Passive Safety Systems & Natural Circulation

A passive safety system has to work after the power is gone and the pumps have stopped. Fewer moving parts mean fewer things to fail, but the driving force is correspondingly weak, so how much heat it can actually remove has to be known precisely.

First, natural circulation. For external reactor vessel cooling, which floods the outside of the vessel in a high-power reactor after the core has melted, we solved the whole system with the MARS-KS system code and used CFD alongside it to estimate the natural circulation flow rate (Nuclear Engineering and Technology, 2022). A system code alone struggles to carry the local resistance that the geometry creates.

Second, setting a direction with no moving part. A vortex-type passive fluidic diode flows easily one way and poorly the other; we designed that geometry by topology optimization (2019). Valves belong here too: the transient behaviour of an SMR passive safety valve as it opens was solved with CFD and weighed against what a system code can do with the same problem (2025), and the load on an emergency depressurization valve was analysed as well (NTHAS-13, 2024).

Third, heat pipes. They move heat by phase change and so need no pump, but an alkali-metal heat pipe starts up with its working fluid frozen. We solved that startup with a conduction-based model (Applied Thermal Engineering, 2025). Placing vapour chambers in a fuel assembly to spread heat more evenly was compared against conventional flow designs (2025), and a transportable heat-pipe latent heat storage system for microreactors is the subject of a patent application (2022). Work on non-nuclear experimental capabilities for microreactors, carried out at Idaho National Laboratory, connects to the same thread (Nuclear Technology, 2023).