Nuclear cruise missile explodes

[Enthalpy]

Explosion and reported brief radioactive contamination at a Russian missile test facility in Nyonoksa, 47km from Severodvinsk
https://www.bbc.com/news/world-europe-49301438
https://thebarentsobserver.com/en/security/2019/08/latest-rosatom-says-five-employees-killed-blast-while-testing-isotope-and-liquid
https://www.france24.com/en/20190810-russia-says-five-died-missile-test-explosion
https://www.rt.com/russia/466194-rosatom-killed-arkhangelsk-blast/
https://www.thedrive.com/the-war-zone/29356/russia-admits-mysterious-missile-engine-explosion-involved-nuclear-isotope-power-source
the newspapers of varied origins and quality seem to have very few sources, Rosatom and the Defence ministry, plus unverified videos. 5 dead, 3 wounded.

The cruise missile is supposedly a Burevestnik, said to be propelled by a nuclear reactor for big range, as shall the Poseidon torpedo be
https://en.wikipedia.org/wiki/9M730_Burevestnik
https://en.wikipedia.org/wiki/Status-6_Oceanic_Multipurpose_System
https://www.youtube.com/watch?v=okS76WHh6FI

but the reports on the explosion tell instead
"liquid-propellant engine"
"isotope power source" for a propulsion system
which isn't necessarily a nuclear reactor, where fission happens.

One source fitting the wording would be a big amount of natural or artificial short-lived isotopes that heat the air that passes by in a ramjet, or hydrogen from a tank. This needs a huge radioactivity obtained from a big reactor few hours to weeks before flight. Not convenient.

Or maybe isotopes only heat the propellant before it burns with air, in a bizarre pumping cycle for ramjet or scramjet
https://www.chemicalforums.com/index.php?topic=91121.msg333349#msg333349 and next ones
but I see essentially drawbacks to that. While pumping for a scramjet isn't trivial, chemical or thermal paths bring less worries.

Explanations, suggestions...?