Two New US Nuclear Reactor Designs Go Critical.

[NNadir]

Valar Atomic’s Ward 250 Becomes Second Reactor to Go Critical Under DOE Pilot Program

Excerpts:

Valar Atomics has achieved self-sustaining criticality and completed zero-power testing at Ward 250, its Gen IV tri-structural isotropic (TRISO)-fueled modular high-temperature gas reactor (HTGR), at the Utah San Rafael Energy Lab in Emery County. The project is the second advanced reactor to go critical under the Department of Energy’s (DOE’s) Reactor Pilot Program and the first DOE-authorized reactor built and operated outside the national laboratory system.

The milestone, confirmed by DOE’s Office of Nuclear Energy at about 4:30 p.m. MDT on June 18, involved a “zero-power fueled criticality demonstration,” the DOE said...

...Ward 250’s criticality comes less than two weeks after Antares Nuclear’s Mark-0 became the first advanced reactor to go critical under the Reactor Pilot Program on June 4 at INL. Antares Nuclear’s Mark-0 is a sodium heat-pipe-cooled microreactor fueled by high-assay low-enriched uranium (HALEU) TRISO fuel compacts.


Ward 250, an HTGR rated at 100 kWt initial test power and scalable to 5 MWe, uses helium coolant and TRISO fuel particles in Advanced Gas-Cooled Reactor (AGR) compacts, according to an October 2025 Valar quality-assurance program description. The document says Ward 250 incorporates passive safety features and builds on WardZero prototype technology, while the co-located Valarin Fuel Fabrication Facility is designed to manufacture TRISO-coated particle fuel embedded in graphite compacts using German HOBEG technology with modern process improvements. Kiewit Nuclear Solutions served as the project’s engineering, procurement, and construction contractor...

Both reactor types are TRISO based. I'm certainly not a TRISO kind of guy, since I favor used nuclear fuel reprocessing, but the TRISO fuel type seems to be very popular, featured in the Kairos commercial reactor now under construction in Tennessee.

This said, there are zero nuclear reactors that are inferior to any other energy source, including the land and material profligate so called "renewable energy" systems, which depend on access to fossil fuels, and fossil fuels themselves.

Zero power criticality doesn't mean all that much, the real task is powered criticality.

An advantage to TRISO fuels is that they are inherently high temperature fuels. Raising the temperatures of nuclear reactors under low pressure conditions is key to moving nuclear energy beyond mere electricity generation to process intensive roles that can eliminate fossil fuels entirely.