# 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 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 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 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.

## History

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.[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]

Since 2001, the Travelling wave reactor was systematically discussed in INES 1, in 2004, INES-2 in 2006 and INES 3 in 2010 meetings in Japan, and was called "CANDLE" Reactor an abbreviation from Constant Axial shape of Neutron flux, nuclides densities and power shape During Life of Energy production reactor, as proposed by Sekimoto in 2001 and 2005, published in Progress in Nuclear Energy. In 2010 Popa-Simil publishes in INES-3 proceeding the paper "advanced Nuclear Reactor from Fiction to Reality", where he discuses the case of micho-hetero-structures improved Travelling Wave Reactor, further detailed in Plutonium Futures meeting, in 2010 the paper "Plutonium Breeding In Micro-Hetero Structures Enhances the Fuel Cycle", describing a TWR with deep burnout enhanced by plutonium fuel channels, and multiple fuel flow.

In 2010 a group from Terra Power applies for the patent EP 2324480 A1 following WO2010019199A1 "Heat pipe nuclear fission deflagration wave reactor cooling" where in order to be accepted calls the traveling or singular wave "deflagration" as it is moving with about 1-4 inch per year, and introduces the heat pipe cooling already applied in space reactors built at LANL and INL since 2000, or even earlier, using the flaws and weaknesses of USPTO. 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.[7] Bill Gates featured TerraPower in his 2010 TED talk.[8]

## Reactor physics

Papers and presentations on the TerraPower TWR[9][10][11] 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:

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

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.[10]

## Fuel

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.[11] 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"[citation needed] 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.[10]