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Using nuclear fission as a method of propulsion has been researched and developed since the early 20th century. From the moment the first nuclear submarine, the USS Nautilus, was commissioned in 1954 by the US Navy, the concept proved its potential, achieving feats of sustained speed and time submerged that no other conventional submarine of its time could hope to match.
Such achievements would not have been possible without the power of nuclear fission and the material science used to harness it safely. This article will analyze the materials used in a modern-day reactor of a 2018 HMS Astute submarine, to highlight the material science required to make this technology a reality.
How Nuclear Fission Works
To understand why certain materials are chosen for nuclear reactors, an explanation for how nuclear reactors work is required.
Nuclear reactors are powered by a process called nuclear fission, in which a neutron is fired into the nucleus of a ‘heavy’ atom, causing it to become unstable and split. This releases energy as heat and other neutrons absorbed by other atoms, repeating the cycle. Released energy heats surrounding water into steam. At this point, the reactor acts essentially as a steam engine. The steam turns a turbine (in this case, propelling the submarine), before being condensed back into water and sent back to the reactor’s core.
The HMS Astute uses a Pressurized Water Reactor (PWR), instead of a Boiling Water Reactor (BWR). A PWR has two water circuits. The reactor core heats up the highly pressurized primary circuit. Despite being heated beyond its typical boiling point, the tremendous pressure keeps the water in its liquid form, known as ‘super-heated water’. The secondary circuit heats and converts into steam by this primary circuit. However, a BWR only has a single circuit. This explanation provides an insight into the extreme conditions that these reactors are required to withstand.
The Outer Layers of The Reactor (Vessel and Shielding)
The vessel of the reactor is the component that houses the core itself. This vessel undergoes some of the most extreme conditions in the entire reactor: it must withstand extremely high temperature and pressure, while continually being struck by stray neutrons from the fission process within. Its material must be exceedingly robust so that it does not suffer any cracks or other significant structural failures. If this structure is compromised, nuclear radiation would be released into the environment. In a space as enclosed as a submarine, such a scenario would be hazardous.
Engineers use specifically treated materials to create the vessel. In the case of the HMS Astute, a low alloy manganese molybdenum steel is used. The manganese and molybdenum content significantly improves the steel’s mechanical properties such as strength.
The alloy is also highly weldable, which is a desirable characteristic, as welding is the process of choice when creating vessels. Welding is better at handling large loads than most other joining methods as it distributes the load more evenly, minimizing singular points of high stress.
The alloy is also resistant to neutron irradiation, which typically causes materials to become brittle, deteriorating both its weldability and toughness. The material is usually quenched and tempered, further improving its strength.
Around the vessel is further shielding, to lower the number of neutrons and gamma rays that may escape. The HMS Astute reactor has two primary shields. The lower half comprises a solid-immovable lead tank of freshwater, which absorbs most gamma rays. Hydrogen in fresh (light) water has a relatively high absorption cross-section, as it can form deuterium. Water is also a good moderator as it reduces the high energy of neutrons moving through it, reducing their speed.
The upper half of the shielding is flexible, as it is made of many large blocks of polyethylene. Like light water, polyethylene has a very high hydrogen content, giving it a high absorption cross-section. Similarly, its scattering power - the ability to reduce an incoming neutrons energy - is considerably increased. The upper primary shielding must be movable, as engineers will insert fuel and boron control rods into the vessel via a removable top.
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Inside the Core (Fuel Rods, Control Rods, and Moderator)
Within the core, there are fuel rods filled with radioactive pellets. Most commonly, these pellets are made of enriched uranium dioxide, which is made into a powder and then turned into small ceramic-like pellets using high pressure and heating (a process known as sintering).
The HMS Astute fuel pellets are filled with helium to better transfer heat and prevent overheating. It also contains clad in a corrosion-resistant alloy with a low neutron absorption rate so that it does not absorb the outgoing neutrons and prevent the fission process.
Enrichment of the uranium dioxide refers to a process that increases the percentage of the U-235 isotope. Natural uranium typically has around 0.7% U-235. The rest of its content is the non-fissile U-238 isotope. Enrichment can increase this percentage to as high as 5%, making it much more efficient in undergoing fission.
Nuclear fission, if not properly controlled, will grow exponentially; one neutron can split an atom and release two neutrons. This growth has the potential to meltdown or even explode a reactor’s core. Many control rods are placed between the fuel rods to prevent this, and can be dropped in at any time to stop the fission process.
Control rods are usually made of boron, as it has a high neutron absorption rate and a high melting point. Furthermore, the reactor's temperature is moderated by using the primary circuit’s water as a coolant. The water also acts as a moderator, lowering the neutron’s speed to increase the likelihood of a collision occurring, instigating a fission reaction.
With these materials together, the HMS Astute is capable of propulsion from one of the most efficient energy sources currently known. As propulsion and energy methods progress, material science adapts to make them possible, and the scientific feat of the nuclear reactor is no exception.
References and Further Reading
History (2010) USS Nautilus—world’s first nuclear submarine—is commissioned. [Online] Available at: https://www.history.com/this-day-in-history/uss-nautilus-commissioned (Accessed on 29 September 2020).
Naval History and Heritage Command (2015) Nautilus IV (SSN-571) [Online] Available at: https://www.history.navy.mil/research/histories/ship-histories/danfs/n/nautilus-ssn-571-iv.html (Accessed on 29 September 2020).
Suzuki, K. (1998) Reactor Pressure Vessel Materials. [Online] International Atomic Energy Agency. https://inis.iaea.org/collection/NCLCollectionStore/_Public/30/013/30013703.pdf?r=1 (Accessed on 29 September 2020).
Bel V [Online] Reactor. Available at: https://www.belv.be/index.php/en/ct-menu-v-nuclear/ct-menu-v-nucleartechnology/ct-menu-v-pwrinoperation/ct-menu-v-pwrinoperation-reactor(Accessed on 29 September 2020).
World Nuclear [Online] Uranium Enrichment. Available at: https://www.world-nuclear.org/information-library/nuclear-fuel-cycle/conversion-enrichment-and-fabrication/uranium-enrichment.aspx (Accessed on 29 September 2020).
Afework, B., et al. Neutron moderator. [Online] Energy Education. Available at: https://energyeducation.ca/encyclopedia/Neutron_moderator (Accessed on 29 September 2020).
Zhang, X., Yang, M., Zhang, X., Wu, H., Guo, S. and Wang, Y. (2017) Enhancing the neutron shielding ability of polyethylene composites with an alternating multi-layered structure. Composites Science and Technology, 150, pp.16-23. https://doi.org/10.1016/j.compscitech.2017.06.007
Gates, J., 2018. Astute Class Nuclear Submarine, 2010 To Date (Owners' Workshop Manual). Yeovil: Hayes Publishing.