Submarines are kept afloat with the aid of ballast tanks, which can be filled with air. These ballast tanks consist of three groups; located in the stern, the bow and the mid-part of the submarine.

To enhance a submarine’s surfacing capabilities, the ballast tanks located in the mid-part of the submarine have one regular valve and one emergency valve. When diving, the air is pushed out of the mid-part ballast tanks through the regular valve. But air can still escape from the ballast tanks when the valves are sealed if the ballast tanks are leaky or if the valve seal is damaged. Repairs to the ballast tanks’ system are usually carried out in a dock.

Air from the ballast tanks can be released from a moored submarine in stormy weather. The ballast tanks are then filled with water and the floating capability of the submarine is reduced. All submarines are designed to stay afloat even when one of the compartments or two of the ballast tanks on the starboard or portside are flooded. Most of the first generation submarines have leaky ballast tanks, which increases the risk of sinking.

To keep the submarines afloat the ballast tanks have to be refilled periodically with air from onshore facilities. Each submarine is also designed to have its own high-pressure air to use in emergency situations.

The hull of a submarine has hundreds of external valves, which require regular maintenance. Maintenance can only be carried out in dock. If the valves are damaged, water can then seep into the submarine’s compartment. If the adjacent ballast tanks are also damaged, the submarine can sink.

The sinking of a laid-up submarine with spent nuclear fuel onboard represents a threat to the environment. In order to enhance the nuclear safety of laid-up submarines, their primary circuit is secured with a solidifying matter, the electricity cables to the control rods are cut off and the control grid is set to a fixed position.

Those measures can provide nuclear safety in regular situations but in the event of an emergency, such as if a submarine sinks or capsizes, they may prove to be insufficient. It is unclear whether water can penetrate into the primary circuit and, hence, into the reactor, as a consequence of the vessel sinking and the ensuing increase of pressure with increasing depth. Should such a situation occur, it could lead to radioactive contamination of the water and greatly complicate defuelling efforts when the submarine is raised.

Under ordinary circumstances, the probability of an uncontrolled chain reaction is close to zero. But what happens when a submarine capsizes and hits the seabed is hard to predict. The development of an uncontrolled chain reaction is possible in such situation if, for example, the control grid is torn off its fixed position.

Past experience indicates that the most high-risk work is in refuelling or defuelling the reactor. Approximately 50 different technical operations are carried out during the process, 25% of which may potentially expose the operators to radiation. The risks are higher during defuelling of first generation submarines, as their reactor installations do not meet any modern safety requirements.

In the 1990s, a safer method was developed for removing spent nuclear fuel from pressurised water reactors on submarines. First, the reactor tank is emptied of the water before the work begins. This water slows the neutrons inside the reactor. By removing the water from the tank, the risk of an uncontrolled chain reaction in the reactor core is reduced. The drawback of this method is the level of radiation in the reactor compartment increases dramatically because there is no longer any water present to moderate the neutrons. Subsequently, extra measures must be taken to prevent the exposure of the workers to radiation. So this method of defuelling can only be carried out on submarines that have been laid-up for a number of years and the level of radiation has fallen naturally.