How much 20% enriched uranium does Iran need for their 5 MW thermal Tehran Research Reactor (TRR) and how much have they produced? Is Iran producing more 20% enriched uranium than is needed and can it be used to produce a nuclear weapon?

(Modified 2/25/13 and 2/26/13)

How much 20% enriched uranium does Iran need for their 5 MW-thermal Tehran Research Reactor (TRR) and how much have they produced? Is Iran producing more 20% enriched uranium than is needed and can it be used to produce a nuclear weapon?

(Front image Ahmadinejad viewing TRR fuel element (9)).

Iran’s nuclear program began in 1957 under a US-Iran cooperative agreement. The US provided the Iranian government a $350K grant as part of the Atoms for Peace program and a 5 MW research reactor, the TRR. The research reactor originally had fuel plates with 93% enriched uranium and it began operating in 1967. The US provided Iran education and technical support in the nuclear field up until 1979-1980. In 1980 when I was at the University of New Mexico nuclear engineering school we had a number of Iranian students in the program that had been sent to the US to study nuclear engineering with scholarships provided by the Shah’s government.

In 1987, Argentina’s Applied Research Institute converted the TRR to operate with fuel enriched to 19.75% uranium rather than the 93% enriched uranium it was originally designed for. The 19.75% enriched uranium has roughly 20% of its fuel enriched in the isotope of U-235 and according to the IAEA standard is classified as low enriched uranium (LEU). In comparison, 93% enriched uranium, with 93% of the material enriched in U-235, is weapons-grade material and could be used to make a nuclear weapon.

The TRR is used to produce medical isotopes. Due to questions over Iran’s nuclear program, the government has reduced access to the international market of medical radioisotopes and therefore must produce them domestically to meet their nation’s needs. The research reactor is getting old and they are building the IR-40, or heavy water research reactor (HWRR) as a replacement for the TRR.

One of the primary concerns is that the IR-40 and TRR research reactors could also be used to produce plutonium that would be used for a nuclear weapon. The 5MW TRR is assessed to be able to produce approximately 0.77 kgs of Pu239/yr or a fraction of the 6 kgs (1), defined as a significant quantity, by the IAEA. It would take approximately 8 years to produce to produce 6 kgs of plutonium using the TRR.

The fuel form for the TRR research reactor is different from fuel form for light water reactors and it can be confusing to understand the differences and the fuel loading. A light water reactor uses cylindrical fuel rods which contain UO2 fuel pellets stacked in a column with a large empty space above to hold gaseous fission products released during operation. The TRR uses Material Test Reactor (MTR)-type fuel elements which consists of fuel plates that are rectangular in shape with fuel, U3O8-aluminum, placed in between two sheets of aluminum and pressed together. Nineteen of these plates are placed within a square aluminum casing to create a single fuel element. The control rods, used to control the power within the reactor, are made similarly but they have few plates with uranium and less uranium in each of the plates. Within the reactor core there are 22 fuel elements and 4 control rods.

Fuel Element Overview (2)

  1. 19 fuel plates per fuel element
  2. 14 fuel plates per control fuel element
  3. 22 fuel elements in the core
  4. 4 control fuel elements (or 5 control fuel elements (per a more recent publication (3))
  5. 8 x 7.7x 7.5 cm fuel element dimensions
  6. 0.007 thickness of fuel in the plate
  7. 0.15 thickness of the plate overall

20% enriched uranium

  1. 0.28 kg of U235 per fuel element (not uranium total)
  2. 0.214 kg of U235 per control fuel element (not uranium total)

Therefore calculating out the amount of U-235 and uranium in the core:

  1. 7.23 kgs of U235 in the fuel elements and control elements. (mod’d 2/26/13 from 6.4 to 7.23)
  2. 36.61 kgs of uranium total in the core (at 20% enriched U235) (mod’d 2/26/13 from 32 kgs to 36.61).

