Small but mighty? Small nuclear reactors are supposed to be more efficient, safer and more flexible than large nuclear power plants – thanks, among other things, to passive cooling without electricity or pumps. Researchers have now tested how effective such a passive cooling system is in practice in a special facility in Switzerland. It turned out that without active circulation of the air in the containment, cooling through condensation only works to a limited extent. The measurement data now helps to optimize concepts.
Small nuclear reactors, so-called Small Modular Reactors (SMR), are seen by supporters as a climate-friendly source of electricity and a useful supplement to renewable energies. The compact small power plants produce an electrical output of around 300 megawatts and use less but more highly enriched nuclear fuel. Because they consist of prefabricated components, they can be built faster and cheaper. There are currently around 80 SMR plants being planned or under construction worldwide, but most of the concepts for such small reactors currently only exist on paper.
How well does passive cooling work?
A feature of many of these modular small reactors are innovative cooling systems: some use salts or special gases instead of water, others are designed to be completely passively cooled. Instead of electrically operated pumps, purely physical effects such as condensation, gravity or differences in density ensure the necessary heat exchange and cooling. However, before such cooling systems are approved and used, they must be extensively tested.
Such tests are possible in the PANDA test facility at the Paul Scherrer Institute (PSI) in Switzerland. The five-story-high facility includes several large containers in which the conditions in reactor cores, pressure vessels and other reactor parts can be simulated. Instead of nuclear fuel, an electric heater generates the necessary heat; gas mixtures can be taken from over 80 points in the system and analyzed with a mass spectrometer.
“So far, the developers of simulations could not be sure that their calculations correspond to reality,” says lead author Yago Rivera Durán from the Paul Scherrer Institute. “We are closing this gap with PANDA.”
With condensation against overheated steam
In the current test, Rivera and his colleagues examined a question central to nuclear power plants: What happens if the reactor core overheats and steam is released into the surrounding containment? If this vapor is not cooled efficiently and quickly, the pressure can rise so much that the container gives way or even explodes. In common nuclear power plants, this is prevented by electricity-powered water cooling systems. However, if the power goes out, they no longer work – as was the case with the Fukushima nuclear disaster in 2011.
That’s why some small reactors rely on passive cooling through condensation. The core element is vertical metal pipes that contain cold water. When the hot steam comes into contact with the pipe, it condenses and gives off its heat to the water in the pipe. The warm water rises up the pipe and gives off its heat to a water reservoir above the safety container. The water that cools down again sinks down the pipe and can now absorb further heat. This creates a cooling circuit that works without pumps or electricity – or so the theory goes.
Rivera and his team have now examined in detail for the first time whether such condensation cooling also works in practice. Your measurement data shows how the physical processes take place inside a system on the scale of an SMR nuclear power plant.
Lack of movement in water vapor
The measurements have already revealed some crucial differences to theory. The superheated water vapor was distributed less evenly in the replica containment container than expected. There were no strong convection currents that constantly brought new, fresh steam to the cooling pipe. “Instead, the measurements on the cooling tube show very low gas velocities, so the condensation is primarily controlled by local diffusion of steam and non-condensable gases,” report the researchers.
As a result, the gases separate in the safety container: the hot steam collects in the upper area, and after a while there is almost only air in the lower part. This means that condensation only occurs in the upper part of the cooling tube, which severely limits the cooling performance. The water vapor also only reaches the surface of the pipe slowly, which also reduces the cooling performance. If these effects are not taken into account, the system can dissipate heat less effectively.
(Video: PSI Paul Scherrer Institute)
Further measurements planned
“These results are relevant for the development and evaluation of containment guidelines,” emphasize the researchers. The measurement data can now help to optimize passive cooling systems and also help to specify computer simulations of the processes taking place there. The data thus provides an important basis for the development of future generations of reactors.
The new measurements mark the beginning of an international benchmark initiative involving 25 institutions in different countries. In it, experimental results are used to check and improve simulation methods. Rivera’s team is also already planning a follow-up project in which they want to examine the long-term autonomous operation of passive safety systems in small modular reactors in more detail.
Source: Yago Rivera Durán (Paul Scherrer Institute, Villigen, Switzerland) et al., Nuclear Engineering and Design, 2026; doi: 10.1016/j.nucengdes.2026.114919