On Wednesday, the US Nuclear Regulatory Commission announced that it had issued its first construction approval in nearly a decade. The approval will allow work to begin on a site in Kemmerer, Wyoming, by a company called TerraPower. That company is most widely recognized as being financially backed by Bill Gates, but it’s attempting to build a radically new reactor, one that is sodium-cooled and incorporates energy storage as part of its design.

This doesn’t necessarily mean it will gain approval to operate the reactor, but it’s a critical step for the company.

The TerraPower design, which it calls Natrium and has been developed jointly with GE Hitachi, has several novel features. Probably the most notable of these is the use of liquid sodium for cooling and heat transfer. This allows the primary coolant to remain liquid, avoiding any of the challenges posed by the high-pressure steam used in water-cooled reactors. But it carries the risk that sodium is highly reactive when exposed to air or water. Natrium is also a fast-neutron reactor, which could allow it to consume some isotopes that would otherwise end up as radioactive waste in more traditional reactor designs.

The reactor is also relatively small compared to most current nuclear plants (245 megawatts versus roughly one gigawatt), and incorporates energy storage. Rather than using the heat extracted by the sodium to boil water, the plant will put the heat into a salt-based storage material that can either be used to generate electricity or stored for later use. This will allow the plant to operate around renewable power, which would otherwise undercut it on price. The storage system will also allow it to temporarily output up to 500 MW of electricity.