The TRR research reactor is designed to be flexible and allow the fuel elements and other materials such as target materials for the production of specific radioisotopes to be placed within the core in different positions therefore there is not a single configuration but rather a flexible arrangement. The core consists of a 9 by 6 rectangular grid with 54 locations that can accommodate the 22 fuel elements and 4 control rods.

In 1990 Argentina provided fuel for Iran’s TRR with 20% enriched uranium. According to the 1990 IAEA INFCIRC Argentina provided 115 kgs of uranium in:

  1. 65 standard fuel elements
  2. 12 control elements and 3 instrumented fuel assemblies (4)

Therefore given each core has 22 fuel elements, the 65 fuel elements provided by Argentina is around 3 full core loadings for the TRR.

How Much 20% Uranium-235 has Iran Produced up to May 25th 2012?

Based upon the May 25th, 2012 IAEA report (5) the total amount of UF6 with 20% enriched uranium produced to date:

  1. 110.1 kgs at the Pilot Fuel Enrichment Plant (PFEP)
  2. 35.5 kgs at the Fordow Fuel Enrichment Plant (FFEP)

That is a total of 145.5 kgs UF6 produced of which 67.5% of the 145.5 kgs of UF6 is uranium and the rest fluoride and impurities.

  1. 98 kgs of uranium produced with 20% enriched uranium out of the 145.5 kgs of UF6.

120 kgs of uranium is the amount of Iranian’s claim they need for fuel production for TRR (6). This is close to the 116 kgs (115.8 kgs) received from Argentina in 1992 in the form of 65 fuel elements, 12 control rods, and 3 instrumented fuel assemblies.

To reach 120 kgs of uranium Iran needs to produce an additional:

  1. 22 kgs uranium or
  2. 32 kgs more of UF6 to meet total of 177.6 kgs of UF6 equal to 120 kgs of enriched uranium.

Conversion from UF6 to U3O8 – the fuel material

At the Fuel Plate Fabrication Facility (FPFF) they used 45.5 kgs of UF6 to produce 14 kgs of U3O8. Converting to uranium –there is about 12 kgs of uranium in 14 kgs of U3O8 – or 85%.

The process to convert from UF6 to U3O8 was~39% efficient with 61% losses that require recovery of the uranium. How they recover the uranium and how quickly is a question that is important in determining how much UF6 is needed. Therefore they may need 60% more UF6 if they don’t have an adequate recovery system but this seems extreme.

  • ~142 kgs of U3O8 required to get the 120 kgs of uranium or an addition 128 kgs of U3O8 needs to be produced.

How Much Enriched Uranium is Needed?

The reactor fuel elements provided by Argentina for approximately 3-full TRR cores lasted around 20 years – from 1992 to 2012. Which implies that each core lasts around 6 to 7 years depending on the power level and the amount of time the reactor is in use.

Production of the fuel plates and the fuel elements is probably being done in a batch job. What this means is it is easier to produce three cores or even four or five all at once and store them rather than to stop the production and then make one core every six or seven years.

Therefore depending upon how you set up your assumptions – you can state that Iran has produced more 20% enriched uranium than is needed for one TRR core loading OR they need to produce additional 20% enriched uranium to produce enough fuel for multiple fuel loadings and store it for use over the next twenty years or so.


In January 2012 Iran announced that they had completed their first domestically produced fuel element for the TRR. Iran began enriching up to 20% U-235 for the production of the TRR fuel elements on February 2, 2010 after the US and other nations refused to provide fuel elements for the TRR reactor. For a good overview of the discussions during this period go to the NTI Iran timeline (7).

To produce a qualified fuel element for the TRR is a difficult task and could take several years of testing. There are issues of material quality, welding, and quality assurance that must be met to produce a viable fuel element. Taking this in to account, it may be possible that the initial fuel elements produced by Iran could only operate for shorter period of time within the range of 1 to 2 years rather than the 6 to 7 years of operation for the fuel provided by Argentina. This would imply that indeed they need significantly more 20% enriched uranium as they develop their fuel fabrication capabilities. Or they may have to operate at lower power levels as they work through the technical issues.


