Decreasing the Risk of Iran’s 5% Enriched Uranium Hexafluoride Stockpile

Dr. F. Dalnoki-Veress
March 3, 2015

Read the full report on a new option for Iran’s uranium stockpile:
A Crazy idea? Isotopic Denaturing of Iran’s 5% UF6 Stockpile with Reprocessed Uranium

The Nuclear Negotiations

Iran's Uranium Stockpile: Worker at Uranium Conversion Facility in Iran

Worker at Uranium Conversion Facility in Iran, Source:

The most challenging problem in the negotiations between Iran and the P5+1 is dealing with the scope of the enrichment program envisaged by Iran. Iran has stated that it requires enough enrichment capacity to be able to provide fuel for the Bushehr reactor requiring at least 100,000 SWU/year, which is an order of magnitude more capacity than Iran currently has installed. However, the P5+1 are likely to insist on an annual production limit a factor of 20 times less.  Another challenge is the quantity of low enriched UF6 (uranium hexafluoride) that currently exists in Iran’s stockpile or may be produced in the future, which could be re-enriched to weapons grade in a breakout scenario.

A New Proposal: Isotopic Denaturing

We propose the possibility of denaturing low enriched UF6 (such as the near 5% material) with foreign (Russian) reprocessed uranium (RepU) in order to decrease the risk of the current and future stockpiles of enriched UF6. In a first step, 7952.9 kg of near-5% UF6 is blended down to 3% using 6950 kg natural uranium. In a second step the material is blended with 2483.6 kg 17% RepUF6 (with >4.9 ppm U-232) to bring the material back up to 5% U-235 enriched material. This material is significantly less attractive for use in a nuclear weapon if further enriched to weapons grade. [1] The  reason for this is that U-232 present in reprocessed uranium is enriched along with the U-235 increasing the number of neutrons both through spontaneous fission but also due to the  (α,n) reaction on light element contaminants in the material.The advantage to Iran is obtaining at least twice the amount of 5% enriched material which can be used for the Iranian nuclear program, but the disadvantage is that the enhanced radioactivity of the material needs to be managed in the civilian nuclear program.

Isotopic denaturing is the process of artificially changing the natural isotopic composition of a chemical element in order to give it new, desirable properties. Weapons grade U-235 is easily denatured by adding enough U-238 (blending down to 20% enrichment) to the material to increase the neutron count rate so that a ms free of neutrons becomes unlikely. However, this material is not truly denatured because it can be re-enriched to weapons grade. We suggest an alternate approach where reprocessed uranium is blended with UF6 making it highly impractical for further enrichment to weapons grade for constructing a bomb while still allowing the LEU UF6 fabrication into fuel and for reactor use. In fact, the neutron emission rate increases when enriched to weapons grade rather than decreasing. Denaturing uranium with reprocessed uranium was suggested before in different contexts and I draw upon these publications as an option for consideration for Iran. [2] In order to do this calculation we follow the following steps (sorry, this is a bit technical):

  • Derive expression for the masses and concentrations for a 2-step blended material based on the initial feed mass of 7952.9  kg near-5% U-235 mass.
  • Calculate neutron production as a function of the concentration of U-232 due to alpha particle activity from U-232 daughters interacting through the (α,n) reaction on assumed impurities present if the UF6 is converted to metal form (assumed composition of metal as on the Novosibirsk Chemical Concentrate Plant website). We use SRIM 2008 to calculate the α-particle energy loss, TENDL-2013 for the cross-sections of the impurities, and ORNL’s Scale 6 to calculate the quantity of U-232 daughters after an assumed 1 year of decay.
  • Calculate fraction of U-232 in 5% blended UF6 if it is further enriched to weapons grade U-235. The ratio of the fraction of U-232 in 90% enriched material and the 5% enriched material we call γ.
  • Knowing γ and the number of neutrons produced as a function of U-232 concentration, we can derive the required concentration to produce a neutron-free ms making construction of a bomb more difficult. [3]
  • We calculate the dose from a 5 kg sphere of 90% U-235 enriched uranium including the added U-232. We calculate a dose of 178 mrem/hr. It is not self-protecting at this level but it would complicate the manufacturing process into a bomb. We also find that the dose from the near-5% blend is a factor of 22 less. This is more manageable as long as precautions are in place to protect the workers and environment. [4]

A Topic Worth Introducing

The Iranian Government may be hard to convince that a reprocessed uranium blend is acceptable since the material may be perceived as “inferior” to enriched natural uranium, and this may be judged to be discriminatory. However, we inject this into the current discussion on Iran’s nuclear program since this approach may be part of the formula for Iran to “save face” and alleviate tension between Iran and P5+1. Especially because the government desires high quantities of enriched UF6 for their civilian nuclear program. Material that has been denatured in the way we discuss does not eliminate the threat from the material but would make it more difficult for use in a bomb unless a dedicated re-enrichment program is employed to remove the U-232.


[1] See “By the Numbers Page” for the current status of the Iranian program and the concept map at:

[2] P. N. Aleseev et al, The Concept of the Use of Recycled Uranium for Increasing the Degree of Security of Export Deliveries of Fuel for Light-Water Reactors, Physics of Atomic Nuclei, v 73, no 14, pg 2264-2270, 2010. E. F. Kryuchkov et al, Evaluation of Self-Protection of 20% Uranium Denatured with 232U Against Unautorized Reenrichment, Nuclear Science and Engineering, 162, p. 208-213, 2009. See also Chapter 14 by Kryuchkov et al. in Nuclear Power – Deployment, Operation and Sustainability, edited by P. Tsvetkov, 2011.

[3] A. Glaser, On the Proliferation Potential of Uranium Fuel for Research Reactors at Various Enrichment Levels, Science and Global Security, 14, pg 1–24, 2006.

[4] IAEA, Safety of Uranium and Plutonium Mixed Oxide Fuel Fabrication Facilities, Specific Safety Guide, No. SSG-7, 2010.

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