User:Ntouran/twr

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.

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. 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 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.

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. 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:

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

The plutonium then fissions to produce two fission products, an average of around 2.5 neutrons, and about 200,000,000 electron volts (eV) of energy, which is converted to 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. 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.

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 these stockpiles represent 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 electricity 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 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.