The job of the coolant is to remove heat from the very hot fuel Balls and to transfer it out of the reactor to the water heat-exchanger. The water heat-exchanger is known as the Steam Generator because the heated water comes out as high temperature steam which then goes to the turbines. The Helium is returned to the reactor.

Some SMR-class reactors are cooled with Lead or a Lead-Bismuth alloy, others with liquid Sodium metal, and yet others with a liquid salt. In each case, the job of the coolant is to move heat from the inside to the outside of the reactor.

So, heat transfer properties are important in choosing a coolant, and metals like Lead or Sodium transfer heat well. But there are other practical considerations as well, like: What happens when the reactor cools down for a maintenance inspection?

You can’t allow Lead to solidify inside the reactor. The same holds true for Sodium, and for that matter, for salt although the melting point of the salt is quite low. Another factor is; what if the coolant leaks out for any reason? Or, how do you handle it when it is unloaded intentionally for a maintenance inspection? Sodium reacts violently with air or water, and the Lead would be heavy and hot. A liquid salt also presents similar problems. In all cases our team felt that the simplicity of dealing with Helium was a major factor compared to the other possible coolant options.

Helium is also not corrosive and in fact is completely inert and safe to handle.

Other factors were also considered, like; you can see through Helium. That means that an optical probe can be inserted into the reactor or coolant-line for a visual inspection without removing the coolant.

Some SMR development companies chose a coolant such as Lead because they already had some existing experience in that technology from previous work that they had done, such as developing power packs for nuclear submarines. We had no such constraints and so could make a free choice for a commercial civilian use.

Yet another factor is that in the extremely unlikely event of an accident in which the coolant runs out of the reactor, Helium would pose a minimal hazard to emergency responders. Helium is perfectly safe for people, and even hot Helium would expand and cool rapidly, in contrast to very hot Lead or Sodium.

So, after careful consideration of all the factors; from cooling efficiency to ease of maintenance and inspections, we chose Helium as the best option.

A fortuitous development in South Africa has also been that one of the world’s largest deposits of Helium has been discovered in South Africa and is currently being developed by the company Renergen. https://phys.org/news/2021-10-helium-south-africa-gold.html