Rajan Wilcox
Nuclear reactors have been powering naval vessels since the USS Nautilus first put to sea in 1955. Since then, the US Navy has accumulated over 5,400 reactor years of operation and 128,000,000 miles safely steamed. The US and UK have historically used highly enriched uranium (HEU) to fuel their naval reactors, while France and China have opted for low-enriched uranium (LEU). The amount of fuel required depends on the enrichment level, with HEU-fuelled reactors needing less volume to produce the same amount of power as LEU-fuelled reactors. For example, a US Navy submarine can run for 10-33 years without refuelling, while a French submarine fuelled with LEU needs to be refuelled every 10 years.
| Characteristics | Values |
|---|---|
| Type of Reactor | Pressurized water reactors (PWRs) |
| Fuel Type | Highly enriched uranium (HEU) |
| Uranium Enrichment | >20% U-235, with current U.S. submarines using fuel enriched to at least 93% |
| Refueling Frequency | 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 |
| Fuel Efficiency | 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 |
| Reactor Development | The U.S. Navy has developed 27 different plant designs, installed in 210 nuclear-powered ships, and has accumulated over 5,400 reactor years of operation |
| Reactor Safety | The U.S. Navy has never disclosed a reactor accident, but has suffered at least one coolant loss accident |
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What You'll Learn
Fuel enrichment and core life
The fuel used in naval reactors is typically highly enriched uranium (HEU) with an enrichment level of over 90% U-235. This high enrichment results in a smaller and quieter reactor core, increased power density, and extended core life. The core life of naval reactors is designed to be long, with refueling required only after 10 or more years, and new cores designed to last 25 years in carriers and 10-33 years in submarines.
The long core life is achieved through the use of high uranium enrichment and the incorporation of a "burnable neutron poison". This burnable poison is gradually depleted as non-burnable poisons, such as fission products and actinides, accumulate. The loss of the burnable poison counterbalances the creation of non-burnable poisons, resulting in stable long-term fuel efficiency. Additionally, the use of high-density, low-enriched uranium (LEU) fuels has been proposed as an alternative to HEU. LEU fuel would have a lower enrichment of U-235, typically just below the 20% threshold, which defines weapon-usable HEU.
While LEU fuel has the potential to reduce the proliferation risks associated with HEU, it also presents some challenges. One of the main challenges is the size and weight of the reactor core. LEU cores may require a larger reactor pressure vessel and a larger submarine to provide additional buoyancy and offset the increased weight of the reactor compartment. Additionally, the use of LEU fuel may require a return to mid-life refueling, as opposed to the current practice of refueling every 10 years or more.
The feasibility of adopting LEU fuel in naval reactors is currently under investigation by organizations such as the US Department of Energy's Office of Defense Nuclear Nonproliferation and the Office of Naval Reactors. These organizations are exploring advanced LEU fuel designs and reactor concepts that could potentially balance the benefits of extended core life and reduced proliferation risks while mitigating the challenges posed by core size and refueling intervals.
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Fuel type and reactor design
The fuel type and reactor design used in naval reactors vary depending on the country and specific requirements. Here is an overview of the fuel type and reactor design considerations for naval reactors:
Fuel Type:
- Highly Enriched Uranium (HEU): Currently, naval reactors in the United States, the United Kingdom, Russia, and India use highly enriched uranium (HEU) fuel with an enrichment level of ≥20% U-235. The U.S. and UK naval reactors use weapon-grade HEU with 93-93.5% U-235. This high enrichment level contributes to long core life, allowing for extended periods between refuelling, typically over a decade.
- Low Enriched Uranium (LEU): There have been studies and efforts to transition naval reactors from HEU to LEU fuel. LEU fuel with an enrichment level of around 19.75% U-235 is considered, as it falls below the 20% threshold above which uranium is deemed weapon-usable. LEU fuel has the potential to offer a 50% increase in uranium density, which could reduce the volume of fuel required. However, the use of LEU may require larger reactor pressure vessels and modifications to submarine design to accommodate the increased weight of the reactor compartment.
- Other Fuels: Alternative fuel options, such as uranium-molybdenum alloys, USi3, and uranium metal alloyed with molybdenum and silicon, are also being considered for their potential advantages in terms of uranium density and core volume optimization.
Reactor Design:
- Pressurized Water Reactors (PWRs): PWRs are the standard reactor type for naval applications. They use ordinary water as both a coolant and moderator. PWRs have a primary cooling circuit with high-pressure water flowing through the reactor core and a secondary circuit for steam generation to drive the turbine. This design provides high power density in a small volume, making it ideal for naval vessels.
- Designations: Naval reactors in the U.S. are designated with a three-character naming system. The first letter indicates the type of ship the reactor is intended for (A for aircraft carrier, C for cruiser, D for destroyer, and S for submarine). The second letter represents the reactor's generation or designer. For example, S9G denotes a submarine (S) with a ninth-generation (9) reactor designed by General Electric (G).
- Long Core Life: Naval reactors are designed for long core lives, typically 10 or more years, with new cores designed to last up to 25 years in carriers and 10-33 years in submarines. This is achieved through high uranium enrichment and the use of "burnable neutron poison," which maintains stable long-term fuel efficiency by counterbalancing the accumulation of non-burnable poisons.
