Uranium Enrichment

The very low concentration of U-235 (the fissile isotope) is problematic because most conventional nuclear reactors require a higher proportion of fissile material. Most commercial reactor fuel is enriched to between 3% and 5% U-235, a product known as reactor-grade uranium. (Nuclear weapons require uranium enriched to greater than 90% U-235 (World Nuclear Association 2013a).) How is the U-235 iso­tope enriched? The methods of enrichment must take advantage of the sole differ­ence between the two isotopes: the fact that U-235 has 92 protons and 143 neutrons while U-238 has 92 protons and 146 neutrons. Even with a difference of only three neutrons, these two isotopes can be separated and the proportion of U-235 increased.

The first step in the enrichment process is the conversion of uranium oxide to uranium hexafluoride (UF6), a gas at room temperature (Equations 9.12a and 9.12b).

UO2(s) + 4HF(g) ^ UEt(s) + 2H2O(g)

The 235 UF6 and 238 UF6 can then be separated by one of two main methods: effu­sion (often less accurately referred to as diffusion) or centrifugation. While the effu­sion method is rapidly becoming obsolete, it still accounted for processing of 25% of the enriched U-235 supply in 2010. The basis of enrichment by diffusion is the rate at which the two isotopic UF6 gases diffuse through a thin membrane. Graham’s law (Equation 9.13) quantifies this property; r and r2 are the diffusion rates for the two gases of masses M1 and M2:

(9.13)

Thus, based on the miniscule difference in the mass of the two uranium isotopes, gaseous diffusion technology can enrich the uranium after several passes through the membrane.

The centrifugation method is projected to account for up to 93% of the enriched uranium supply by 2017 and is the more efficient process since centrifugal force is pro­portional to mass, not the square root of mass (World Nuclear Association 2013a). In this process, the mixture of UF6 gaseous isotopes is spun in cylinders at 50-70,000 rpm and centrifugal forces separate the two, concentrating the slightly heavier 238UF6 closer to the cylinder wall. The gas centrifuge technology consumes only about 5% as much electricity as the gaseous diffusion technology, but it should be noted that the enrich­ment process accounts for almost one-half the cost of nuclear fuel. The carbon footprint of nuclear power, although relatively small, can be mostly attributed to the enrichment process as the electricity it requires typically comes from coal-fired power plants.

New methods of uranium enrichment are under development and a “third-gener­ation” laser-based process is nearing commercial implementation. In this process, a laser precisely tuned to ionize a U-235 atom (once again in a UF6 feedstock) creates [235UF6]+ which is then separated from [238UF6] by ionic attraction to a negatively charged collector plate. For all of these separation methods, the next step in the nuclear fuel cycle is conversion of the enriched uranium hexafluoride gas to uranium dioxide (Equation 9.14):

Щ(я) + 2^O(g) + H2(g) ^ UO2(s) + 6HF(g)

The enriched uranium dioxide is then fabricated into sintered ceramic pellets that are then manufactured into fuel rods.

Updated: September 26, 2015 — 7:38 am