[RetroTechie]

Unless someone has more details, "Subcritical" is being generous:

Wednesday's test used chemicals and radioisotopes to "validate new predictive explosion models" that can help detect atomic blasts in other countries, Bloomberg reported, citing the Department of Energy.

Conventional explosives, some radioactive tracers mixed in, and see what can be detected above ground? To improve understanding of how an actual nuclear blast elsewhere may be detected? (say, apart from seismic data or other sources). Or something like that.

In short: not a nuclear blast in any way, shape or form. Fox title is clickbait fluff.

[isthatafact]

I would not call it clickbait. "nuclear test" is a term that has a specific meaning, and it seems that most people would read the headline as meaning that the US has set off a nuclear (fission or fusion) bomb, unless I am completely reading it wrong.

I would consider the headline to be disinformation.