Recent statements by U.S. government officials have provided new estimates of when Iran could be expected to have the ability to manufacture a nuclear weapon. Specifically they have indicated that it would take Iran a year or more to be able to produce enough highly enriched uranium (HEU) for one nuclear weapon. Further that even with this material it would take Iran three to five years to turn this material into a nuclear explosive device.
Both of these estimates seem to be very optimistic from a U.S. point of view. Based on what the IAEA has been reporting on Iran’s centrifuge enrichment effort, Iran could have enough low enriched uranium to process into one weapon’s worth of HEU by the end of July and could then produce this HEU by mid-November—meaning that Iran could produce enough HEU for one nuclear weapon (20 kilograms) in only seven months. If Iran continues to improve its enrichment capability, the time required would be even shorter. To produce a nuclear explosive device would take no more than one year based on historical evidence from the U.S. nuclear weapons program. Since these two efforts (producing the HEU and producing the explosive device) could be carried out concurrently, Iran is at most one year away from producing a nuclear explosive device should it desire to do so.
The key statements were made by General James Cartwright, Vice Chairman of the Joint Chiefs of Staff, Lieutenant General Ronald Burgess, Director of the Defense Intelligence Agency and Michele Flournoy, Undersecretary of State for Political Affairs during congressional hearings on April 14, 2010 though similar statements attributed to “U.S. officials” were reported the day before in a Reuters article.
Both Cartwright and Burgess stated that it would take Iran one year to produce one nuclear weapon’s worth of HEU once it decided to do so. An even longer estimate was reported in the Reuters article: “the current view of the intelligence community” is that such a capability would occur in “the next few years”. Sources in the Reuters article cited “technical problems” at Iran’s enrichment plant at Natanz. Undersecretary Flournoy also stated that regarding their enrichment program Iran has “been having some technical problems”.
These assessments are hard to square with the accelerating progress that the IAEA has been reporting regarding Iran’s centrifuge enrichment program. I have analyzed this issue in detail in prior writings.
Iran is using its enrichment facility at Natanz to build up a stockpile of 3.5% enriched uranium. I have calculated that once this stockpile reaches 1,900 kilograms, Iran will have the ability to produce one weapons worth of HEU by batch recycling the 3.5% enriched uranium through the Natanz enrichment plant. As of January 29, 2010, Iran had produced about 1,400 kilograms of 3.5% enriched uranium. Throughout most of 2009, Iran was producing a steady 57 kilograms of 3.5% enriched uranium per month. However, during the latest period for which there is information (November 23, 2009 to January 29, 2010), the Iranian production rate increased by 37% to 78 kilograms of 3.5% enriched uranium per month.
If Iran has continued to produce at the same rate, then by mid-April Iran would have a stockpile of about 1,600 kilograms of 3.5% enriched uranium and it will take Iran only about 3.5 months (roughly the end of July) to produce the final 300 kilograms required to reach the 1,900 kilogram goal. This material could then be batch recycled at the enrichment facility at Natanz to produce one weapons worth of HEU. This process could be accomplished in about 3.5 months, meaning that Iran could have the HEU by mid-November of this year. These estimates assume no further increase in Iran’s enrichment capability, even though further increases seem to be inevitable i.e. Iran could likely achieve this goal even sooner than mid-November. Given the straightforward nature of these calculations, it is hard to see why U.S. government officials believe that it will take Iran one or more years to achieve this goal. Further given the 37% increase in Iran’s production of 3.5% enriched uranium, it is puzzling that these same government officials are reporting “technical problems” with Iran’s enrichment effort.
In the congressional hearings General Cartwright went on to say that even if Iran were to already have the HEU necessary to produce a nuclear weapon that it would take Iran “three to five years” to be able to produce a nuclear explosive device. Cartwright called this “an historical estimate” based on other countries’ experiences though he did not specify which countries he had in mind. Again however, this seems to be a very optimistic estimate from a U.S. point of view.
Looking at the histories of the five established nuclear powers (United States, Russia, United Kingdom, France and China), none of these countries first produced the fissile material (HEU or plutonium) for the weapon and only then developed the means of detonating it. Rather the production of the fissile material and the development of the explosive device were done in parallel. For all of these programs the production of the fissile material was the hardest, costliest and most time consuming part of the endeavor.
