Interesting hypothesis, but I’m not convinced. I would need to see some kind of backup for this sentence:

Authorities later identified them as housings for VM-4SG nuclear reactors.

Russia has a rail connection to North Korea and most submersible reactors are small enough to transport by rail. We can determine the dimensions of a submarine-based reactor using this article: “Rare Look At Nuclear Reactor Inside Russian Ballistic Missile Submarine”. Using the foot of the sailor for reference, we can conclude it’s pretty compact - it would fit on a rail carriage. This seems to remove a rational motive for transporting by sea, putting the component within reach for other states.

The parts visible on the aft deck were very likely to be nuclear reactor parts - reactor covers for a Project 10510 icebreaker they’re building. Large ones, not something you put on a submarine. The only possible consumer would be a large vessel.

In the middle of the ship, there were two large cranes. Also intended for Vladivostok, which was suffering from logistical backlog. Consequently, we can be pretty sure it was sailing towards the Far East and likely would have visited Vladivostok first - to get rid of the cranes and reactor covers.

What was in the hold - I wonder if anyone knows.

North Korea is in the far east, there’s no denying that.

It should be noted that North Korea is building a nuclear submarine. It seems to resemble the ancient Russian “Golf” class ships, but isn’t a direct copy. So they would definitely need nuclear reactor parts, but the ship seems mostly completed - it should already have a reactor inside.

Technical note: in a type 10510 icebreaker, two reactors sit on the top deck side by side (for ease of maintenance) and Ursa Major / Sparta / whatever its name currently was - it was carrying two components resembling a reactor cover. In a nuclear submarine, the reactor (or several) is embedded deep in the hull. When you start painting the hull of a submarine (and NK has started), you generally must have a reactor installed. This does not preclude them wanting a better reactor for a next generation sub.

Back when it sank, it was said that it was carrying cranes needed at the Vladivostok port for expanding the Russia’s ability to export over the sea to southeastern Asia.

Might have been true as well, but it’s interesting that this kind of important piece of information managed to go missing for so long because it looked like there was already an answer to why the ship was on the way. And also, to why it was sunk.

Sink them all.

Spanish investigators have confirmed that the Russian cargo vessel Ursa Major, which sank off the coast of Cartagena in December 2024, was carrying undeclared nuclear reactor components likely bound for North Korea.

According to La Verdad, the ship was part of Russia’s shadow fleet and took an unusual route from St. Petersburg to Vladivostok through the Mediterranean.

Though the ship’s manifest listed only empty containers and port equipment, aerial images revealed two large, undeclared containers at the stern. Authorities later identified them as housings for VM-4SG nuclear reactors.

What does a housing for a nuclear reactor look like?

“Ghost Ships” are very high risk, high reward operations. You can’t say anything when they get sunk because they never were supposed to have existed. Makes them wonderful targets.