Explained: Downblending – the key process in the US-Iran nuclear deal
The United States and Iran have signed a memorandum of understanding (MoU) to end hostilities, in which Tehran pledges not to develop nuclear weapons in exchange for sanctions relief and access to a $300 billion development fund. However, the MoU lacks specifics on resolving the nuclear dispute beyond a commitment to downblend Iran’s uranium stockpile.
Iran possesses hundreds of kilograms of highly enriched uranium and retains the technical capacity to produce more. Paragraph 8 of the MoU states that both sides agree to dispose of enriched material through a mechanism involving downblending on site under International Atomic Energy Agency (IAEA) supervision. U.S. President Donald Trump said the U.S. would “go and get” Iran’s highly enriched uranium and “dilute” it either in America or in Iran.
Downblending is a technology that reduces the purity of uranium, making it harder to use for nuclear weapons. Natural uranium consists mainly of uranium-238 (99.28%) and uranium-235 (0.72%). Only uranium-235 is fissile, meaning it can sustain a nuclear chain reaction. Enrichment increases the concentration of uranium-235, while downblending reverses this process by mixing enriched uranium with depleted or natural uranium to produce a lower concentration.
The goal is to reduce the amount of uranium-235 available for a bomb and increase the “breakout time” – the time needed to produce enough weapons-grade material. This is a cornerstone of nuclear non-proliferation. For example, the 2015 Iran deal limited enrichment to 3.67% uranium-235, sufficient for power generation but not for weapons.
The downblending process involves several steps:
- Uranium hexafluoride (UF6), stored as a solid in steel cylinders, is heated in an autoclave to 80–110°C, turning it into a gas.
- A blendstock – depleted uranium (0.2-0.3% uranium-235) or natural uranium (0.7%) – is also converted to gas.
- The two gases are pumped into a mixing junction called a blending tee. Engineers use thermal mass flow meters to control the ratio of feedstock to blendstock, ensuring the final product has the desired enrichment level (e.g., below 5%).
- The gas mixture passes through an online enrichment monitor (OLEM), which detects gamma rays emitted by uranium-235 (energy signature at 186 keV) to verify the concentration.
Successful downblending reduces Iran’s stockpile of high-enriched uranium, lowering the risk of weaponisation. International trust in Iran’s nuclear intentions will depend on transparent implementation and IAEA verification.
