Traveling wave reactor
A traveling-wave reactor (TWR) is a type of fourth-generation nuclear reactor that nuclear engineers anticipate can convert fertile material into usable fuel through nuclear transmutation in tandem with the burnup of fissile material. TWRs differ from other kinds of fast-neutron and breeder reactors in their ability to utilize fuel efficiently without uranium enrichment or reprocessing, instead directly using depleted uranium, natural uranium, thorium, spent fuel removed from light water reactors, or some combination of these materials. The name refers to the fact that fission does not occur throughout the entire TWR core, but remains confined to a boundary zone that slowly advances through the core over time. TWRs could theoretically run, self-sustained, for decades without refueling or removing any spent fuel from the reactor.
Traveling-wave reactors were first proposed in the 1950s and have been studied intermittently since that time. The concept of a reactor that could breed its own fuel inside the reactor core was initially proposed and studied in 1958 by Saveli Feinberg, who called it a "breed-and-burn" reactor. Michael Driscoll published further research on the concept in 1979, as did Lev Feoktistov in 1988, Edward Teller/Lowell Wood in 1995, Hugo van Dam in 2000, and Hiroshi Sekimoto in 2001.
No TWR has yet been constructed, but in 2006, Intellectual Ventures launched a subsidiary named TerraPower, LLC to model and commercialize a working design of such a reactor, which has since come to be called a "traveling-wave reactor". TerraPower has developed TWR designs for low- to medium- (300 MWe) as well as high-power (~1000 MWe) generation facilities. Bill Gates featured TerraPower in his 2010 TED talk.
Reactor physics 
Papers and presentations on the TerraPower TWR describe a pool-type reactor cooled by liquid sodium. The reactor is fueled primarily by depleted uranium-238 "fertile fuel", but requires a small amount of enriched uranium-235 or other "fissile fuel" to initiate fission. Some of the fast-spectrum neutrons produced by fission are absorbed by neutron capture in adjacent fertile fuel (i.e. the non-fissile depleted uranium), which is "bred" into plutonium by the nuclear reaction:
Initially, the core is loaded with fertile material, with a few rods of fissile fuel concentrated in the central region. After the reactor is started, four zones form within the core: the depleted zone, which contains mostly fission products and leftover fuel; the fission zone, where fission of bred fuel takes place; the breeding zone, where fissile material is created by neutron capture; and the fresh zone, which contains unreacted fertile material. The energy-generating fission zone steadily advances through the core, effectively consuming fertile material in front of it and leaving spent fuel behind. Meanwhile, the heat released by fission is absorbed by the molten sodium and subsequently transferred into a closed-cycle aqueous loop, where electric power is generated by steam turbines.
Unlike light-water reactors (LWRs), TWRs use only a small amount (~10%) of enriched uranium-235 or other fissile fuel to initiate the nuclear reaction. The remainder of the fuel consists of natural or depleted uranium-238, which can generate power continuously for 40 years or more and remains sealed in the reactor vessel during that time. TWRs require substantially less fuel per kilowatt-hour of electricity than do LWRs, owing to TWRs' higher fuel burnup, energy density and thermal efficiency. A TWR also accomplishes most of its reprocessing "on the fly" within the reactor core. Spent fuel can subsequently be recycled after simple "melt refining", without the chemical separation of plutonium that is required by other kinds of breeder reactors. These features greatly reduce fuel and waste volumes while enhancing proliferation resistance.
Depleted uranium is widely available as a feedstock. Stockpiles in the United States currently contain approximately 700,000 metric tons of depleted uranium, which is left as a byproduct of the enrichment process. TerraPower has estimated that the Paducah enrichment facility stockpile alone represents an energy resource equivalent to $100 trillion worth of electricity. TerraPower has also estimated that wide deployment of TWRs could enable projected global stockpiles of depleted uranium to sustain 80% of the world's population at U.S. per capita energy usages for over a millennium.
In principle, TWRs are capable of burning spent fuel from LWRs, which is currently discarded as radioactive waste. Spent LWR fuel is mostly depleted uranium and, in a TWR fast-neutron spectrum, the neutron absorption cross-section of fission products is several orders of magnitude smaller than in a LWR thermal-neutron spectrum. While such an approach could actually bring about an overall reduction in nuclear waste stockpiles, additional technical development would be required to realize this capability.
TWRs are also capable, in principle, of reusing their own fuel. In any given cycle of operation, only 20–35% of the fuel gets converted to an unusable form; the remaining metal constitutes usable fissile material. Recast and reclad into new driver pellets without chemical separations, this recycled fuel can be used to initiate fission in subsequent cycles of operation, thus displacing the need to enrich uranium altogether.
Traveling wave vs. standing wave 
The breed-burn wave in TerraPower's TWR design does not move from one end of the reactor to the other but gradually from the inside out. Moreover, as the fuel's composition changes through nuclear transmutation, fuel rods are continually reshuffled within the core to optimize the neutron flux and fuel usage at any given point in time. Thus, instead of letting the wave propagate through the fuel, the fuel itself is moved through a largely stationary burn wave. This is contrary to many media reports, which have popularized the concept as a candle-like reactor with a burn region that moves down a stick of fuel. By replacing a static core configuration with an actively managed "standing wave" or "soliton", however, TerraPower's design avoids the problem of cooling a highly variable burn region. Under this scenario, the reconfiguration of fuel rods is accomplished remotely by robotic devices; the containment vessel remains closed during the procedure, and there is no associated downtime.
Concept criticism 
Kirk Sorensen of Flibe Energy has criticized the TWR as "a particularly difficult implementation" of the fast breeder reactor, which he characterizes as "already hard to build in the first place", as well as for plans for its eventual nuclear decommissioning. Dr. Robert Hargraves, who is on the Flibe Energy Board of Advisors, lauded the goal of addressing energy poverty globally with the TWR, but briefly highlighted that its projected cost of energy production, "competitive with [conventional] nuclear power", wasn't as low as fossil fuels (e.g. coal).
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- THORIUM REMIX 2011 (YouTube video; comments begin about 1:00:25)
- Board of Advisors :: Flibe Energy
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- "TerraPower Reactors Approach the Nuclear Ideal" by Intellectual Ventures Lab
- TerraPower: How The Traveling Wave Nuclear Reactor Works Inc 4 diagrams from IV
- Terrapower.com Company website