Once the uranium has been enriched to 3-4% 235U—depending on the specific requirements of a reactor—it is converted to uranium oxide (UO2) and then has to be made into fuel pellets that fit in the reactor. This process is identical to that used at the Melox facility in France to make MOX fuel (see Chapter 9). The only difference is that the MOX fuel pellets are made of reprocessed plutonium oxide mixed with the uranium oxide, instead of solely uranium oxide in normal nuclear fuel pellets. Basically, the uranium oxide powder is compressed under extremely high pressure to form pellets about the size of a pencil eraser. The pel­lets are heated to a high temperature to drive off any water and oils, ground to extremely uniform sizes, cleaned and inserted into zirconium alloy tubes 12 to 14 feet long. The tubes are filled with helium gas, sealed, tested for integrity, and are now fuel rods. The fuel rods are assembled into a matrix of 14 x 14 or up to 17 x 17, depending on the reactor, and ultimately about 200 fuel assemblies go into the reactor core. One-third of the fuel assemblies are used up and switched out every 18 months or so.

The energy density of a nuclear fuel pellet is remarkable. Each pellet weighs about 7 grams and contains the same energy as 140 gallons of oil, 157 gallons of regular gasoline, 17,000 cubic feet of natural gas, and 1,780 pounds of coal (40). A couple of handfuls of pellets are equivalent to a train car full of coal. This energy density is what makes nuclear reactors so efficient. The entire fleet of 104 US reac­tors, each producing about one GW of power, uses only 19,000 tons of uranium in a year (41). But the United States produces only about 1,500 tons of uranium per year, so where does the rest come from?