Sure, it’s conventional explosive with radioactive markers to test the detection capabilities of their equipment. I was being polite with my earlier comment, in case I had missed something in the article, but I guess I didn’t.

The Wikipedia page on Explosives gives some reading on the differences between the two. Suffice it to say, chemical and nuclear explosions are fundamentally different things; breaking/reforming of molecular bonds to produce heat and energy, compared to splitting or fusing atoms themselves, which releases FAR more energy than those molecular reactions since the bonds holding atomic nuclei together are so much stronger. If we say that a nuclear explosion is literally any explosion + radioisotopes, then you could buy some uranium online, tape it to a brick of plastic explosive, and say you’ve got yourself a nuke. Maybe someone should tell Iran they don’t need to waste all that money on centrifuges.

You’re half right though, in that this probably is a response to Russia, but demonstrating your ability to detect underground nuclear tests is not at all the same thing as actually conducting one.