-standing gap between theoretical computer models and physical reality to aid future reactor licensing and safety verification.
The research team used the unique PANDA test facility to gather empirical proof that natural physical phenomena can reliably stabilise a reactor containment vessel during emergency scenarios without electricity or mechanical pumps.
PANDA is a German acronym for Passive Nachzerfallswärmeabfuhr und Druck-Abbau Testanlage. Which translates as Passive Residual Heat Removal and Pressure Relief. The name perfectly describes the core engineering purpose of the facility: testing how a nuclear containment structure can naturally vent pressure and dissipate core decay heat without relying on electricity or external water pumps.
Until now, the simulation of complex cooling processes for SMRs has required experimental data that has so far been limited. At PSI’s PANDA test facility, researchers have for the first time investigated passive cooling systems for small modular reactors under realistic conditions. The experiments, carried out with scientific support from cooperation partners in more than ten countries, provide high-resolution measurement data that can be used to validate such systems in simulations.
The PANDA test facility features highly specific, precise scaling laws, mechanical blueprints, and multi-compartment operational dynamics. PANDA is deliberately engineered to act as a full-scale model for time and thermodynamics, meaning system pressure and temperatures change at exactly the same rate as a real-world nuclear accident.
The test facility extends over five floors, reaching a height of 25 metres. It consists of six containers, with a total volume of roughly 500 cubic metres, in which processes that occur in nuclear reactors can be realistically simulated. PANDA contains no radioactive material. The steam, which reaches temperatures of up to 200 degrees Celsius and pressures as high as 10 bar, is generated by an electric heater with a power output of 1.5 MW. At more than 80 different points, gas mixtures from different areas of the facility can be extracted and analysed with a mass spectrometer.
The six containers include:
- A reactor pressure vessel (RPV), which houses the 1.5 MW electrical heater rods. It generates high-pressure steam to simulate the core decay heat of a shut-down reactor.
- Two drywells replicate the high-dry containment zone surrounding the reactor core. The two vessels are bridged by a one-metre-wide connection pipe.
- Two wetwells (suppression chambers) – vertical cylinders with a diameter of roughly 3.8 metres – contain water to a depth of 4 metres (45 cubic metres of water per vessel). The remaining upper volume of the 10.1-metre tall wetwell vessel is left intentionally empty to act as a gas expansion space for steam and air. The two wetwells are connected by two four-metre-long cross-pipes (one liquid, one gas) to equalise sudden pressure spikes.
- A Gravity-Driven Cooling System (GDCS) replicates an emergency reservoir. It dumps emergency water back into the RPV using simple gravity when pressure drops.
The experiment at PSI looked at what happens if, in an accident, steam is released from the reactor into the power plant’s outer containment structure? This steam has to be cooled, or it will increase pressure on the containment structure. In conventional reactors, active safety measures such as water spray systems, which require pumps and valves, handle these tasks. They dissipate heat and keep the pressure in the containment vessel under control. However, these systems depend on a reliable power supply. If that fails, their function can be impaired. Therefore, researchers are increasingly looking into passive means of cooling steam.
The project team at PSI, led by Dr Yago Rivera Durán, tested a closed cooling circuit. This consisted of a vertical pipe, approximately six metres high, through which cold water flows. If steam were to escape into the containment vessel during an incident, it would strike the cold surface of the pipe, condense there, and drip back into the reactor as liquid water.
The heat released in this process is transferred to the water inside the pipe. Because warm water is less dense than cold water, it naturally rises and releases its heat to a water reservoir. The cooled water then flows back down. This creates a natural cycle based solely on the density difference between warmer and colder water – entirely without pumps or electricity.
Previous experiments had already shown that such systems work. The PSI team has now presented, for the first time, highly detailed measurement data showing precisely how the physical processes inside a system on the scale of a nuclear power plant would unfold. Using high-speed cameras, the researchers even documented in detail tiny droplets of water that condense on the surface of the pipe.
For the first time, the researchers were able to observe how gases inside the containment vessel separate: More air collects in the lower section, while more steam remains at the top. This finding is important for both reactor design and computer simulations. If this effect were not taken into account, the system would be less effective at dissipating heat.
Researchers also tracked tiny particles in the gas and demonstrated that it moves very slowly near the pipe. In this area, therefore, condensation is determined not by larger currents, but primarily by diffusion: The water vapour reaches the surface of the pipe only slowly and condenses there. This means that the cooling process is highly dependent on local conditions.
Many SMR designs can be replicated using the PANDA facility. There are some 1,450 sensors able to provide valuable data. “Until now, researchers developing simulations couldn’t be certain that their calculations matched reality,” said Dr Durán. “We’re closing the gap with PANDA.” This will make data crucial for safety assessments and the licencing of future reactors available for the first time.
“The latest publication marks the launch of an international benchmarking initiative based on PANDA data,” PSI said. “Twenty-five institutions are already participating in this global collaboration, using the experimental results to verify and improve their simulation methods. A follow-up project, PANDA-2, will build on this work and focus even more intensely on complex scenarios as well as the long-term autonomous operation of passive safety systems. This international project is currently expected to run until 2030, while national and EU projects are already planned well into the 2030s.”
