Executive Summary

The Paul Scherrer Institute PSI has collected high-resolution measurement data on passive cooling systems for small modular reactors in its PANDA research facility for the first time. These systems utilize physical effects such as density differences instead of pumps or electricity to remove heat in emergencies. The experiments were conducted with partners from over ten countries, and the results were published in the scientific journal Nuclear Engineering and Design. The data obtained closes a critical gap for validating simulations of future reactor generations and forms the foundation for an international benchmark initiative with already 25 participating institutions.

People

  • Yago Rivera Durán (Project Team Leader, PSI Center for Nuclear Engineering and Sciences)

Topics

  • Nuclear energy and reactor safety
  • Small modular reactors (SMR)
  • Passive safety systems
  • Experimental research and data validation
  • International scientific cooperation

Clarus Lead

While small modular reactors are considered a key technology for future energy supply, reliable experimental data on the functioning of their passive safety systems has been lacking to date. The PSI measurements directly address this validation gap and enable simulation developers for the first time to compare their calculations against real laboratory data – a prerequisite for regulatory approvals. The international benchmark initiative signals that the nuclear energy industry considers these research results crucial for the market maturity of modular reactors.

Detailed Summary

The experiment focused on a central safety scenario: when steam from the reactor core is discharged into the outer containment during an accident, this steam must be cooled to prevent pressure buildup. The PSI team tested a closed cooling loop consisting of an approximately six-meter-high vertical pipe through which cold water flows. The escaping steam hits the cold surface, condenses there, and drips back as water. The released heat is transferred to the inner water; since warm water is lighter, it naturally rises, releases heat to a reservoir, and flows back down cooled – a self-regulating cycle without external energy input.

The high-resolution measurement data were novel: Using high-speed cameras, researchers documented for the first time how condensed water droplets form on the pipe surface. They demonstrated that gases in the safety container separate spatially – air collects at the bottom, steam at the top – an effect that is crucial for cooling efficiency. By tracking tiny particles, they also showed that gas flow near the pipe is very slow and condensation there occurs primarily through diffusion, not through larger flows. These local conditions significantly influence cooling performance.

The PANDA facility itself is a globally unique infrastructure: it extends 25 meters across five floors, contains approximately 500 cubic meters of container volume, and 1,450 sensors. A 1.5-megawatt heater generates steam up to 200 degrees Celsius and 10 bar pressure. Gas mixtures can be extracted at over 80 locations and analyzed by mass spectrometry. This flexibility enables the replication of several dozen different SMR concepts. The recent publication launches an international benchmark initiative in which 25 institutions already participate to validate their simulation methods against PANDA data. The follow-up project PANDA-2 will run until 2030 and will focus on more complex scenarios and autonomous long-term operation.

Key Findings

  • Passive cooling systems for small modular reactors function reliably without pumps or external power supply, based on physical effects such as density differences.
  • High-resolution experimental measurement data from PSI close a critical validation gap for computer simulations and regulatory approval procedures.
  • The international benchmark initiative with 25 institutions signals that these research results are considered crucial for the market maturity of modular reactors.

Critical Questions

  1. Data Quality and Scalability: To what extent are the measurement data from the PANDA laboratory facility transferable to industrial nuclear power plants at full scale, particularly with more complex geometries and higher pressures than tested in the experiment?

  2. Long-Term Behavior and Material Compatibility: The experiments demonstrate short-term condensation processes. Do the data also provide insights into long-term behavior, material fatigue, or chemical reactions between steam, water, and container materials under realistic operating conditions?

  3. Research Independence: PSI works in cooperation with Swissnuclear (association of Swiss nuclear power plant operators). How is it ensured that research results are published and evaluated independently of the economic interests of operators?

  4. Alternative Cooling Concepts: Were passive systems from other manufacturers or concepts also tested in the experiments, or do the measurements focus on specific SMR designs already cooperating with PSI?

  5. Failure Scenarios and Limits: Under what conditions (e.g., extreme temperature gradients, pipe contamination, gas mixture anomalies) could the passive cooling system reach its limits, and were these limits experimentally investigated?

  6. Regulatory Acceptance: Which regulatory bodies (e.g., ENSI in Switzerland, international regulators) have already signaled that the PANDA data are sufficient for approval procedures, or are further experiments required?

Source List

Primary Source: [Passive Cooling Systems for Small Modular Reactors: PSI Research] – https://www.news.admin.ch/de/newnsb/E8a18StpfCE2o1vZz3754

Scientific Publication: Rivera, Y. et al. (2026). «Experiments addressing Passive Containment Cooling Systems for Small Modular Reactors in the PANDA facility». Nuclear Engineering and Design, DOI: 10.1016/j.nucengdes.2026.114919

Supplementary Source: Paul Scherrer Institute PSI – Press Release: https://www.psi.ch/de/news/medienmitteilungen/kuehlen-ohne-pumpen-neue-messdaten-fuer-modulare-reaktoren

Verification Status: ✓ 07.05.2026

This text was created with the support of an AI model.
Editorial responsibility: clarus.news | Fact-checking: 07.05.2026