There was some sort of event, involving a significant release of radiation, at the Nenoksa naval base:

On the morning of Thursday, August 8, something exploded at the Nenoksa Naval Base in Russia, not far from the city of Severodvinsk. This article is a good summary of what we knew by Friday. Since then, the Russian government has said that a radioactive source was involved in the explosion, along with liquid rocket fuel. Reports have gone back and forth on whether radiation detectors in Severodvinsk detected anything. Five more people have been reported dead. Sarov/VNIIEF, one of the Russian nuclear weapons laboratories, has released a statement, which some folks are rushing to translate.

The translation that I have seen of this video shows it not to be particularly informative, but it does reveal that there was an incident, and there were fatalities.

The New York Times coverage is similarly anodyne, though it does reveal a spike in radiation, albeit one that stays below recommended limits, at a nearby town.

The indications are that this is not a nuclear warhead, both Russian and US nuclear warheads have been designed to survive things like a rocket motor explosion, so it would imply that it was a test of some sort of nuclear propulsion system, along the lines of the 9M730 Burevestnik nuclear powered cruise missile announced by Putin last year.