Nuclear Propulsion

On-board reactors for submarines and aircraft carriers

In addition to conventional land-based power generating reactors, nearly 220 on-board reactors around the world supply the energy needed to propel 150 nuclear vessels and submarines. Nuclear energy is particularly suitable for ships that need to be at sea for long periods without refueling, such as aircraft carriers, cruisers and submarines or icebreakers lost in remote parts of northern Siberia.

The advantage of Nuclear propulsion is to be completely independent of air. Nuclear submatines do not have to surface frequently like conventional submarines. Another advantage : current generations of nuclear submarines never need to be refueled throughout their lifespans, génerally of 25 years .

However, while onboard reactor technology has proven to be ideal and robust for submarine propulsion, it remains prohibitive in size, weight, and costs for the propulsion of merchant ships.

 

Specificities of the naval propulsion nuclear reactors

The purpose of propulsion instead of electricity production, the particularities inherent to warships in the case of submarines and aircraft carriers, and the cramped housing of the nuclear reactor affect the detailed design and technology.

Especially in the case of submarines, auxiliary facilities of on-board reactors such as cooling, ventilation, safety and control systems, are coexisting with the ordinary functionalities of the ship.

While the power of an reactor generating electricity varies slowly, a ship may have to react rapidly during its mission or to escape a threat. The reactor must be able to respond quickly, reliably and safely to the events inherent to the life of submarines or aircraft carriers.

Availability is also essential. For a submarine, the loss of propulsion at sea can be a very serious event that could lead to the loss of the vessel.

The spent fuel is the source of radioactive waste similar to that of power reactors, but in a smaller quantity. The submarine or aircraft carrier reactor core is replaced during a refit. Once its fuel spent, the core of a submarine or aircraft carrier is replaced. Spent fuel is stored in pools or dry storage casks until radioactivity declines. It may be finally reprocessed or ends in an underground repository.

Features of on-board reactors

Today, the technology adopted for nuclear propulsion is mainly that of pressurized water reactors (PWRs). The thermal power of these reactors ranges from 10 MWth (thermal megawatts) for prototypes, up to 200 MWth for larger submarines and 300 MWth for aircraft carriers and other surface vessels such as Russian cruisers of the Kirov type. Nuclear powered vessels have generally an electrical power in the range of 35-50 MW (megawatts).

The fuels used are very highly enriched uranium oxides. The enrichment at the beginning of the naval propulsion era was up 80 to 95%, in order to have the maximum instantaneous power (fast dives). It is now rather close to 15%.

These reactors should allow variable speed regimes adaptable to the operational circumstances, going from the ordinary cruising speed of submarines to ultra fast speeds in case of diving and intervention : The reactor shoild be able to pass from 10% to 100% of its power in 30 seconds to 1 minute. The cruising speed of large surface vessels is of the order of 25 to 30 knots.