|Fig. 1: Schematic diagram of energy extraction from depleted uranium (U-238).  (Source: Wikimedia Commons)|
With the momentum toward meaningful action on global climate change such as the climate talks get under way in Paris, many countries have been investing in low-carbon energy sources to fuel their economies. From wind turbines and solar panels to new forms of biofuels and hydropower, a mix of generation types will be vital to meeting global energy demand in a responsible and sustainable way. However, it has been argued that the current renewable energy sources is still struggling with satisfying the continuously increasing demand of energy without using the conventional energy resources, for instance, fossil fuel, which give major negative impacts on the environment.
To both meet the global energy demand and minimize the negative environmental impact, Bill Gates and a group of like-minded visionaries decided that the private sector needed to take action.  Recently, Bill Gates announced a new coalition of global billionaires to back companies commercializing clean energy innovations.  The action made by the new coalition was to organize an expert group of scientists and engineers in order to analyze all energy generation technology options from a total systems perspective. The group insisted that the best option for addressing the world's growing energy challenges was to expand the utilization of nuclear energy, in a more suitable form, on a global basis. Consequently, Bill Gates mentioned TerraPower in his speech at the exclusive tech conference TED in 2010, it was the first time that many had heard of the nuclear project associated with the company called TerraPower. 
TerraPower uses a Traveling Wave Reactor (TWR) design. Even though TWR technology has been studied since the 1990. TerraPower tries to develop a practical design for travelling wave nuclear reactors. Depleted uranium is currently a waste byproduct of the enrichment process. TWR burns fuel made from the depleted uranium by the uniquely designed TWR where the depleted uranium are gradually converted the waste material from the enrichment process through a nuclear reaction without removing the fuel from the reactor's core.  It has been argued that the TWR can sustain this process indefinitely, generating heat and producing electricity. More detailed physics behind TWR will be discussed in the next section.
As a self-fueling source of energy, TerraPower argued that they use proven fast reactor technology, high-performance computing simulations and real testing in current fast reactor test facilities to make the traveling wave reactor (TWR) concept a reality.  As mentioned above, TWR is a uniquely designed nuclear reactor operating for an extended period of time by using only depleted uranium (U- 238) as fuel. For conventional nuclear energy plants, large atom (U-235) split (fissions) and creation of a sustained chain reaction release the neutrons and heat. U-238 cannot sustain a chain reaction in today's light water reactors hence, U-238 is considered as a by-product (waste) of the enrichment process in the light water reactor.
In contrast to the today's light water reactor, TerraPower announced that they utilized (but, has not been demonstrated) a method to extract energy from U-238, making it a perfect source of energy for the TWR as shown in Fig.1. In other words, the reactor technology of TerraPower theoretically use a small amount of enriched uranium at the beginning of the process, but then the nuclear reactor will run on the waste product and can make and consume its own fuel. TWR is powerful enough because the reactor does not have to be refueled or have its waste removed until the end of life of the reactor (theoretically a couple hundred years).  Using waste uranium reduces the amount of waste in the overall nuclear life cycle, and extends the available supply of the world's uranium for nuclear by many times.
|Fig. 2: Possible engineering embodiment of TWR. (Courtesy of TerraPower)|
In 2006, TerraPower launched an effort to develop the first practical engineering embodiment of a breed-and- burn fast reactor, producing a design concept known as a traveling-wave reactor (TWR).  Conceptual TWR design shown in Fig.2 is a liquid sodium-cooled fast reactor. The innovative reactor consists of a cylindrical reactor core (3) submerged in a large sodium pool (6) in the reactor vessel (2), which is surrounded by a containment vessel and dome (1) that prevents loss of sodium coolant in case of an unlikely leak from the reactor vessel. The pumps circulate primary sodium coolant through the reactor core exiting at the top and passing through intermediate heat exchangers (7) located in the pool. In the event that grid power is not available, decay heat is removed using two dedicated safety class decay heat removal systems: the Reactor Vessel Air Cooling System (RVACS) and the Auxiliary Cooling System (ACS) (8), which operate entirely by natural circulation with no need for electrical power. In case of emergency, the gravity will activate the safety and control rod (5) to be mechanically inserted into core for the fission control. Based on the extensive pre-conceptual study that evaluated various core configurations and compositions, the core design with an approximate cylindrical core geometry composed of hexagonally shaped fuel bundles, containing a combination of enriched and depleted uranium metal alloy fuel pins clad in ferritic martensitic steel tubes plays a key role to achieve an effective breed and burn process in TWR and becomes major distinguishing feature of the TWR from other fast reactor designs. 
Compared to the conventional reactors capturing only about 1 percent of the energy potential of their fuel, TWR represents a new class of nuclear reactor. Conceptually, TWR reactor design allows TWR to utilize depleted uranium as their primary fuel. These innovations greatly simplify the nuclear fuel cycle by eliminating or reducing the need for enrichment, reprocessing, and waste storage and disposal. Fissile fuel is both produced and then consumed in-reactor, greatly improving the fuel efficiency of the TWR and resource availability for the reactor. Even though it has been reported that TerraPower has not yet produced a working prototype six years after it was founded. I hope that operation of a traveling wave reactor can be demonstrated soon and commercial deployment can begin less than 10 years.
© Wonjin Yun. 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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