A nuclear submarine is a vessel powered by a nuclear reactor, typically a pressurized water reactor fueled by enriched uranium. This propulsion system provides virtually unlimited range and high speeds. Many of these vessels also carry nuclear weapons, such as ballistic missiles or torpedoes, adding a second source of radioactive material. Since the advent of nuclear propulsion, several nations have lost nuclear submarines to accidents at sea. The primary concern regarding these sunken vessels is the long-term risk of radioactive contamination to the marine environment. This article explores the radioactive material, the potential for sudden accidents, and the chronic environmental hazard posed by these submerged wrecks.
Sources of Radiation and Potential Acute Risks
The radioactive inventory of a sunken submarine comes from two distinct sources: the reactor core and any nuclear warheads onboard. The reactor core contains a high volume of radioactive material, primarily the uranium fuel and highly radioactive fission products created during operation. The fuel is highly enriched uranium, designed to maximize power output and core lifespan.
The warheads contain highly enriched fissile material like plutonium or weapons-grade uranium, though in smaller quantities than the reactor fuel. Warheads are designed to be stable, meaning the primary risk is the release of fissile material, not a nuclear explosion. The most immediate, though low-probability, danger is an unintended chain reaction, known as a criticality accident. This occurs if the fissile material is rearranged to form a critical mass, causing a sudden, localized burst of radiation and heat. A non-nuclear explosion, such as from conventional weapons, could also pose an acute risk by breaching the reactor vessel and scattering radioactive debris.
The Threat of Long-Term Radionuclide Leakage
The substantial, long-term risk comes from the gradual degradation of the submarine’s protective barriers in the harsh marine environment. The reactor is encased within a thick, steel pressure vessel and the submarine’s hull, which acts as the initial shield. Over decades and centuries, saltwater corrosion slowly eats away at the hull and internal structures, eventually allowing seawater to contact the radioactive material.
Once containment is breached, specific radionuclides dissolve and disperse into the ocean water and sediment. The most concerning materials are the long-lived fission products and transuranic elements, which pose a chronic hazard. Cesium-137 and Strontium-90 are highly water-soluble, allowing them to disperse widely. Plutonium isotopes remain hazardous for a very long time due to half-lives measured in tens of thousands of years.
These radioactive substances can enter the marine food web through bioaccumulation, where organisms absorb contaminants from the surrounding water and sediment. Although the ocean provides significant dilution, the contamination risk is highly localized to the immediate area around the wreck site. This localized contamination can potentially affect deep-sea ecosystems and commercial fishing grounds. Assessments suggest that significant corrosion and large-scale leakage could begin within 20 to 30 years of sinking for some Soviet-era submarines.
Tracking and Mitigation Efforts
A total of eight nuclear submarines are known to have been lost at sea: two by the United States and six by the former Soviet Union or modern Russia. These wrecks are primarily located in the North Atlantic and Arctic Oceans, particularly the Barents and Kara Seas. The risk profile of each wreck varies significantly based on the reactor design, the vessel’s depth, and the amount of radioactive material remaining.
International bodies and national governments, particularly the US and Russia, maintain programs to monitor the most high-risk wrecks. Monitoring is often conducted using remote operating vehicles (ROVs) to take sediment and water samples, checking for elevated radionuclide levels. For wrecks deemed to pose an unacceptable risk, like the Russian submarines K-27 and K-159, mitigation efforts have been proposed or discussed.
Mitigation Strategies
Mitigation strategies range from leaving deep-sea wrecks undisturbed due to the difficulty of recovery, to more active measures. These include stabilizing the site with protective structures or attempting salvage. Some salvage operations, such as the lifting of the Kursk, have successfully removed the bulk of the vessel. The long-term goal for many sites is to prevent the dispersal of radioactive materials until the shorter-lived radionuclides have decayed significantly.