Step 3: Refine It
Because U-238 and U-235 differ in weight by only about five billionths of a quadrillionth the weight of a paper clip, one go in a centrifuge is not enough to separate the two and fully enrich the hex, so it must move through additional centrifuges. How these connect to one another—a configuration of rows known as a cascade—can yield clues to the fuel's future uses. At each row, the uranium is slightly enriched and the U-235-rich hex moves up to the next row. Since power-plant uranium requires less enrichment, cascades for reactor-grade material have fewer rows, thus are "shorter." Uranium intended for nuclear weapons, however, requires that more than 90 percent of the hex be enriched, so it uses taller, narrower rows.