Front End of Cycle

- Exploration

The nuclear fuel cycle starts with exploration for uranium and the development of mines to extract uranium ore. A variety of techniques are used to locate uranium, such as airborne radiometric surveys, chemical sampling of groundwater and soils, and exploratory drilling to understand the underlying geology. 

Once uranium ore deposits are located, the mine developer usually follows up with more closely spaced in fill, or development drilling, to determine how much uranium is available and what it might cost to recover it.

- Uranium Mining

When ore deposits that are economically feasible to recover are located, the next step in the fuel cycle is to mine the ore using one of the following techniques: 

• underground mining
• open pit mining
• in-place (in-situ) solution mining
• heap leaching

Before 1980, most U.S. uranium was produced using open pit and underground mining techniques. 

Today, most U.S. uranium is produced using a solution mining technique commonly called in-situ-leach (ISL) or in-situ-recovery (ISR). This process extracts uranium that coats the sand and gravel particles of groundwater reservoirs. 

The sand and gravel particles are exposed to a solution with a pH that has been elevated slightly by using oxygen, carbon dioxide, or caustic soda. The uranium dissolves into the groundwater, which is pumped out of the reservoir and processed at a uranium mill. 

Heap leaching involves spraying an acidic liquid solution onto piles of crushed uranium ore. The solution drains down through the crushed ore and leaches uranium out of the rock, which is recovered from underneath the pile. Heap leaching is no longer used in the United States.

- Uranium Milling

After uranium ore is extracted from open pit or underground mines, it is refined into uranium concentrate at a uranium plant. The ore is crushed, pulverized and ground into a fine powder. Chemicals are added to the fine powder, which causes a reaction that separates uranium from other minerals. Groundwater from solution mining operations is circulated through resin beds to extract and enrich uranium. 

Despite the name, the concentrated uranium products are usually a black or brown substance called yellowcake (U₃O₈). Mined uranium ore typically yields 1 to 4 pounds of U₃O₈ per ton of ore, or 0.05% to 0.20% yellowcake. Solid waste from pits and underground mining operations is called mill tailings. Treated water from solution mining is returned to the subterranean reservoir where the mining process is repeated.

- Uranium Enrichment

After conversion, the UF₆ gas is sent to the enrichment plant, where the individual uranium isotopes are separated to produce enriched UF₆, which contains U-235 at a concentration of 3% to 5%. 

The United States uses two uranium enrichment processes: gas diffusion and gas centrifugation. The United States currently has an operating enrichment plant that uses a gas centrifuge process. The enriched UF6 is sealed in tanks and cooled and solidified before being transported by train, truck or barge to the nuclear reactor fuel assembly plant. 

Atomic vapor laser isotope separation (AVLIS) and molecular laser isotope separation (MLIS) are novel enrichment techniques currently under development. These laser-based enrichment processes can achieve higher initial enrichment (isotope separation) factors than diffusion or centrifugation processes, and can produce enriched uranium faster than other techniques.

- Uranium Reconversion and Nuclear Fuel Manufacturing

Once uranium is enriched, it can be converted into nuclear fuel. In a nuclear fuel manufacturing facility, solid UF₆ is heated to a gaseous state, and the UF6 gas is then chemically processed to form uranium dioxide (UO₂) powder. The powder is then compressed and formed into small ceramic fuel pellets. The pellets are stacked and sealed into long metal tubes about 1 cm in diameter to form fuel rods. The fuel rods are then bundled together to form a fuel assembly. There are approximately 179 to 264 fuel rods per fuel assembly, depending on the reactor type. A typical reactor core contains between 121 and 193 fuel assemblies.