Iran’s Nuclear “Breakout” Time Reduced to 3-4 Months

The April 2020 U.S. State Department Compliance Report, Adherence to and Compliance with Arms Control, Nonproliferation, and Disarmament Agreements and Commitments, states that Iran has “progressively expanded its uranium enrichment activities and stockpile of enriched uranium, key factors in determining the amount of time required to produce enough fissile material for a nuclear weapon or device, should Iran decide to pursue nuclear weapons.”¹ This time period is often referred to as the time needed to produce enough weapon-grade uranium for a nuclear weapon, or as breakout time. In line with previous conventions used by the Institute, 25 kilograms of WGU represents an amount sufficient for a single nuclear weapon.²

Breakout predictions are intended to represent a realistic minimum time Iran would need to produce its first 25 kilograms of WGU, alternatively referred to as a credible worst-case estimate. They account for system limitations, centrifuge breakage, and other inefficiencies, but not for all the various problems or delays of the type that have been encountered by Iran’s program, which could lengthen the time needed for enrichment further.

The breakout estimate is not a “best” or average estimate of Iran’s breakout, since such estimates are associated with high uncertainties, plagued by an Iranian centrifuge program in significant flux and one surrounded with great secrecy about its breakout capabilities. Just as important, calculational methodologies that emphasize simple use of “ideal cascade” calculations should be avoided, as they lead to serious underestimates of the breakout timelines and are non-plausible in the Iranian context. Similarly, not all of Iran’s LEU stocks are suitable for use in a breakout, an issue discussed below and in the Annex.

It needs to be remembered that the purpose of the breakout estimate today is to measure the risk posed by Iran’s increasing nuclear enrichment capabilities. As part of a risk assessment for a nuclear Iran, where the risk cannot be quantified over all possibilities, including assigning probabilities to these possibilities, a realistic worst case estimate becomes a supportable way of quantifying risk, with the important proviso that a case should not be incompatible with existing knowledge about Iran’s nuclear program. Likewise, cases should be avoided that arbitrarily assign difficulties to Iran achieving a breakout, given that Iran’s centrifuge program can always do worse, from the point of view of lengthening breakout timelines, but it can also do better than such expectations.

^{1. U.S. State Department, Bureau of Arms Control, Verification and Compliance, Executive Summary of the 2020 Adherence to and Compliance with Arms Control, Nonproliferation, and Disarmament Agreements and Commitments (Compliance Report), April 2020, https://www.state.gov/2020-adherence-to-and-compliance-with-arms-control-nonproliferation-and-disarmament-agreements-and-commitments-compliance-report/ ↩}

^{2. Weapon-grade uranium is defined as uranium enriched to at least 90 percent U-235. The amount of 25 kilograms above referring to the entire amount of uranium should not be confused with what is defined as a “significant quantity” of weapon-grade uranium by the International Atomic Energy Agency (IAEA), 25 kilograms of uranium-235 in more than 90 percent enriched uranium. At 90 percent enriched, a significant quantity would correspond to 27.8 kilograms of weapon-grade uranium. Moreover, a first nuclear weapon may require more or less than 25 kilograms of WGU, depending on its design and the rate of losses in preparing weapons components from the weapon-grade uranium hexafluoride, which is the output of a centrifuge plant.}