|Fig. 1: Sodium-cooled Fast Reactor. (Source: Wikimedia Commons)|
As the world's population continues to grow, the need to produce clean, safe, and sustainable energy that meets basic electricity production and primary energy needs will continue to increase. Amongst different emerging energy technologies, nuclear energy has the potential to become a prominent low-emission supplier. Currently, most nuclear power plants use Generation III reactors; however, next generation nuclear reactors (i.e., Generation IV) have the potential to serve as an opportunity to further develop the technology's sustainability and efficiency.  The Generation IV International Forum (GIF), an international collective representing 14 countries, has led the necessary R and D to develop the next series of innovative nuclear energy systems to address several large-scale nuclear use challenges, such as: [2,3]
Conservation of uranium resources
Reduction in radioactive waste
Deployable while minimizing risk of nuclear weapons proliferation
The GIF's first action was to select six promising Generation IV nuclear reactor concepts that could mitigate some of the large-scale nuclear challenges.  The six chosen concepts were Sodium-cooled Fast Reactors (SFR), Gas- cooled Fast Reactors (GFR), Lead-cooled Fast Reactors (LFR), Molten Salt Reactors (MSR), Very High Temperature Reactors (V/HTR), and Supercritical-Water-cooled Reactors (SCWR). These technologies will take several decades to deploy and are predicted to be implemented towards the middle of the twenty-first century.  Out of the six potential technologies, Sodium-cooled Fast Reactors (SFR) have received concentrated research efforts from some countries, as seen by the 652 million euro budget granted to the ASTRID (Advanced Sodium Technological Reactor for Industrial Demonstration) project by the French Alternative Energies and Atomic Energy Commission (CEA) in 2010. [4,5]
A Sodium-cooled Fast Reactor (SFR) is a type of nuclear reactor that utilizes molten sodium metal as the reactor coolant as it allows for a high power density with a low coolant volume.  An SFR can achieve a core power density of around 300 MW/m3 compared with Pressurized Water Reactors (PWR) that achieve 100 MW/m3.  Furthermore, a sealed coolant system is needed as the sodium is highly reactive with air and water; however, the oxygen-free system prevents corrosion.  As can be seen in Fig. 1, the primary sodium coolant does not directly exchange its heat with the water. Instead, a sodium-sodium heat exchanger is used where the secondary molten sodium stream can exchange its heat with water to generate steam.
Unlike some nuclear reactors that utilize thermal neutrons, an SFR uses fast neutrons, which are neutrons that have not completely thermalized, to convert U-238 into plutonium. Given the reactor's closed fuel cycle and generation of plutonium, a fissile fuel, the reactor can also be used as a breeder to regenerate fuel. This process requires additional recycling processes to ensure that the fuel is properly developed and qualified for use. [3,4] Because of this, SFRs have the potential to become an attractive energy source for countries interested in managing their nuclear supply and nuclear waste.
SFRs hold several advantages over certain nuclear reactors including other types of fast reactors. Indeed, the systems use of liquid metal provides a multitude of advantages due to the physical properties of the molten metal coolant. For example, the metal's high thermal conductivity and heat capacity creates a large thermal inertia against overheating if coolant flow is lost.  In addition, sodium-based systems do not serve as neutron moderators, unlike water, which allows the use of fast neutrons. Lastly, sodium can be operated at near atmospheric pressure since its boiling point is higher than the reactor's operating temperature. 
As mentioned above, sodium is highly reactive with air and water. A sodium leak could lead to the production of toxic sodium-oxide aerosols and explosions caused by sodium fires due to the lack of the water-fail safe that current nuclear reactors utilize.  Early SFRs, constructed before the establishment of the GIF, have also suffered from corrosion and sodium leaks that resulted in runaway nuclear reactions and sodium explosions. One example of this was the Michigan Enrico Fermi Atomic Power Plant, which experienced a sodium explosion that delayed the reactor's repairs after a partial core meltdown in 1966. 
In addition, sodium's fast and exothermic reaction with water produces sodium hydroxide and hydrogen that can damage the generator or cause a hydrogen explosion. Furthermore, the fast neutrons in the core can activate sodium, causing it to become radioactive. However, the half-life of activated sodium is only 15 hours. 
© Arturo Rojas. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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 "Overview of Generation IV (Gen IV) Reactor Designs," Institut de Radioprotection et de Sûreté Nucléaire, Report IRSN 2012/158, September 2012.
 G. De Clercq, "The Key to Nuclear's Future or an Element of Doubt," Reuters, 13 Oct 14.
 C. Grandy, "US Department of Energy and Nuclear Regulatory Commission - Advanced Fuel Cycle Research and Development Seminar Series," Argonne National Laboratory, ANL-AFCI-238, August 2008.
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