The Spanish newspaper La Verdad has reported that the Russian ship Ursa Major—which sank in the Mediterranean Sea, 60 miles south of Cartagena, on December 23, 2024—was transporting the hulls of two VM-4SG nuclear reactor cores from Russia to North Korea.
The ship sank due to a mysterious underwater explosion, best explained as sabotage.
VM-4SG reactors are used in the nuclear power plant of Project 667BDRM SSBNs, an advanced variant of the Delta submarine. Designed by the Rubin Central Design Bureau and built at Sevmash in Severodvinsk between 1984 and 1992, seven boats were commissioned.
These submarines feature a double-hulled configuration for enhanced survivability, measuring 167.4 meters in length, with a beam of 11.7 meters and a submerged displacement of 18,200 tons. Powered by two OK-700A nuclear propulsion units, they achieve submerged speeds of up to 24 knots and operational depths of 320 meters, with unlimited range and 90-day endurance.
The OK-700A propulsion units (nuclear steam-generating propulsion plants) use two VM-4 series reactors.
The VM-4SG, a specific upgraded variant of the VM-4 reactor, powers second-generation submarines such as the Project 667BDR (Delta III, using VM-4S) and 667BDRM (Delta IV, using VM-4SG), providing reliable, long-endurance operation for strategic ballistic missile roles.
VM-4SG reactors use enriched uranium fuel (typically 20% U-235, though some variants reach 40%) in a compact, modular design with pipe-within-pipe coolant loops for reduced size and enhanced safety.
There is some skepticism over the La Verdad report. Many Russian analysts point out that the reactor is currently not in production.
Indian Connection
Notably, bmpd TC has reported that the VM-4SG reactor is part of the propulsion unit of the Indian Arihant-class SSBN. Since Arihant-class submarines remain in production, it is likely that VM-4SG reactors remain in production.
Officially, the Arihant-class submarines are equipped with Indian-made CLWR-B1 nuclear reactors. However, it was only in 2021 that the Nuclear Power Corporation of India Limited (NPCIL) and Larsen & Toubro Special Steels and Heavy Forgings Limited announced completion of a facility in Hazira to produce nuclear reactor hulls. So far, there has been no report of a hull being manufactured at the facility. Despite the secrecy surrounding India’s nuclear submarine development and manufacturing, the lack of such an announcement appears odd, particularly keeping in mind Indian media hype over self-sufficiency in defence manufacturing.
According to bmpd, Indian sources acknowledge that the Arihant CLWR-B1 reactor is “heavily based on the Soviet VM-4/4SG reactor.” However, it is well known that the CLWR-B1 reactor vessels are manufactured in Russia, along with several other critical submarine components.
The four Arihant-class submarines will be followed by a clean-sheet, new-design SSBN with a 13,300-tonne displacement, carrying twelve K-6 6,000-km-range ballistic missiles. The new design, referred to as the S-5-class submarine, will be powered by CLWR-B2, a 190-MW reactor designed by the Bhabha Atomic Research Centre (BARC).
It is being reported that CLWR-B2 is based on the design of the Soviet VM-11 (OK-650B) submarine reactor, used on Soviet third-generation submarines, including the Project 971 Akula submarines that the Indian Navy leased from Russia.
The OK-650B reactor is rated at approximately 190 MW thermal output. It drives a steam turbine delivering around 43,000–50,000 shaft horsepower (shp) to a single seven-bladed propeller, enabling submerged speeds of up to 33–35 knots and unlimited range.
Conclusion
La Verdad cited details from an official Spanish maritime investigation (report 8059/24-Escora) into the sinking of the Russian cargo ship.
According to the report, the ship was carrying two undeclared, blue-tarped containers at the stern, each weighing approximately 65 tons, that did not appear on the manifest.
Aerial imagery from Spanish rescue operations, while the ship was still afloat but listing, revealed the blue containers. Crew interrogation led to the identification of the undeclared cargo as VM-4SG reactor casings.
The hull breach (a 50 cm × 50 cm inward-bent hole) and seismic data from the explosions pointed to the use of a supercavitating torpedo to sink the ship. The seismic data were consistent with 1.6–1.8 magnitude waves generated by 20–50 kg of TNT.
The ship was tracking an unusual 15,000 km route from St. Petersburg to its declared destination of Vladivostok. Onboard heavy-lift cranes were incongruous with Vladivostok’s facilities but suitable for underdeveloped ports. As such, investigators concluded that the cargo was bound for Rason, North Korea—a port near the Russian border with rail links.
The logically appearing conclusion would be that the ship was sunk by a US/NATO submarine to prevent North Korea from acquiring SSN/SSBN capability.
No independent corroboration of the La Verdad report, such as public photos of the reactors or official Spanish government statements, has emerged.
North Korea is in the process of developing nuclear-powered submarines (SSN/SSBN capabilities). In January 2021, Kim Jong Un announced at the 8th Congress of the Workers’ Party of Korea that the design of a new nuclear-powered submarine was in the final examination stage as part of a five-year weapons development plan.
Notably, North Korea remains upbeat about its SSN/SSBN plans, suggesting that there may have been no setbacks.
In March 2025, state media released photos of Kim inspecting a partially constructed nuclear-powered strategic guided missile submarine, described as a “significant advancement” in naval nuclear capability.
In December 2025, North Korea unveiled the completed hull of an 8,700-ton nuclear-powered SSBN, amid reports of potential Russian assistance in reactor technology.
Developing a nuclear propulsion unit for a submarine is a challenging task. Designing and testing a propulsion unit is one thing; manufacturing it is quite another challenge.
Finally
Fortunately for India, all Arihant-class submarines have already been launched, ensuring that there will be no immediate impact from the loss of two VM-4SG nuclear reactors—if they were indeed lost. However, the Ursa Major sinking starkly underscores the urgency for India to accelerate the development of fully indigenous capabilities for designing and manufacturing nuclear reactors for submarine propulsion.
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