|Fig. 1: Molten Salt Reactor.  (Source: Wikimedia Commons - courtesy of the U.S. Department of Energy)|
Transatomic Power is a company in Cambrige, MA that is currently working on the development of a newly designed nuclear reactor. Nuclear reactors, like many other power plants, produce power by generating heat to create steam, which is then used to power a turbine. The main thing that differentiates nuclear reactors, is the way in which they produce this heat. For example, fossil fuel plants burn coal, oil, or natural gas to produce heat, a mechanism that is cheap and effective but emits massive amounts of carbon dioxide which pollutes our environment. Nuclear reactors use a technique called nuclear fission, which is either a nuclear reaction or a radioactive decay process where the nucleus of an atom splits into smaller parts. This split releases massive amounts of energy in the form of gamma rays and only uses a small amount of fuel. 
Today, the majority of nuclear reactors are light water reactors, which use normal water, as opposed to heavy water, as a coolant and neutron moderator. Heavy water, or deuterium oxide, is a form of water that has an increased amount of the hydrogen ion deuterium. Deuterium has a neutron and a proton, while a normal hydrogen atom just has one proton. This extra neutron makes the atom twice as heavy, which is why they refer to this water as heavy water. Light water reactors use solid uranium oxide pellets as fuel that are trapped within a small metal framework. The core of the reactor is surrounded by water that is heated by the nuclear fission reactions, turned to steam, and then evaporated into a connected pressure vessel. This steam energy powers a turbine which is connected to a generator so that electricity can be produced. After the the steam goes through the turbine, it condenses back into water and is taken back into the pressure vessel. River water is usually used to cool the condenser and is then refed back into the river once it is used to cool the system. Light water reactors, however, can only use up to about 4% of their fuel, and the rest remains radioactive for hundreds of years. In addition, the reactors use electricity continuously to pump water over their core to keep it cool which makes them inactive if the plants generator goes out. 
Molten salt reactors are nuclear fission reactors that use a molten salt mixture as the coolant and fuel source instead of water. These reactors can run at much higher temperatures than light water reactors which increases their thermodynamic efficiency. The molten salt water reactor is by no means a new concept, but lately there has been a renewed interest in this technology in hopes that it can provide a more efficient fuel source. Transatomic Power's reactors use uranium dissolved in liquid salt to fuel fission. Molten salt's boiling point is much higher than the temperature of the reactor fuel which helps it act as a type of thermostat. As the reactor heats, the molten salt expands allowing the fuel to spread around and slow down the nuclear reaction. Because of the inherent structure of liquid, liquid fuel is much more resistant to damage from radiation than solid materials. Fig. 1, to the right, shows how the closed fuel cycle of molten salt reactors can facilitate the more efficient burning of plutonium. Transatomic Power claims that with proper filtration, their liquid fuel can remain in their reactors for decades which would allow much more energy to be extracted from them. Another benefit of molten salt reactors is that they do not need active cooling like the light water reactors. The Transatomic reactor includes a "freeze valve" that, if the reactor lost all electric power, would allow for it to gradually stop its reactions in a safe way. [1,3,4]
Transatomic Power's molten salt reactor is a new spin off an old idea. The technology was first tested in the 1960s at Oak Ridge National Laboratory. Though the reactor was tested for six years, it was expensive and required highly enriched fuel, so the technology was never fully developed. The Transatomic reactor has been modified to eliminate the use of graphite and burn sources for fuel that would be considered waste to many other reactors. This reactor is still in the research stage, and Transatomic Power has yet to even develop a prototype for their new reactor design. Without more funding, it is possible that Transatomic Power's molten salt reactor will remain in the research phase for a long time. 
© Phoebe Morgan. 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.
 J. Serp et al., "The Molten Salt Reactor (MSR) in Generation IV: Overview and Perspectives," Prog. Nucl. Energ. 77, 308 (2014).
 R. Perry et al., "Development and Commercialization of the Light Water Reactor, 1946-1976," Rand Corporation, R-2180-NSF, June 1977.
 Y. Kelaita, "Molten Salt Reactors," Physics 241, Stanford University, Winter 2015.
 E. Strickland, E, "Transatomic Power Building a "Walk-Away Safe" Reactor," IEEE 6840794, IEEE Spectrum 51, No. 7, 25 (July 2014).
 "Generation IV Nuclear Energy Systems: Ten-Year Program Plan, Vol. I," Idaho National Laboratory, March 2005, p. 42.