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Traveling wave reactor

Traveling wave reactor

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A traveling-wave reactor, or TWR, is a type of conceptual nuclear reactor
Nuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...

 that theorists speculate can convert fertile material
Fertile material
Fertile material is a term used to describe nuclides which generally themselves do not undergo induced fission but from which fissile material is generated by neutron absorption and subsequent nuclei conversions...

 into fissile
In nuclear engineering, a fissile material is one that is capable of sustaining a chain reaction of nuclear fission. By definition, fissile materials can sustain a chain reaction with neutrons of any energy. The predominant neutron energy may be typified by either slow neutrons or fast neutrons...

 fuel as it runs using the process of nuclear transmutation
Nuclear transmutation
Nuclear transmutation is the conversion of one chemical element or isotope into another. In other words, atoms of one element can be changed into atoms of other element by 'transmutation'...

. TWRs differ from other kinds of fast-neutron
Neutron temperature
The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy, usually given in electron volts. The term temperature is used, since hot, thermal and cold neutrons are moderated in a medium with a certain temperature. The neutron energy distribution is...

 and breeder reactor
Breeder reactor
A breeder reactor is a nuclear reactor capable of generating more fissile material than it consumes because its neutron economy is high enough to breed fissile from fertile material like uranium-238 or thorium-232. Breeders were at first considered superior because of their superior fuel economy...

s in their ability to, once started, reach a state whereafter they can achieve very high fuel utilization while using no enriched uranium
Enriched uranium
Enriched uranium is a kind of uranium in which the percent composition of uranium-235 has been increased through the process of isotope separation. Natural uranium is 99.284% 238U isotope, with 235U only constituting about 0.711% of its weight...

 and no reprocessing
Nuclear reprocessing
Nuclear reprocessing technology was developed to chemically separate and recover fissionable plutonium from irradiated nuclear fuel. Reprocessing serves multiple purposes, whose relative importance has changed over time. Originally reprocessing was used solely to extract plutonium for producing...

, instead burning fuel made from depleted uranium
Depleted uranium
Depleted uranium is uranium with a lower content of the fissile isotope U-235 than natural uranium . Uses of DU take advantage of its very high density of 19.1 g/cm3...

, natural uranium
Natural uranium
Natural uranium refers to refined uranium with the same isotopic ratio as found in nature. It contains 0.7 % uranium-235, 99.3 % uranium-238, and a trace of uranium-234 by weight. In terms of the amount of radioactivity, approximately 2.2 % comes from uranium-235, 48.6 % uranium-238, and 49.2 %...

, thorium
Thorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....

, spent fuel
Spent nuclear fuel
Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor...

 removed from light water reactor
Light water reactor
The light water reactor is a type of thermal reactor that uses normal water as its coolant and neutron moderator. Thermal reactors are the most common type of nuclear reactor, and light water reactors are the most common type of thermal reactor...

s, or some combination of these materials. The name refers to the design characteristic that fission does not happen in the entire TWR core, but takes place in a fairly localized zone that advances through the core over time. TWRs could theoretically run, self-sustained, for decades without refueling or removing any used fuel from the reactor.


Traveling-wave reactors were first proposed in the 1950s and have been studied intermittently since. 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
Intellectual Ventures
Intellectual Ventures is a private company notable for being one of the top-five owners of U.S. patents, as of 2011. Its business model has a focus on developing a large patent portfolio and licensing these patents to companies. Publicly, it states that a major goal is to assist small inventors...

 launched a subsidiary named TerraPower, LLC to model and commercialize a practical engineering embodiment of such a reactor, which has since come to be called a traveling-wave reactor. TerraPower has developed TWR designs for low- to medium-power (300-MWe) and large power (~1000-MWe) application. Bill Gates
Bill Gates
William Henry "Bill" Gates III is an American business magnate, investor, philanthropist, and author. Gates is the former CEO and current chairman of Microsoft, the software company he founded with Paul Allen...

 featured TerraPower in his 2010 TED
TED (conference)
TED is a global set of conferences owned by the private non-profit Sapling Foundation, formed to disseminate "ideas worth spreading"....


