This event shows the use of nuclear propulsion already in cargo ships, a technology well mastered by the Russians (remember their use in Arctic icebreakers thereof). Reactors are always sensitive to cosmic events emitting relativistic neutrons. I checked and found a few sources from various observatories of a cosmic event on the day of the sinking, with locations of the observatory matching. The first sighting is from Las Palmas in Spain.

And is confirmed from Moscow State university.

The absence of plume suggests the use of a fast neutron reactor. If it were crematory there would be a black plume rising. A water reactor would be burning due to water reactivity in contact with the plume. An anonymous source from Russian forces actually confirmed it was a fast neutron system. Fast neutron reactors burst once but absence of moderation and decompaction by the blast mean that delayed neutrons are unlikely to trigger a secondary blast within (in spite of water entry through the hole in the case of this ship).