The development of the explosive device itself was paced so as to be ready whenever the fissile material was. For all of these programs, once sufficient fissile material had been produced, it usually took only a few months until the finished weapon was produced. A nuclear test occurred soon thereafter.
However, there was an incident in the U.S. nuclear weapon program which does allow one to estimate how long it would take to produce a nuclear explosive device assuming that the production of the fissile material is not an issue. Specifically the discovery of Pu-240 led to a crisis in the U.S. effort to develop plutonium based nuclear weapons. (For more details of these events see the Appendix). However, in only 11 months (from August 1944 to July 1945) the U.S. was able to develop and test an implosion nuclear device which used plutonium as its fissile material. This is far less than the three to five years that Cartwright has estimated.
One could argue about how applicable the U.S. experience is to the current situation of Iran but in general it seems that Iran is in a better position today to quickly produce a nuclear explosive device than the U.S. was in 1944. On the one hand, the U.S. did probably have more resources and better people than Iran does today. But on the other hand in 1944 no one had ever produced an implosion device and how to even do so was unclear. Today, however, general descriptions of such devices are widely available. Further Iran won’t have to start from scratch. The 2007 NIE clearly indicated that at least until the fall of 2003, Iran was doing some work on designing and producing a nuclear explosive device.
Some Europeans as well as the IAEA are concerned that some aspects of a weaponization effort are still continuing.
And it is unknown how much aid on weapons development that Iran may have received from Pakistan and/or North Korea. On the whole an estimate of one year or less seems far more plausible than General Cartwright’s three to five years.
Difficult choices need to be made if Iran is to be prevented from acquiring a nuclear weapon capability. Sound decisions cannot be made if they are based on very rosy estimates of how long it will take Iran to achieve key milestones on the path to this goal. The fact is that Iran could have a nuclear weapon in at most one year if it chooses to do so. Unless urgent action is taken to arrest Iran’s nuclear weapon development program, the West will need to start planning for Iran’s new status as a defacto nuclear weapons state.
The Pu-240 Crisis and the U.S. Development of Implosion Nuclear Weapons
As was discussed in the text, the Pu-240 crisis which occurred during the U.S. development of nuclear weapons during World War II provides a key insight into how long it might take Iran to produce a nuclear weapon. This appendix will provide details of this incident.
In December 1942, President Roosevelt approved a large scale effort to produce nuclear weapons. There were two way to produce the fissile material needed for such weapons. One required the production of HEU by enriching the U-235 content of uranium from its natural concentration of 0.7% to 80% or more. The other involved the production of plutonium (mainly Pu-239) by irradiating natural uranium in reactors and then chemically separating the resultant plutonium from the spent fuel. Since the U.S. development of nuclear weapons, all other countries aspiring to acquire nuclear weapons have initially focused their efforts on the production of only one or the other of these two fissile materials but the U.S., having abundant resources and facing the large uncertainties of being the first country to develop such weapons, pursued the production of both fissile materials simultaneously.
The explosion of a nuclear weapon requires the generation of a supercritical mass of fissile material. There were two ways to produce this supercritical mass. One is the gun method where one subcritical mass of fissile material is fired as an artillery projectile into another subcritical mass of fissile material, producing the necessary supercritical mass and nuclear explosion. The other is the implosion method where a subcritical mass of fissile material is surrounded by high explosives. These explosives are detonated simultaneously compressing the fissile material. The reduced surface area of the compressed fissile material causes it to become supercritical. From the beginning it was recognized that of the two methods, implosion was the superior one as it would permit more efficient use of fissile material in nuclear weapons. However, in 1943 no one knew how to make this method work and it was decided to focus the main effort of research on the gun method which involved the utilization of well-developed conventional artillery technology.
