The pyrometallurgical process (“pyro” for short) extracts from used fuel a mix of transuranic elements instead of pure plutonium, as in the PUREX route. It is based on electroplating— using electricity to collect, on a conducting metal electrode, metal extracted as ions from a chemical bath. Its name derives from the high temperatures to which the metals must be subjected during the procedure. Two similar approaches have been developed, one in the U.S., the other in Russia. The major difference is that the Russians process ceramic (oxide) fuel, whereas the fuel in an ALMR is metallic.
In the American pyroprocess, technicians dissolve spent metallic fuel in a chemical bath. Then a strong electric current selectively collects the plutonium and the other transuranic elements on an electrode, along with some of the fission products and much of the uranium. Most of the fission products and some of the uranium remain in the bath. When a full batch is amassed, operators remove the electrode. Next they scrape the accumulated materials off the electrode, melt them down, cast them into an ingot and pass the ingot to a refabrication line for conversion into fast-reactor fuel. When the bath becomes saturated with fission products, technicians clean the solvent and process the extracted fission products for permanent disposal.
Thus, unlike the current PUREX method, the pyroprocess collects virtually all the transuranic elements (including the plutonium), with considerable carryover of uranium and fission products. Only a very small portion of the transuranic component ends up in the final waste stream, which reduces the needed isolation time drastically. The combination of fission products and transuranics is unsuited for weapons or even for thermal-reactor fuel. This mixture is, however, not only tolerable but advantageous for fueling fast reactors.
Although pyrometallurgical recycling technology is not quite ready for immediate commercial use, researchers have demonstrated its basic principles. It has been successfully demonstrated on a pilot level in operating power plants, both in the U.S. and in Russia. It has not yet functioned, however, on a full production scale.
The operating capabilities of thermal and fast reactors are similar in some ways, but in others the differences are huge. A 1,000-megawatt-electric thermal-reactor plant, for example, generates more than 100 tons of spent fuel a year. The annual waste output from a fast reactor with the same electrical capacity, in contrast, is a little more than a single ton of fission products, plus trace amounts of transuranics.
Waste management using the ALMR cycle would be greatly simplified. Because the fast-reactor waste would contain no significant quantity of long-lived transuranics, its radiation would decay to the level of the ore from which it came in several hundred years, rather than tens of thousands.
If fast reactors were used exclusively, transportation of highly radioactive materials would occur only under two circumstances— when the fission product waste was shipped to Yucca Mountain or an alternative site for disposal and when start-up fuel was shipped to a new reactor. Commerce in plutonium would be effectively eliminated.
Some people are advocating that the U.S. embark on an extensive program of PUREX processing of reactor fuel, making mixed oxides of uranium and plutonium for cycling back into thermal reactors. Although the mixed oxide (MOX) method is currently being used for spoiling excess weapons plutonium so that it cannot be employed in bombs—a good idea—we think that it would be a mistake to deploy the much larger PUREX infrastructure that would be required to process civilian fuel. The resource gains would be modest, whereas the long-term waste problem would remain, and the entire effort would delay for only a short time the need for efficient fast reactors.