|Fig. 1: Schematic of a modular helium reactor. (Source: Wikimedia Commons)|
In much of the world, the United States in particular, the nuclear energy boom has come to a halt. Beyond the large capital costs for nuclear reactors, the regulations surrounding building such large machines have driven new investment in large reactors to a near stand-still. Although at least partially justified by the nuclear disasters such as Chernobyl and Three Mile Island, the regulatory burdens prohibit investment. Current estimates for these reactors are on the order of $10 billion, upfront costs. On the other hand, new developments in small, so- called "modular" reactors, provide great promise to deliver a new nuclear energy revolution. These reactors are small, mobile, safer, and offer to the free market a more attractive investment opportunity. They build on passive cooling technologies that significantly mitigate the potential for melt-downs and other nuclear disasters.  Hyperion Power, for instance, brought an approximately $30 million to market that has the capability to power 20,000 homes.  These reactors pose the potential to provide power to developing nations with low proliferation risk, bring independent power to military bases, and generate electricity in extremely remote locations.
One of the first of such modular technology designs is the gas turbine- modular helium reactor (GT-MHR). This reactor is characterized by its intrinsic safety, where a melt-down is not possible, it has an extremely high radiation energy conversion factor, and its waste is relatively clean compared by other competing reactor designs. For thermal energy conversion, it uses a gas-turbine system that employs the high-efficiency Brayton cycle. This sort of cooling cycle design was enabled by developments in large aviation and industrial turbines along with high-temperature high-strength alloys for the pressure vessels. These vessels designs were crucial as the reactor is modular in nature.  A schematic of a modular helium reactor is seen in Fig. 1.
Regarding the system's safety, the core is protected passively. Due to the use of non-oxidizing fuel particles, strong graphite construction materials, and a negative temperature coefficient of reactivity, the reaction actually shuts itself down should the cooling system begin to malfunction. The choice of non-oxidizing fuel is important so that should the core crack under heat pressure, the fuel will not ignite or release toxic fuels in a Chernobyl-style fire and explosion.  Furthermore, the choice of helium as a coolant means that coolant leaks pose no reactivity threat. This is in contrast with the many, currently-in-use, sodium cooled nuclear fission reactors. Due to the inherent safety characteristics of the reactor, after depressurization of the coolant loop, the reactor system passively shuts itself down and maintains a safe state for tens of hours. This occurs in the worst-possible case in which even the mechanical means for lowering the reactivity and removing heat fail. During this time period, ways of recovering mechanical or electrical control over the reactor can be employed. 
Beyond their power generation benefits, the modular helium reactors can be used in the destruction of weapons grade plutonium. The gas turbine modular helium reactor has the ability to destroy 90% of weapons grade plutonium-239 and 65% of the total plutonium in a single reactor cycle. During this process, electricity is still generated at efficiencies of nearly 50%. The single cycle efficiency is important as it reduces proliferation risks during the shuffling of core material. Recycling reactors, in contrast to modular reactors, require easy access to the core material itself. Thus, these reactors can also play a crucial role in the destruction of Russian and United States stockpiles of weapons-grade nuclear material. 
Due to the inherent safety and proliferation security offered by completed enclosed and encased modular helium reactors, they are poised to play an important role in powering the future. Their negative temperature coefficient of reactivity and passive cooling systems make these some of the safest available reactor designs. More recently, the Idaho National Laboratory initiated its Next Generation Nuclear Plant program to "support commercialization of the high temperature gas-cooled reactor."  Their designs are all based around many of the key elements in the original proposals for gas turbine modular helium reactors. As a result, it is clear that the technology behind modular helium reactors will help play a role in providing cheap and abundant power for the future, while helping to tackle one of the world's great threats, nuclear proliferation.
© Kevin Fischer. 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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