Advanced new reactor designs offer the potential to transform the nuclear power industry, providing safe, carbon-free electricity that addresses the plethora of challenges arising from decarbonization of power grids worldwide.

One such design pathway utilizing tri-structural isotropic (TRISO) coated fuel as the energy source within high-temperature gas-cooled reactors (HTGRs) — commonly called pebble bed reactors — could soon emerge as one of the most promising pathways for nuclear power stations of the future.

Potential Game Changers

A pebble bed nuclear power plant currently being designed by X-energy for Energy Northwest in Washington state could become the first to utilize TRISO fuel in an HTGR design in the U.S. With a total net rated capacity of 320 megawatts electrical (MWe), four X-energy Xe-100 Generation IV reactors could receive regulatory approval and be brought online by 2028.

X-energy has received funding from the U.S. Department of Energy (DOE) under the Advanced Reactor Demonstration Program that was authorized in the Energy Act of 2020. A total of $3.2 billion was authorized for nuclear demonstration projects with funding available over the fiscal year 2022-27 budget cycles. The program is intended to speed demonstration of the most viable advanced reactor designs through cost-shared partnerships. In addition, $6 billion in funding has been authorized by the Infrastructure Investment & Jobs Act, targeting microreactors, small modular reactors and various types of advanced nuclear reactors.

TRISO Is a Leap Forward

The TRISO fuel design uses fissile uranium-235, the same material used in conventional light water reactors (LWRs) in service today. However, instead of fuel rods, uranium in this design is placed inside a fuel kernel composed of three layers of refractory carbon and silicon carbide material. This uranium oxycarbide (UCO) kernel measures about 0.855 millimeters in diameter, about the size of a poppy seed. Then, about 18,000 of these tiny TRISO particles are embedded inside a graphite sphere, measuring about 6.35 centimeters in diameter. The uranium encased in these tennis-ball-sized fuel pebbles is impervious to extraction for other uses such as enrichment to weapons-grade plutonium.

The Xe-100 continuously recycles about 200,000 of these fuel pebbles through a gravity feed reactor core. As fuel pebbles reach the bottom of the reactor vessel, they are extracted mechanically and checked. Those with remaining life are recirculated back to the top of the cylindrical reactor vessel, while those that are spent are removed and routed for storage in a dry cask containment system.

This continuous recycling process means there is no need for refueling outages like those needed every 18 months at conventional light water reactor plants. Thus, over the estimated 60-year design life of the Xe-100 reactor, the plant would be a highly available (approximately 95%) power source for the grid.

Safe Energy Alternative

Unlike conventional reactors, the Xe-100 design uses no water for cooling. Instead, it uses pressurized helium that circulates inside the reactor core as the coolant to keep temperatures within a stable range and provide the heat transfer mechanism. As an inert gas, helium is an incredible conductor without becoming radioactive.

TRISO fuel is designed to withstand temperatures four times greater than those present in conventional LWR nuclear facilities. Once encased within the graphite pebble, each individual fuel particle acts as a microscale containment that cannot melt down.

This avoids the potential safety issues faced by LWRs when the active coolant and moderator (water) is lost, and fuel rods begin to overheat and melt down. When a pebble bed reactor is shut down, there isn’t any heat left to circulate through the core because the fuel itself has limited the nuclear chain reactions inside of the reactor. Combining this fuel with an appropriate reactor design reduces the need for large concrete structures, because it is an inherently safe advanced nuclear power station. Advanced reactors are walk-away safe, meaning that they don’t need any additional fail-safe mechanisms for removing residual heat from the core.

Process Heat Is Ancillary Benefit

The heated helium circulates through an adjacent vessel containing water, superheating it into steam at about 1,000 degrees Fahrenheit. This steam then turns a turbine to produce carbon-free electricity.

An Xe-100 plant can also be used as a source of process heat for a wide range of industrial applications such as district energy configurations for military bases, refineries, manufacturing, hospital and university campuses, desalination and hydrogen production (referred to as pink hydrogen).

Baseload and Load Following Capacity

The Xe-100 is designed to ramp up or down at approximately 5% per minute in response to load conditions. The optimal range is between 100% and 40% of net capacity, but the reactor is able to operate at stable power levels down to 25% of capacity.

Conventional nuclear power stations struggle with ramping up or down because load changes require reactivity changes in the reactor, which are a challenge to manage. Thus, nearly all are considered baseload units because they operate most efficiently when running at full output capacity.

Because of its reactor design, the Xe-100 can function as a flexible power generating station, much like many advanced class gas-fueled facilities that can counter the effects of intermittent renewable generation on the grid.

New fuel and innovative reactor designs like the Xe-100 will undoubtedly become a prominent part of the advanced nuclear reactors needed to move safely toward a decarbonized grid.


See how we are supporting X-energy with a range of engineering and studies for its HTGR design project.

Learn More

Jim Jurczak is project management director for the Energy Group at Burns & McDonnell. In a career spanning over 30 years, Jim has overseen design, construction and startup of some of the company’s largest power plant projects, including fossil-fueled combined-cycle facilities and nuclear power projects.