X-energy has work ahead in quest to build TRISO-X fuel facility, Xe-100 reactor

November 10, 2021, 3:00PMNuclear News
The TRISO-X fuel pebble shown here contains TRISO particles—HALEU-bearing kernels of oxide and carbide in alternating layers of pyrolytic carbon and silicon carbide. (Image: X-energy)

X-energy and Centrus Energy announced last week that they have completed the preliminary design of the TRISO-X fuel fabrication facility and have signed a contract for the next phase of work. The planned facility would produce TRISO fuel particles and pack those particles into fuel forms, including the spherical graphite “pebbles” needed to fuel X-energy’s Xe-100 high-temperature gas reactor.

But before TRISO-X can produce its first pebble, X-energy must have a final design and a license application approved by the Nuclear Regulatory Commission, an application for what would be the only Category II fuel fabrication facility in the country, built specifically to handle high-assay low-enriched uranium (HALEU).

Regulatory engagement: Documentation, inspections, and meetings that support X-energy’s licensing of the TRISO-X fuel facility are underway. X-energy submitted a topical report, “TRISO-X Pebble Fuel Qualification Methodology, Revision 2” to the NRC on September 2. On October 8, the NRC confirmed by email to TRISO-X that the submission had passed a completeness review and that “the NRC staff expects to issue a draft safety evaluation by July 22, 2022.” A previous version of the same topical report had been submitted in April and discussed in a June meeting between X-energy and the NRC.

On October 4, the NRC approved the TRISO-X Facility Safeguards Information Plan. The approval followed an August 31 visit by NRC staff to an X-energy facility in Oak Ridge, Tenn., where NRC staff inspected the company’s security structures, computer systems, equipment, and components and interviewed staff.

Pebble cycle: The first Xe-100 power plant, a high-temperature gas-cooled pebble-bed reactor capable of producing 320 MWe in its standard four-module configuration, is to be built by 2027 near Richland, Wash., through a cost-shared award from the Department of Energy’s Advanced Reactor Demonstration Program.

According to design documentation included in X-energy’s September 2 fuel qualification submittal to the NRC, the Xe-100’s active core volume “is filled with approximately 224,000 spherical graphite fuel elements, or pebbles, each containing about 19,000 TRISO-coated particles. . . . Fissions within the coated particles create the nuclear heat which is conducted to the pebble’s surface. A helium circulator transports the helium heat transport fluid through the pebble bed, carrying heat from the pebbles to the steam generator.”

Fuel pebbles are loaded into the core through a central tube at the top of the reactor pressure vessel while the reactor is operating. “A fuel discharge system at the bottom of the core removes spent pebbles through the bottom of the RPV for assessment by the Burn-up Measurement Systems (BUMS) that evaluates each pebble for physical damage and burn-up, and can return them to the top of the RPV, where they are again loaded into the core, or send them to spent fuel storage.” A typical pebble goes through the process multiple times and is removed from the reactor before reaching burn-up limits. When those limits are reached, the pebbles are inventoried and stored in spent fuel casks.

Brought to you in part by the U.S. government: TRISO fuel is not new, having been demonstrated in high-temperature gas-cooled reactors in the United Kingdom, the United States, Germany, and elsewhere. But to fuel U.S. advanced reactors, the DOE launched the Advanced Gas Reactor (AGR) Fuel Development and Qualification Program in 2002 help establish a domestic, commercial TRISO fuel fabrication capability in the United States and to generate fuel performance data. X-energy’s work is supported by data obtained through the AGR qualification program demonstrating fuel performance under a range of operating conditions.

To learn more, read Two decades of DOE investment lays the foundation for TRISO-fueled reactorsby Paul Demkowicz, of Idaho National Laboratory, and John Hunn, of Oak Ridge National Laboratory, in the August 2021 issue of Nuclear News.

“Poised to break ground”: On August 9, X-energy announced that it will expand its fuel division as a wholly owned subsidiary, called TRISO-X, to commercially develop its proprietary tristructural isotropic (TRISO) fuel.

“X-energy is ready to begin commercial production after years of refining its TRISO fabrication processes,” the company said. “TRISO-X is poised to break ground on a commercial facility in order to meet the Xe-100 deployment schedule that sees at least four Xe-100 modules coming online in Washington state by 2027.”

The company is also advancing plans for a second fuel fabrication facility in Canada to serve customers that could include Ontario Power Generation, which is reportedly considering the Xe-100 design for the Darlington new nuclear project.

Categorically: The Xe-100 would use HALEU enriched to 19.75 percent uranium-235, which puts the material in the NRC’s Category II (between 5 and 20 percent U-235). While Category III facilities can fabricate light water reactor fuel enriched to below 5 percent U-235, HALEU fuel must be fabricated in a Category I or II facility.

X-energy is not the only U.S. company actively developing commercial TRISO fuel fabrication capabilities. On November 10, 2020, BWX Technologies announced that it had restarted a TRISO fuel production line in Lynchburg, Va. As a Category I facility, BWXT’s facility is licensed to fabricate uranium fuel at all levels of enrichment.

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