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.

[ret5hd]

1. question:

why design a reactor to need helium…when that is in short supply and needed in other fields (medicine, physics, etc)

btw, i am 5 yrs old…conduct yourself accordingly.

[NNadir]

2. I agree that helium is in short supply. Thus I do not in general support helium cooled reactors nor do I expect...

...that I can support fusion reactors for much the same reason, dependence on helium.

This said, any nuclear reactor of any type is superior to any fossil fuel or so called "renewable energy" device.

Helium in a nuclear reactor is used in a closed system and in theory, if not in practice, does not demand anything beyond a single recharge. This is not in practice realistic, since helium is about as bad as hydrogen in a propensity for leaking, having low viscosity.

For gas cooled nuclear reactors, I favor carbon dioxide, which has been industrially utilized in British AGCRs.

Many TRISO designs do not rely on helium cooling, although TRISO fuel was originally developed for reactors that were. The Kairos reactor for example is TRISO fueled but heat transfer is carried out in a molten salt, not helium.

I am not up to speed on the nature of the Antares nor the Valar reactors. I am merely thrilled that any nuclear reactors are being built and tested.

Again, I'm not a TRISO kind of guy. Left to my own devices, I tend to favor metallic, both liquid and solid, nuclear fuels, as well as in some cases, carbides, oxides, and nitrides that are in relatively pure form. Carbides and nitrides are easily reprocessed, as they used to say in television and magazine ads, "just add water." My deepest preference is for metallic fuels, followed by nitrides and carbides. The background on my computer is, nonetheless, the plutonium/carbon phase diagram, albeit connected with my fascination with the low melting point of plutonium and its eutectics with neptunium and iron, as well as the less desirable ternary eutectic with cobalt and cerium.

A little more about helium:

All of the helium on Earth is derived from the decay of actinides and actinide alpha decay products, beginning with uranium and thorium, including elements that are subactinide nuclei such as radium, radon, polonium, thallium and lead exhibiting alpha decay. It took many billions of years for this helium to accumulate in the rocky material in which it forms. We are going through it with abandon. Every time I see a party balloon, my heart breaks.

There are certain actinides, such as ²³⁸Pu, ²³²U, ²⁴¹Am, ²⁴²Am, ²⁴²Cm, among many others up to californium isotopes that are potential sources of helium. This said the same energy to mass ratios that make nuclear fuels vastly superior to all other energy materials in an environmental sense, means that they can only be a tiny source of helium, even if we are to include ³He from the decay of tritium.

I hope this response is to your liking. I will say that for a 5 year old you are very precocious, more precocious than many of my correspondents in forums here who could benefit by advancing beyond the 5 year old intellectual and emotional level. Obviously, as a 5 year old, in the next 50 or 60 years if you live so long, you will advance way beyond anything I can tell you. You may, as I have more or less done for fission products and actinides, be able to memorize those portions of the table of nuclides that are involved in the realization of nuclear energy's obvious superiority to all other forms of energy.

On an even more serious note, the depletion of helium, as is the case with other critical elements, is a very, very, very serious matter, far beyond a joke. Anyone who is five years old will have to live with the consequences of my generation's profligate use of resources. I am inclined to feel very guilty to what we have done to all of the toddlers on the planet, as well as infants and adolescents. I hope a five year old can survive hearing the profanity I am about to use: My generation has fucked the future and we lack the decency to even care.

I'm appalled.

Have fun in Kindergarten and first grade.

[ret5hd]

3. TY...and i also cringe when i see a party balloon.