“The biggest problem is swelling,” said Kevan Weaver, a physicist and TerraPower’s director of technology development. “The neutrons knock an atom out of the lattice, and leaves a hole, and then the holes coalesce and form voids, and the part swells.”
So TerraPower’s engineers are experimenting with different types of metals, at different temperatures. In December, they will put thousands of samples into a Russian reactor that will irradiate them for six years, with neutrons of the same energy that TerraPower’s reactor would have. At the end of this decade, they will see how the metals’ strength was changed, and predict whether the metal will survive for 30 years.
Another problem is that when uranium is split, some of the fragments are gases. This is tolerable in current fuels, but no fuel could hold a 30-year accumulation.
Designing the core of the reactor is an additional problem. TerraPower engineers call it a “traveling wave reactor,” because the area in which the uranium-238 has been converted to plutonium and can be fissioned travels through the core like a wave.
However, every time the designers change the thickness or type of metal they are using, the flow of neutrons will change, too, and the 30-year life of the core is so long that the inventory of fission products, some of which absorb neutrons, will also change, as some unstable materials give off radiation and transmute themselves into something else.
To allow the neutrons to travel at a speed that is best for converting waste uranium into plutonium fuel, the reactor uses sodium, not water, to moderate the neutrons’ speed and carry off the usable heat. However, hot sodium burns on contact with air.
TerraPower is not alone in pursuing a reactor that will turn waste uranium into energy, and if such a concept can be commercialized, Gates might not be the first to do it. General Atomics, which has decades of experience in nuclear power, but is probably best known for producing the Predator drone, is pursuing what it calls an “energy multiplier” reactor module on the same general principal. However, General Atomics, which is based in San Diego, Californai would use helium, not sodium, potentially simplifying some problems.
“You just set it up, let it burn, and it goes,” General Atomics senior vice president John Parmentola said.
Like TerraPower, General Atomics is courting the Chinese.