A traveling-wave reactor (TWR) can transmute fertile materials such as natural uranium, depleted uranium, thorium and spent nuclear fuel into usable fuel while burning fissile materials. The name comes from the fact that fission occurs in a boundary zone in the reactor core. The zone of fusion advances so slowly that it may be possible for a TWR to run unattended for decades without the need to remove or replace the fuel. Although TWRs have been discussed on and off for decades since the 1950s, no TWR has ever been constructed.
In 2006, TerraPower was launched by Intellectual Ventures in Bellevue Washington. TerraPower is working on designs for TWRs in the small modular reactor range of up to three hundred megawatts and the full-sized reactor size of one gigawatt or larger. In 2015, TerraPower signed a memorandum of understanding with the China National Nuclear Corporation for the joint development of a TWR. Current plans of TerraPower call for a six hundred megawatt demonstration plant by 2022 and commercial plants of eleven hundred and fifty megawatts by the late 2020s.
The TerraPower plans call for a pool-type reactor design cooled by liquid sodium. The primary fuel will be depleted U-238 with a small amount of U-235 or another type of fissile fuel to initiate a reaction. Some of the fast spectrum neutrons generated by the fissile material are absorbed by U-238 atoms which converts them to plutonium which can then be burned.
The core of the reactor is initially loaded with fertile material. A few rods of fissile material are placed in the center of the core. A series of concentric zones form around the center of the core. The first zone contains fission products and left-over fuel. The second zone is where the fission of the bred fuel takes place. The third zone is the breeding zone where neutron capture creates fissile material from the fertile materials. The fourth zone contains fresh fertile material which has not yet been transmuted.
The fuel rods are constantly moved around so that the zones do not actually move through the fuel like a candle burning down a wick. Instead, there is a sort of standing wave pattern that holds the four concentric zones in one place while the fuel itself is moved. Robots move the fuel around so that the reactor remains sealed and no human intervention is required. The heat generated by the fission zone is captured by the liquid sodium and then transferred to a closed water loop. The steam in the closed water loop turns a turbine to created electricity.
The TWR has higher fuel burn up, energy density and thermal efficiency than a conventional light water reactor. A load of natural or depleted uranium should be able to burn and generate power for forty years or more while sealed in the reactor core. Eventually spent fuel can be replaced by simple “melt refining” of new fuel without the need for chemical separation of the fissile material from the fertile material. These features of TWRs reduce the volume of fuel required to produce electricity, the volume of waste produced and the danger of proliferation of materials that could be used to make nuclear weapons.
With respect to fuel for TWRs, it is estimated that there are up to seven hundred thousand metric tons in the U.S. TerraPower has estimated that there is so much depleted uranium in the world that eighty percent of the global population could be provided with the current levels of electricity available in the U.S. for a thousand years. TWRs could also burn the spent nuclear fuel that is piling up around the world at nuclear power plants. With some simple reprocessing, TWRs should be able to burn their own waste.
Critics have said that building a TWR will be difficult and expensive from an engineering standpoint. Decommissioning a TWR will be especially difficult. Finally, the power produced will be competitive with other nuclear power systems but will not be competitive with power from cheap fossil fuels and future renewable systems.
