# Traveling wave reactor: Wikis

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# Encyclopedia

A traveling-wave reactor, or TWR, is a kind of nuclear reactor that can convert fertile material into fissile fuel as it runs using the process of nuclear transmutation. TWRs differ from other kinds of fast-neutron and breeder reactors in their ability to use little or no enriched uranium, instead burning fuel made from depleted uranium, natural uranium, thorium, spent fuel removed from light-water reactors, 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.

## History

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.[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]

No TWR has yet been constructed, but in 2006, Intellectual Ventures 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.[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, but requires a small amount of enriched uranium 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), converting it into plutonium by the nuclear reaction:

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

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. 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.

## Fuel

Unlike light-water reactors (LWRs), TWRs can be fueled at the time of construction with enough depleted uranium to produce full power for 60 years or more.[11] 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.[10]

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.[12] TerraPower has estimated that these stockpiles represent an energy resource equivalent to \$100 trillion worth of electricity.[11] 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 electricity usages for over a millennium.[13]

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 are 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 separations, this recycled fuel could be used to start fission in additional TWRs, thus displacing the need to enrich uranium altogether.

## References

1. ^ S.M. Feinberg, “Discussion Comment”, Rec. of Proc. Session B-10, ICPUAE, United Nations, Geneva, Switzerland (1958).
2. ^ M.J. Driscoll, B. Atefi, D. D. Lanning, “An Evaluation of the Breed/Burn Fast Reactor Concept”, MITNE-229 (Dec. 1979).
3. ^ L.P. Feoktistov, “An analysis of a concept of a physically safe reactor”, Preprint IAE-4605/4, in Russian, (1988).
4. ^ E. Teller, M. Ishikawa, and L. Wood, “Completely Automated Nuclear Power Reactors for Long-Term Operation”, Proc. Of the Frontiers in Physics Symposium, American Physical Society and the American Association of Physics Teachers Texas Meeting, Lubbock, Texas, United States (1995).
5. ^ H. van Dam, “The Self-stabilizing Criticality Wave Reactor”, Proc. Of the Tenth International Conference on Emerging Nuclear Energy Systems (ICENES 2000), p. 188, NRG, Petten, Netherlands (2000).
6. ^ H. Sekimoto, K. Ryu, and Y. Yoshimura, “CANDLE: The New Burnup Strategy”, Nuclear Science and Engineering, 139, 1–12 (2001).
7. ^ K. Weaver, C. Ahlfeld, J. Gilleland, C. Whitmer and G. Zimmerman, “Extending the Nuclear Fuel Cycle with Traveling-Wave Reactors”, Paper 9294, Proceedings of Global 2009, Paris, France, September 6–11, (2009).
8. ^ Bill Gates. Innovating to zero!. TED. Retrieved 2010-02-21.
9. ^ R. Michal and E. M. Blake, “John Gilleland: On the traveling-wave reactor”, Nuclear News, p. 30–32, September (2009).
10. ^ a b Wald, M. (2009-March/April). 10 Emerging Technologies of 2009: Traveling-Wave Reactor. MIT Technology Review.
11. ^ a b c Gilleland, John (2009-04-20). "TerraPower, LLC Nuclear Initiative". University of California at Berkeley, Spring Colloquium. Retrieved October 2009.
12. ^ United States Department of Energy, “Depleted UF6 Inventory and Storage Locations”. Accessed October 2009.
13. ^ L. Wood, T. Ellis, N. Myhrvold and R. Petroski, “Exploring The Italian Navigator’s New World: Toward Economic, Full-Scale, Low Carbon, Conveniently-Available, Proliferation-Robust, Renewable Energy Resources”, 42nd Session of the Erice International Seminars on Planetary Emergencies, Erice, Italy, 19024 August (2009).