|Fig. 1: Supercritical Water Cooled Reactor.  (Courtesy of the DOE)|
Nuclear power plants have become a reliable solution for energy by producing electricity with minimal carbon emissions at low, stable costs. As the world's energy needs increase due to a heightened standard of living in both developing and developed countries, optimizing nuclear energy has become a major focus for the progression of our energy usage. The Generation IV International Forum (GIF) has led the international collaboration effort in pursuing a more efficient and cost effective solution in nuclear energy. The GIF, consisting of Argentina, Brazil, Canada, France, Japan, the Republic of Korea, South Africa, the United Kingdom and the United States, has set a created a project roadmap to push the modernization of nuclear energy and create a new wave of technology in the next 15 years.
When discussing the modernization of nuclear energy, the GIF is focused on improving the nuclear reactor, the energy conversion system, and the other fuel cycle technologies. The GIF has four areas that they are looking to optimize in this new Generation IV phase.  The first is sustainability. Successful sustainability is defined by the ability to create sustainable energy and provide long-term accessibility to nuclear fuel, while maintaining minimal nuclear waste.  The second goal of modernization is to increase safety and reliability. The idea is to minimize the likelihood and impact of reactor core damage, which will also minimize the need for emergency response. While these upgrades may seem obvious, the GIF must also take into consideration the fiscal repercussions. For this energy source to compete with others, it must have a life cycle cost advantage over other sources of energy as well as a comparable financial risk.  GIF is also focused on the protection of plant once it has been built. They would like to avoid having the technology potentially used as a path to weaponry, as well as being attacked in a form of terrorism. The potential for dangerous use can be very high if they are built in a way that can be shifted into a weapon or if the destruction of the plant could cause major issues to the surrounding environment.
With the aforementioned goals in mind, the Generation IV initiative elicited a large response in terms of proposed models for development. Out of hundreds of reviewed solutions, the GIF was able to come up with six potential solutions, gas-cooled fast reactor (GFR), lead-cooled fast reactor (LFR), molten salt reactor (MSR), sodium-cooled fast reactor (SFR), supercritical-water-cooled reactor (SCWR), and very-high-temperature reactor (VHTR). Between the six different models, there are actually two different types of reactors, thermal reactors and fast reactors.
Gas-Cooled Fast Feactor (GFR): This solution uses helium as the coolant system as opposed to water. The difficulty is that the gas must be held in a high pressure container that has yet to be developed. Once this system is developed, it will actually be able to use the helium as fluid within the system since it does not become radioactive. [2,3]
Lead-Cooled Fast Feactor (LFR): This model uses a single phase fluid that has a high boiling point and does not need a high pressure operation. The design intends to use liquified lead to heat another fluid and create a power cycle. 
Molten Salt Reactor (MSR): This model uses a molten salt mixture as its coolant. This allows them to run at a much higher temperature and efficiency than water-cooled reactors. This design is also inherently safer in comparison to light water reactors in the sense that the fluoride salts cannot form flammable hydrogen. 
Sodium Cooled Fast Reactor: This design uses molten sodium as the cooling agent. It differs from the salt reactor because of its solid fuel core and its completely closed fuel cycle. 
Supercritical Water Reactor: This designs reactor core can be either thermal or fast-neuron spectrum. The use of light water and heavy water are also dependent on the reactors design. In this design the steam will be applied to a turbine, which will produce water fed back into the core. 
Very High Temperature Reactor: In this design, similar to the gas-cooled fast reactor, helium is the cooling agent. Hence the name, this design creates an extremely high temperature, which can be used as energy to produce electricity or as the formation of hydrogen. The fuel cycle in the reactor is not closed and is done with a one-way pass through. 
These project proposals were submitted in 2001 and the process to began to put these solutions into production. The overall roadmap was divided into three distinct phases, the viability phase, the performance phase, and the demonstration phase. The viability phase is an analysis of the basic fundamentals of the proposed designs. This is to ensure there was nothing overlooked and that technical showstoppers are identified and resolved. The next phase is the performance phase. This phase is when the actual materials are tested and optimized when used under realistic conditions. The last phase, demonstration phase, is when the design is completed, licensing occurs and construction and operation are completed.
© Clay Jones. 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.
 "Technology Roadmap Update for Generation IV Nuclear Energy Systems," The Generation IV International Forum, January 2014.
 G. Roberts, "Nuclear Reactor Basics and Designs for the Future," Physics 241, Stanford University, Winter 2013.
 "GIF R&D Outlook for Generation IV Nuclear Energy Systems," The Generation IV International Forum, August 2009.
 J. Buongiorno and P. E. MacDonald, "Supercritical-Water-Cooled Reactor (SCWR)," Idaho National Engineering and Environmental Laboratory, INEEL/EXT-03-01210, September 2003.