On Friday, the Nuclear Regulatory Commission (NRC) announced that it would be issuing a certification to a new nuclear reactor design, making it just the seventh that has been approved for use in the US. But in some ways, it's a first: the design, from a company called NuScale, is a small modular reactor that can be constructed at a central facility and then moved to the site where it will be operated.

The move was expected after the design received an okay during its final safety evaluation in 2020.

Small modular reactors have been promoted as avoiding many of the problems that have made large nuclear plants exceedingly expensive to build. They're small enough that they can be assembled on a factory floor and then shipped to the site where they will operate, eliminating many of the challenges of custom, on-site construction. In addition, they're structured in a way to allow passive safety, where no operator actions are necessary to shut the reactor down if problems occur.

Many of the small modular designs involve different technology from traditional reactors, such as the use of molten uranium salts as the reactor fuel. NuScale has a much more traditional design, with fuel and control rods and energy transported through boiling water. Its operator-free safety features include setting the entire reactor in a large pool of water, control rods that are inserted into the reactor by gravity in the case of a power cut, and convection-driven cooling from an external water source.