# Traveling wave reactor

Numeric simulation of a TWR. Red: uranium-238, light green: plutonium-239, black: fission products. Intensity of blue color between the tiles indicates neutron density

A traveling-wave reactor (TWR) is a type of nuclear fission reactor that 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 use 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 remains confined to a boundary zone in the reactor core that slowly advances over time. TWRs could theoretically run, self-sustained, for decades without refueling or removing spent fuel.

## History

Traveling-wave reactors were first proposed in the 1950s and have been studied intermittently. 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.[1] Michael Driscoll published further research on the concept in 1979,[2] as did Lev Feoktistov in 1988,[3] Edward Teller/Lowell Wood in 1995,[4] Hugo van Dam in 2000[5] and Hiroshi Sekimoto in 2001.[6]

The TWR was discussed at the Innovative Nuclear Energy Systems (INES) symposiums in 2004, 2006 and 2010 in Japan where it was called "CANDLE" Reactor, an abbreviation for Constant Axial shape of Neutron flux, nuclides densities and power shape During Life of Energy production.[7] In 2010 Popa-Simil discussed the case of micro-hetero-structures,[8] further detailed in the paper "Plutonium Breeding In Micro-Hetero Structures Enhances the Fuel Cycle", describing a TWR with deep burnout enhanced by plutonium[9] fuel channels and multiple fuel flow. In 2012 it was shown that fission[10] waves are a form of bi-stable reaction diffusion phenomenon.[11]

No TWR has yet been constructed, but in 2006, Intellectual Ventures launched a spin-off named TerraPower to model and commercialize a working design of such a reactor, which later came 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.[12] Bill Gates featured TerraPower in his 2010 TED talk.[13]

In 2010 a group from TerraPower applied for patent EP 2324480 A1 following WO2010019199A1 "Heat pipe nuclear fission deflagration wave reactor cooling". The application was deemed withdrawn in 2014.[14]

In September, 2015 TerraPower and China National Nuclear Corporation (CNNC) signed a memorandum of understanding to jointly develop a TWR. TerraPower plans to build a 600 MWe demonstration Plant, the TWR-P, by 2018–2022 followed by larger commercial plants of 1150 MWe in the late 2020s.[15]

## Reactor physics

Papers and presentations on TerraPower's TWR[16][17][18] 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:

${\displaystyle \mathrm {^{238}_{\ 92}U+\,_{0}^{1}n\;\rightarrow \;_{\ 92}^{239}U\;\rightarrow \;_{\ 93}^{239}Np+\beta \;\rightarrow \;_{\ 94}^{239}Pu+\beta } }$

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.[17]

## Fuel

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.[18] TWRs require substantially less fuel per kilowatt-hour of electricity than do light-water reactors (LWRs), owing to TWRs' higher fuel burnup, energy density and thermal efficiency. A TWR also accomplishes most of its reprocessing within the reactor core. Spent fuel can 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.[17]