The Swiss PANDA test confirms the applicability of passive cooling for small modular reactors.

Researchers at the Paul Scherrer Institute (PSI) in Switzerland have published study results that for the first time precisely document the operation of passive cooling systems for small modular reactors (SMRs). Using the PANDA facility, the scientists have demonstrated that next-generation reactors can be safely cooled exclusively by the natural convection phenomenon, without the need for pumps, valves, or external power, which radically improves safety as it largely eliminates the risk of reactor meltdown.

PANDA (Passive Nachzerfallswärmeabfuhr und Druck-Abbau Testanlage – roughly: “facility for testing passive removal of residual heat and pressure relief”) is a 25-meter-tall facility that simulates processes in a nuclear reactor in real time, but entirely without radioactive material. With a volume of 500 cubic meters and 1,450 sensors, this is the only place in the world where more than ten different design concepts for SMR reactors currently under development worldwide can be tested.

Key findings: Stratification and diffusion

Although the principle of passive cooling and natural convection has long been known, the PSI team has for the first time gathered high-resolution data that changes existing computer simulations. A closed six-meter loop was tested where steam, condensing on a cold pipe, releases heat that naturally drives hot water up to the reservoir, creating a cooling cycle with no mechanical intervention. The test recorded that gases separate inside the containment building—which is a critically important datum because it directly affects the efficiency of heat removal.

“Until now, researchers could not be sure whether their calculations matched reality. With the PANDA project, we are closing that gap,” says Jago Rivera Duran of PSI, who conducted and signed the research.

Promise: fulfilled

The significance of these tests is, in our view, exceptional: it confirms the correctness of claims that the nuclear industry has been making for years, advocating the development of small modular reactors and which has claimed and – continues to claim – that small modular reactors should be much safer than large-capacity reactors precisely because passive cooling potentially eliminates the worst scenario in the nuclear industry: reactor meltdown. Namely, when a reactor is shut down, thermal energy continues to be produced as a result of natural decay of fission products – and this is a process that cannot be stopped. And this residual heat is roughly around 6% of the reactor's thermal power, which means that in large-capacity reactors it is enormous and it is necessary to “force” cool the reactor for some time even when it is completely shut down. At least two of the three major nuclear accidents (TMI and Fukushima) were precisely the result of a loss of cooling when the reactor had already been shut down, after which reactor meltdown occurred. And one of the promises of small modular reactors is, on the other hand, that due to somewhat lower power, passive cooling could be achieved that would significantly reduce the danger of this “worst” scenario in the nuclear industry.

However, this characteristic of a smaller-power reactor is not completely unknown — for example the Soviet OK-650 reactor developed by OKB (Experimental Design Bureau) Afrikantov, which powered a large number of Soviet submarines and was equipped with a sophisticated passive cooling system and, according to some reports, could even produce energy in some regimes relying solely on natural convection.

By the way – this is an interesting field of research, as evidenced by the fact that the PANDA facility is already booked for tests up to 2030, which indicates the importance of such research for licensing and safety assessments of future nuclear facilities.