There was one problem with the gun method. It produces a supercritical mass relatively slowly compared to the implosion method. If a stray neutron were to start a chain reaction too early, the weapon would predetonate and produce less (perhaps far less) than its design yield. The main source of neutrons was expected to be the result of the reaction of alpha particles (produced by the decay of U-235 or Pu-239) with certain light element impurities in the fissile material. For U-235 this was not seen to be much of a problem. With a 700 million year half-life, it produces alpha particles at a relatively low rate—resulting in a similarly low rate of neutron production. Indeed a U-235 nuclear weapon using the gun method was successfully developed and used against the city of Hiroshima without any prior nuclear test of this design.
For the plutonium gun weapon this problem was more serious. Pu-239 has a 24,000 year half-life and produces alpha particles at a 30,000 times higher rate than does U-235. To deal with this problem it was planned to build a special high velocity gun and at the same time to rigorously purify the plutonium so as to greatly reduce the amount of light element impurities. It was hoped that these two measures would be enough to make a plutonium gun nuclear weapon feasible.
In 1943 very little plutonium actually had been produced. Indeed plans to build large plutonium processing facilities were based on less than one milligram of plutonium that had been produced in the Berkeley cyclotron. This plutonium was almost pure Pu-239.
In the spring of 1944 gram quantities of plutonium became available from the experimental X-10 reactor. Tests on this material showed that reactor produced plutonium (using nuclear reactors was the only way to produce large amounts of plutonium) would inevitably contain several percent of Pu-240. Further tests showed that this Pu-240 would produce large numbers of neutrons through spontaneous fission.
The number of neutrons so produced would greatly exceed the number produced by alpha particle reactions with light element impurities. As a result in July 1944 it was necessary to abandon the development of the plutonium gun weapon.
What was to be done? It seemed fairly certain that eventually a U-235 gun weapon would become available. However, enrichment was proceeding slowly and it was thought that at least in the time frame of World War II, few such weapons could be produced. In addition a number of large facilities to produce and process plutonium at Hanford in Washington State were under construction. These included three large plutonium production reactors—the B, D and F reactors. The B reactor was nearly complete. Were these facilities just to be scrapped?
The only alternative was to make implosion work. In August 1944 Los Alamos was reorganized. Two new divisions (the Weapons Physics Division and the Explosives Division) were created to tackle the implosion problem. As is now known, their efforts were quite successful. On July 16, 1945 a plutonium weapon using the implosion method was tested at Alamogordo, New Mexico. On August 9, 1945 a second such weapon was used against the city of Nagasaki.
As was discussed in the text, these events clearly indicate that the development of a nuclear explosive device can take place in a year or less. Estimates of “three to five years” seem unrealistic and very optimistic from the U.S.’s point-of-view.
Hearing of the Senate Armed Services Committee; Subject: U.S. Policy towards the Islamic Republic of Iran, April 14, 2010 and Reuters, “US Officials see Iran nuclear bomb probable in 3-5 years”, April 13, 2010.
During this time, the number of centrifuges that Iran was using to produce enriched uranium dropped 23% from 4,920 to 3,772. However, during this same time, the enrichment capacity per centrifuge increased by 75% from 0.5 SWU per centrifuge-year to 0.87 SWU per centrifuge-year which explains why Iran’s production of enriched uranium increased 37%.
Albert Wohlstetter et. al., Sword from Plowshares
, University of Chicago Press, 1979.
National Intelligence Estimate, Iran: Nuclear Intentions and Capabilities
, National Intelligence Council, November, 2007.
“Altogether, this raises concerns about the possible existence in Iran of past or current undisclosed activities related to the development of a nuclear payload for a missile.” Implementation of the NPT Safeguards Agreement and relevant provisions of Security Council resolutions 1737 (2006), 1747 (2007), 1803 (2008) and 1835 (2008) in the Islamic Republic of Iran
, GOV/2010/10, February 18, 2010, p.9.
This account is drawn from: Richard G. Hewlett and Oscar E. Anderson, Jr., The New World: A History of the United States Atomic Energy Commission
, Volume I, 1939/1946, WASH 1214, U.S. Atomic Energy Commission, 1972 and David Hawkins, Manhattan District History, Project Y, The Los Alamos Project
, Volume I, Los Alamos Scientific Laboratory, LAMS-2532, 1946.
Pu-239 also undergoes spontaneous fission but the spontaneous fission rate of Pu-240 is 40,000 times higher.