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, but requires a small amount of enriched uranium or other fissile fuel to initiate fission
Nuclear fission
In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts , often producing free neutrons and photons , and releasing a tremendous amount of energy...

. 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), converting it into plutonium
Plutonium is a transuranic radioactive chemical element with the chemical symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation...

 by the nuclear reaction:

Initially, the core is charged with fertile material. A small amount of fissile fuel is added to one end of the core. Once the reactor is started, four zones form in 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
Fertile material
Fertile material is a term used to describe nuclides which generally themselves do not undergo induced fission but from which fissile material is generated by neutron absorption and subsequent nuclei conversions...

. The energy-generating fission zone advances through the core over time, effectively consuming fertile material in front of it and leaving spent fuel behind. Heat from fission is converted into electricity using conventional steam turbines.


Unlike light-water reactors (LWRs), TWRs can be fueled at the time of construction with a small amount, about 10 %, of enriched uranium fuel (U-235) to start the heat-producing nuclear reaction along with enough depleted uranium to produce full power for 60 years or more. TWRs consume substantially less uranium than a LWR per unit of electricity generated due to TWRs higher fuel burnup, higher thermal efficiency and higher fuel density. A TWR also accomplishes reprocessing on the fly, without the need for chemical separation that is typical of other kinds of breeder reactors. These features greatly reduce fuel and waste volumes while enhancing proliferation resistance. The resultant traveling wave then burns through the mostly depleted uranium fuel (U-238), which through a series of reactions, converts into plutonium-239
Plutonium-239 is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 has also been used and is currently the secondary isotope. Plutonium-239 is also one of the three main isotopes demonstrated usable as fuel in...

. The nuclear reaction continues until all the uranium is spent. The concept uses molten sodium as a coolant. The energy is then absorbed and carried away as heat, which ultimately generates the power.

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 produced as a waste byproduct of the enrichment process. TerraPower has estimated that the stockpiles present at just the Paducah enrichment facility
Paducah Gaseous Diffusion Plant
The Paducah Gaseous Diffusion Plant is a facility located in McCracken County, Kentucky, near Paducah, Kentucky, that produces enriched uranium, for nuclear power plants. The plant is now operated by United States Enrichment Corporation, a subsidiary of USEC Incorporated, a publicly traded...

 represents an energy resource equivalent to $100 trillion worth of electricity. Company scientists have 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. This is possible because 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. Additional technical development would be required to realize this capability, however.

TWRs are also capable, in principle, of reusing their own fuel. The used metal fuel from TWRs will still contain a high fissile content. Recast and reclad into new driver pellets without chemical separations, this recycled fuel could be used to start fission in additional TWRs, thus displacing the need to enrich uranium altogether.

Travelling wave vs. standing wave

The breed-burn wave in TerraPower's manifestation of a TWR does not physically move. Instead, fuel is shuffled into the burning region as it breeds enough fissile material to keep the reactor critical for long times. The behavior of the reactor power vs. time represents a soliton
In mathematics and physics, a soliton is a self-reinforcing solitary wave that maintains its shape while it travels at constant speed. Solitons are caused by a cancellation of nonlinear and dispersive effects in the medium...

. This is contrary to many media reports, which are still discussing a candle-like reactor with a power region that moves down a stick of fuel. The standing-wave or soliton behavior maintains most of the benefits of the traditional view of a TWR, giving up stagnant fuel while adding simplicity in cooling. "Once-through" nuclear reactors include "medium burnup (50 GWd/t) and high burnup (100 GWd/t) PWRs, the CANDLE reactor of the Tokyo Institute of Technology, the sustainable sodium-cooled fast reactor (SSFR) by ANL, the fast mixed spectrum reactor (FMSR) by BNL, the ultra-long life fast reactor (ULFR) by ANL, the General Atomics Energy Multiplier Module (EM2), and the traveling wave reactor (TWR) of TerraPower."

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