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Fuel density and reactor size
The core of a naval reactor is much smaller than that of a power reactor, so the fuel is typically more highly enriched. This increases the probability of fission, enabling a sustained reaction. Naval reactors are designed to run for over a decade without refuelling, and some new cores are designed to last 25 years in carriers and 10–33 years in submarines.
The fuel used in naval reactors is typically highly enriched uranium (HEU). However, there is a desire to move away from HEU fuel due to the risk of nuclear proliferation. A 1995 report to Congress recommended the use of low-enriched uranium (LEU) fuel, which would have a 50% increase in uranium density. This would result in an LEU core having twice the volume of an HEU core for the same core life.
However, the use of LEU fuel would require a larger reactor pressure vessel, which would increase the size and weight of the reactor compartment. This could be mitigated by reverting to mid-life refuelling, as is the traditional practice for pre-Virginia class submarines, or by increasing the length of the submarine to provide additional buoyancy. Alternatively, the development of advanced LEU fuel could enable the use of LEU in existing reactor designs without affecting the number of refuellings.
The United States Navy has expressed interest in pursuing LEU fuel options for submarines, and Congress has provided additional funding for research and development in this area. The US Department of Energy's office of Defense Nuclear Nonproliferation has also recommended the investigation of several potential fuel and reactor types, including pressurized water reactors (PWRs) and high-density, low-enriched uranium (LEU) fuels.
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Fuel efficiency and neutron poisons
The core of a naval reactor is much smaller than that of a power reactor, so the fuel is typically more highly enriched to increase the probability of fission. Some naval reactors run on low-enriched uranium, which requires more frequent refuelling. For instance, French and Chinese submarines use low-enriched uranium, while current US submarines use highly enriched uranium (at least 93% U-235).
Naval reactors are designed to run for over a decade without refuelling. This 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, resulting in stable long-term fuel efficiency.
Burnable poisons are neutron absorbers that decay under neutron exposure, compensating for the progressive build-up of neutron absorbers in the fuel as it burns, and allowing higher fuel burn-up. The most well-known burnable poison is gadolinium, which is incorporated into the ceramic fuel pellets of naval reactors. An alternative is zirconium diboride integral fuel burnable absorber (IFBA) as a thin coating on normal pellets.
In a nuclear reactor, the energy released from the continuous fission of fuel atoms is harnessed as heat to produce steam, which drives a turbine for propulsion. Most naval nuclear reactors are of the pressurised water type, which generates steam in a secondary circuit. Water is used as both a coolant and a moderator to slow neutrons.
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Fuel sources and fuel security
Currently, the United States and the United Kingdom primarily use highly enriched uranium (HEU) fuel in their naval reactors. This fuel is typically weapon-grade, with a U-235 enrichment of 93% or higher. The use of HEU fuel provides a high power density in a small volume, allowing for the compact design of naval reactors. However, there are concerns about the proliferation and security implications of using HEU fuel in naval reactors.
In recent years, there has been a growing interest in transitioning to low-enriched uranium (LEU) fuel for naval reactors. LEU fuel has a lower U-235 enrichment, typically below 20%. While LEU fuel may require larger reactor cores and more frequent refuelling, it has the potential to reduce proliferation risks and improve fuel efficiency. The United States Department of Energy has conducted studies to evaluate the feasibility of transitioning to LEU fuel and has recommended further investigation into pressurized water reactors (PWRs) and high-density LEU fuels.
France, China, and Russia have also made advancements in nuclear-powered submarines. France, in particular, has designed its submarines to be refuelled in weeks rather than years, demonstrating an alternative approach to addressing refuelling challenges.
To ensure fuel security, naval reactors utilize long-lasting core designs that can operate for 10 or more years without refuelling. This extended core life is achieved through high uranium enrichment and the use of "burnable neutron poison," which maintains stable long-term fuel efficiency. Additionally, the use of materials like gadolinium in fuel pellets helps compensate for the buildup of neutron absorbers, further extending the fuel life.
In summary, fuel sources and fuel security are critical aspects of naval reactor design. While HEU fuel has been the primary choice for naval reactors in the US and UK, there is a growing interest in transitioning to LEU fuel to address proliferation concerns and improve fuel efficiency. Alternative reactor designs and fuelling strategies, such as those employed by France, also offer promising solutions to refuelling challenges.
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Frequently asked questions
The amount of fuel needed by naval reactors depends on the type of reactor and fuel used. Most naval reactors are of the pressurized water type, with a few attempts at using liquid sodium-cooled reactors. The US and UK use highly enriched uranium (HEU) fuel, while France has designed its nuclear submarines to be refueled in weeks with low-enriched uranium (LEU) fuel.
Naval reactors are designed to run for more than a decade between refuelings, with new cores designed to last 25 years in carriers and 10-33 years in submarines. The French navy has designed its nuclear submarines to be refueled in weeks rather than years.
The refueling frequency of naval reactors is influenced by the type of fuel used and the design of the reactor. For example, the use of burnable poisons like gadolinium in the fuel matrix can extend the usable life of the fuel by compensating for the reduced fuel efficiency caused by accumulating poisons and fissile reduction.