Based upon the negotiations with Iran in June 2012, the reported goals are to establish an agreement with Iran to stop enriching uranium up to the 20% and to send out its stockpile of 20% enriched uranium (8). Given the need to provide fuel elements for the TRR reactor to produce medical isotopes, it will be important to perhaps establish an agreement on the completion of fuel elements with 20% enriched fuel for multiple core reloads and provide the fuel back to Iran as needed over the next twenty years or so.

While it is unclear if Iran would have been willing not to enrich uranium up to 20% U235 if they had been given fuel for the Tehran Research Reactor or not it is clear this is a turning point in the relationship between Iran and the UN over their uranium enrichment program.

Ciao Susan Voss

Follow-on evaluation for Nov. 2012 IAEA report.

(Image above: Fuel plate to fuel element (10).)


  1. Tomanin, A., Peerani, P., and Janssens-Maenhout, G., Pu-breeding feasibility in irradiation channels of research reactors, 4/22/2010, Nuclear Engineering and Design, Vol. 241, Issue 2, 2/2011, pgs. 476-483.
  2. Khalafi, H. and Gharib, M., Calculational tools to conduct experimental optimization in Tehran research reactor, Annals of Nuclear Energy, Vol 26, Issue 17, pgs. 160-161, 11/1999.
  3. Farhadi, K. and Khakshournia, S., Feasibility study for Tehran Research Reactor power upgrading, Annals of Nuclear Energy, 35, (2008), 1177-1184.
  4. IAEA Information Circular, INFCIRC97/Add2, January1990,…hers/infcirc97a2.pdf)
  5. Implementation of the NPT Safeguards Agreement and relevant provisions of Security Council resolutions in the Islamic Republic of Iran, GOV/2012/23, 5/23/12,
  7. NTI Iran timeline,
  8. Rozen, L, IAEA announces new Iran talks, former Iran nuclear negotiator proposes “zero 20% stockpile” plan, The Back Channel,
  9. Ahmadinejad viewing TRR fuel element, 2/16/12,
  10. Batista de Souza, J. and Durazzo, M., Fabrication procedures for manufacturing high uranium concentration dispersion fuel elements, San Paulo,


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  1. Susan Voss says:

    August 30, 2012 IAEA report provides the newest estimates for the quantity of UF6 enriched to 5% and 20% U235:

    1. 6876 kg (+679 kg since the previous report) of UF6 enriched up to 5% U-235 and
    2. 189.4 kg (+43.8 kg since the previous report) of UF6 enriched up to 20% U-235

    Per footnote: GOV/2010/10, para. 8; H.E. Mr Fereydoun Abbasi, Vice President of Iran and Head of the Atomic Energy Organization of
    Iran, reportedly made a statement to the effect that Iran plans to build four to five new reactors in the next few years in order
    to produce radioisotopes and carry out research (‘Iran will not stop producing 20% enriched uranium’, Tehran Times, 12
    April 2011). He was also quoted by the Iranian Student’s News Agency as saying “To provide fuel for these (new) reactors,
    we need to continue with the 20 per cent enrichment of uranium” (‘Iran to build new nuclear research reactors – report’,
    Reuters, 11 April 2011).

    Where does the 20% go?

    Fuel Plate Fabrication Plant: As previously reported. Iran has combined into one facility the activities involving the conversion of UF6 enriched up to 20% U-235 into U3O8 and the manufacture of fuel assemblies made of fuel plates containing U3O8. Between the start of conversion activities on 17 December 2011 and 12 August 2012, Iran has fed into the process 71.25 kg of UF6 enriched up to 20% U-235 and produced 31.1 kg of uranium enriched up to 20% U-235 in the form of U3O8.

    With the increased amount of 20% enriched UF6 and increased uranium enrichment capability there will be concerns over the possibility of Iran’s “breakaway” capability – ie, the ability to quickly enrich uranium from 20% to 90% weapons grade.


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