United States Naval Reactors - Power Plants

Power Plants

U.S. Naval reactors are pressurized water reactors, which differ from commercial reactors producing electricity in that:

• they have a high power density in a small volume and run either on low-enriched uranium (as do some French and Chinese submarines) or on highly enriched uranium (>20% U-235, current U.S. submarines use fuel enriched to at least 93%, compared to between 21–45% in current Russian models, although Russian nuclear-powered icebreaker reactors are enriched up to 90%),
• the fuel is not UO₂ but a metal-zirconium alloy (c.15% U with 93% enrichment, or more U with lower enrichment),
• they have long core lives, so that refueling is needed only after 10 or more years, and new cores are designed to last 25 years in carriers and 10–33 years in submarines,
• the design enables a compact pressure vessel while maintaining safety.

Long core life is enabled by high uranium enrichment and by incorporating a "burnable neutron poison", which is progressively depleted as non-burnable poisons like fission products and actinides accumulate. The loss of burnable poison counterbalances the creation of non-burnable poisons and result in stable long term fuel efficiency.

Long-term integrity of the compact reactor pressure vessel is maintained by providing an internal neutron shield. (This is in contrast to early Soviet civil PWR designs where embrittlement occurs due to neutron bombardment of a very narrow pressure vessel.)

Reactor sizes range up to ~500 MWt (about 165 MWe) in the larger submarines and surface ships. The French Rubis-class submarines have a 48 MW reactor that needs no refueling for 30 years.

The navies of Russia, the US, and the UK rely on steam turbine propulsion. Those of the French and Chinese use the turbine to generate electricity for propulsion. Most Russian submarines as well as all US surface ships since Enterprise are powered by two or more reactors. US, British, French and Chinese submarines are powered by one.

Decommissioning nuclear-powered submarines has become a major task for US and Russian navies. After defuelling, US practice is to cut the reactor section from the vessel for disposal in shallow land burial as low-level waste (see the Ship-Submarine recycling program). In Russia the whole vessels, or the sealed reactor sections, remain stored afloat indefinitely.

Other small, easily field-deployed reactor designs have been developed but have no connection to the U.S. Naval Reactor program. A small reactor was used to supply power (1.5 MWe) and heating to McMurdo Station, a US Antarctic base, for ten years to 1972, testing the feasibility of such air-portable units for remote locations. Two others were installed in Arctic locations, all constructed as part of the US Army Nuclear Power Program. A fourth mounted on a barge provided power and fresh water in the Panama Canal Zone. Russia is well advanced with plans to build a floating power plant for their far eastern territories. The design has two 35 MWe units based on the KLT-40 reactor used in icebreakers (with refueling every 4 years).