Dig uranium out of the ground and 99.27% of it is uranium-238. Only 0.72% is uranium-235, the isotope that can sustain a fission chain reaction. Everything about enrichment follows from that gap.
To enrich uranium means raising the share of uranium-235 and stripping out uranium-238. Reactors and weapons both need it. Where they differ is how far the process goes.
Low and high
Low-enriched uranium, or LEU, is under 20% uranium-235. Most reactors want 3.5% to 4.5%, and LEU fuels the light-water designs that make up most of the world’s nuclear plants. Highly enriched uranium, HEU, starts at 20%. It has been used in weapons, naval propulsion reactors, some research reactors and a few specialized designs. About 2,000 tonnes of it exist worldwide.
A weapon can technically be built with as little as 20%, but the more enriched the uranium, the smaller and lighter the bomb. Countries with nuclear weapons tend to use about 90%, which is called weapons grade. According to the International Atomic Energy Agency, Iran has enriched large quantities to 60%. Going from 60% to 90% is easier than reaching 60% in the first place, since there is less uranium-238 left to remove.
How the separation works
The trouble is that the two isotopes are nearly identical chemically. Uranium-238 is only about 1% heavier. So plants convert uranium to a gas, uranium hexafluoride, and spin it in centrifuges at 50,000 to 70,000 rotations per minute. The heavier molecules drift to the walls. The lighter, uranium-235-rich ones collect nearer the center. One scientist compares it to a salad spinner.
One pass barely moves the numbers, so plants chain many stages into a cascade, each passing a slightly enriched stream forward and a slightly depleted one back.
The same machines make both grades. Centrifuges that turn out reactor fuel can be tuned to make weapons material, which is why centrifuge technology is guarded so closely.
From the Manhattan Project to lasers
Large-scale enrichment began in the Manhattan Project, with gaseous diffusion and electromagnetic isotope separation. Gaseous diffusion later became the commercial standard, then faded. The last big commercial plant, at Paducah in the United States, stopped enriching in 2013. Gas centrifuges replaced it. Modern centrifuge plants need roughly 40 to 50 kWh per separative work unit, against about 2,400 to 2,500 for diffusion.
Laser separation is the candidate for a third generation. Global Laser Enrichment has applied to license a laser facility in the United States, though centrifuges still do all the commercial work.
Before and after
The feedstock starts as ore containing as little as 0.1% uranium. Milling produces yellowcake, about 80% uranium. That gets converted either to uranium dioxide, for reactors that need no enrichment, such as CANDU and RBMK designs, or to uranium hexafluoride for the enrichment plant.
What is left over is depleted uranium, mostly uranium-238. It is less radioactive than natural uranium but very dense, which makes it useful for radiation shielding and armor-piercing weapons. Reprocessing spent fuel yields reprocessed uranium, which typically holds slightly more uranium-235 than natural ore does.
Sources
- Enriched uranium – Wikipedia
- What is uranium enrichment and how is it used for nuclear bombs? A scientist explains
