Innovation will be vital to reducing carbon emissions and solving our climate crisis. The brightest minds in nuclear energy have taken this to heart. With new ideas and approaches, they're designing reactors to do more kinds of work, run more flexibly, and fit easily in locations with a need for carbon-free energy, including cities and factories.

X-energy LLC is one of these innovators and its Xe-100 reactor recently joined TerraPower LLC's and GE Hitachi Nuclear Energy's Natrium system in securing support from the U.S. Department of Energy. X-energy's reactor design is a leading example of the next generation of nuclear.

Xe-100 Is a Gumball Machine That Dispenses Carbon-Free Energy

The Xe-100 is a high-temperature, gas-cooled advanced reactor that produces 80 megawatts of electricity and can be scaled up to 320 megawatts. The signature aspect of this design is the "pebble bed," a giant container that holds about 220,000 fuel pebbles and resembles a gumball machine.

The fuel pebbles are small kernels of uranium coated in concentric layers of heat-resistant materials, including graphite, and wrapped together in polished spheres. The fuel is called tristructural isotropic particle fuel or TRISO for short.

They're inserted from the top of the reactor and can be withdrawn from the bottom, even while the reactor is running so there is no need to shut down for refueling. Today's reactors do that every 18 months to two years.

This Design Is Not Like Your Grandmother's Nuclear Plants

The TRISO fuel is a big departure from the fuel rods of today's reactors, and X-energy plans to produce them for other companies' designs as well. But the pebbles aren't the Xe-100's only innovation.

Current reactors use ordinary water to slow down the neutrons in a reactor core to an optimal speed. The water also carries the heat away so it can be used to make electricity. Water is good for moving heat but it expands a lot when heated, creating high pressures. This limits how hot the designers can make the reactor.

The Xe-100 uses helium, which does not expand nearly as much as water when heated, and it uses graphite to slow down the neutrons. A reactor that is based on graphite and helium can run at much higher temperatures than a water-based model. The helium is raised to 1,049 degrees Fahrenheit, compared to about 572 degrees in a water-based reactor.

What's So Hot About a High Temperature Reactor?

A high-temperature reactor gets more work out of each unit of fuel, and high-temperature steam is a more valuable product; it can assist in more industrial tasks, substituting for fossil gas or coal.

And the change that allows higher heat switching to ceramic-coated fuel pebbles and helium coolant adds an additional safety factor: a tiny, super-strong containment. In current reactors, a containment is an enormous concrete-and-steel structure designed to withstand massive steam pressure in case of a major malfunction. Those buildings work well, but they are expensive. In the X-energy design, the containment is the fuel.

Using fuel pebbles and lower pressure makes the building and the components simpler and less expensive. In addition, the pebbles can easily tolerate big temperature swings, so the reactor can vary its output quickly, essential in a grid with a lot of variable wind and solar energy, where the reactors will make up the ever-changing difference between renewable output and total demand.

The emergency cooling system is much simplified, too and the reactor is installed underground, which is better for dissipating excess heat so the emergency planning zone can be 400 yards, instead of 10 miles.

Xe-100 Is Extending the Uses of Carbon-Free Nuclear Energy

A gas-graphite reactor is not an entirely new idea, but the X-energy design shows many refinements over previous models and it offers features that extend its capabilities beyond those of current plants.  Cutting-edge ideas like these will be critical to the transition from fossil fuels, in the electricity system and beyond. A world with a stable climate needs flexible, carbon-free reactors.