The sinking of the Russian cargo vessel Ursa Majo in the Mediterranean Sea has triggered fresh speculation over covert nuclear proliferation and underwater sabotage, after a Spanish newspaper reported that the ship was transporting sensitive nuclear reactor components allegedly bound for North Korea.
According to La Verdad, the vessel went down roughly 60 miles south of Cartagena on December 23, 2024, following what Spanish investigators described as an unexplained underwater explosion. The newspaper claimed the ship was carrying the hulls of two VM-4SG nuclear reactor cores—technology associated with Russian nuclear-powered ballistic missile submarines—on a clandestine transfer route from Russia to North Korea.
Spanish maritime investigators reportedly discovered that two large, undeclared containers, concealed under blue tarps at the ship’s stern, were not listed on the cargo manifest. Each container weighed approximately 65 tonnes. Aerial imagery taken during rescue operations showed the containers clearly while the ship was still afloat but listing heavily. Subsequent crew interrogations, cited by La Verdad, led investigators to identify the cargo as VM-4SG reactor casings.
The ship’s hull was found to have a 50-by-50-centimetre breach bent inward—damage inconsistent with an internal explosion or collision. Seismic data recorded at the time of the incident reportedly detected shockwaves equivalent to a 1.6–1.8 magnitude event, consistent with the detonation of 20–50 kilograms of TNT. Investigators concluded that the most plausible explanation was sabotage using an underwater weapon, possibly a supercavitating torpedo.
VM-4SG reactors are closely associated with Russia’s Project 667BDRM Delta IV class of nuclear-powered ballistic missile submarines, designed by the Rubin Central Design Bureau and built at the Sevmash shipyard between 1984 and 1992. These submarines feature a double-hulled design for survivability, displace approximately 18,200 tonnes submerged, and are powered by two OK-700A nuclear propulsion units. Each propulsion unit relies on a pair of VM-4 series reactors.
The VM-4SG is an upgraded variant of the VM-4 family, using enriched uranium fuel and compact, modular construction with pipe-within-pipe coolant loops designed to reduce size and enhance safety. The reactor enables long-endurance operations required for strategic deterrence patrols.
The claim that such reactors were being shipped abroad has been met with skepticism among Russian analysts. Some argue that VM-4SG reactors are no longer in production, making their alleged export implausible. However, military analysts at bmpd TC counter that the VM-4SG design remains relevant because it forms the technological basis of India’s Arihant-class nuclear-powered ballistic missile submarines.
Officially, the Arihant class is powered by India’s CLWR-B1 reactor, described as an indigenous pressurized light-water reactor. Yet Indian sources have acknowledged that the CLWR-B1 is heavily based on the Soviet VM-4 and VM-4SG designs. While India inaugurated a reactor hull manufacturing facility at Hazira in 2021, there has been no public confirmation that submarine reactor vessels have been produced there, leading analysts to suggest that key components continue to be manufactured in Russia.
The Ursa Majo’s route has further fueled suspicions. Although officially sailing from St. Petersburg to Vladivostok—a journey of approximately 15,000 kilometers—the ship was equipped with heavy-lift cranes unnecessary for Vladivostok’s modern port infrastructure. Spanish investigators reportedly assessed that the cargo was more likely destined for Rason, a relatively underdeveloped North Korean port near the Russian border with rail connections into the country.
If accurate, the shipment would align with North Korea’s long-stated ambition to develop nuclear-powered submarines. At the 8th Congress of the Workers’ Party of Korea in January 2021, Kim Jong Un announced that the design of a nuclear-powered submarine was in its final review phase. Since then, Pyongyang has released images of what it claims are nuclear-powered submarine hulls under construction, including photos published in March 2025 and the unveiling of an 8,700-ton strategic submarine hull in December 2025.
Despite these developments, there is no independent corroboration of La Verdad’s claims. Spanish authorities have not publicly confirmed the presence of nuclear reactor components on the vessel, nor have photographs of the alleged reactors been released. The lack of official confirmation has led some analysts to caution against drawing definitive conclusions.
Nevertheless, the incident highlights the extreme sensitivity surrounding nuclear propulsion technology. Designing a submarine reactor is only part of the challenge; manufacturing reactor vessels, fuel assemblies, and associated systems requires highly specialized industrial capacity and decades of experience.
For India, the sinking—if it did involve VM-4SG components—would not pose an immediate operational setback, as all Arihant-class submarines have already been launched. However, the episode underscores the strategic risks of dependence on foreign supply chains for nuclear propulsion systems and reinforces the urgency of achieving fully indigenous reactor design and manufacturing capabilities.
Whether the Ursa Majo was the victim of covert interdiction or an accident cloaked in speculation, its loss has exposed the shadowy intersection of naval nuclear technology, geopolitical rivalry, and undersea warfare—an arena where proof is scarce, stakes are high, and silence often speaks louder than confirmation.
