Republicans like Rick Santorum and the "law professor" John Yoo are talking of bombing Iran, with the proffered excuse that Iran is developing an atomic bomb. Just assuming that bombing were justified in such a case, we need to evaluate just where Iran might be along the course of producing a nuclear weapon. As citizens, it's our duty to make this evaluation for ourselves.
Highly-enriched uranium (HEU). 20 kgs of this are
needed for an atomic bomb.
But to do this, we must understand how an atomic weapon is constructed. Unfortunately there is too much a tendency to wrap this question with Gordian knot of technical details which bar understanding. I will attempt to cut through those details.
Composition of matter
All matter in the universe is composed of three particles which are formed into atoms. At the nucleus of an atom are found protons, and in some cases, neutrons. Around the nucleus orbit the electrons. The simplest naturally occurring element is hydrogen, which is composed of a single proton orbited by a single electron.
Now, as protons and neutrons have mass (as opposed to an electron which has effectively none), as the number of protons and neutrons increases, the mass, or atomic weight, of the element increases, with uranium being (with one exception) the heaviest of the naturally occurring elements.
All elements have certain chemical properties, which are the ways that an element will behave in the present of other elements. Hence, oxygen is highly reactive with other elements, whereas argon is not. The chemical properties of an element are controlled by the number of protons and electrons of the element; the number of neutrons does not affect chemical properties.
Isotopes
As it turns out, many elements will have irregular numbers of neutrons in the nucleus. Such irregular atomic nuclei are known as isotopes. Hydrogen for example, has two isotopes, one, known as deuterium, has a proton and a neutron in the nucleus, and the other, known as tritium, has a nucleus composed of a proton and two neutrons. These are called "heavy" hydrogen, and water (H2O) composed of heavy hydrogen is called "heavy" water.
Uranium has several isotopes, all of which have 92 protons. The neutrons however range in number from 141 to 146. The most common form, known as uranium 238 (U-238) has 146 neutrons (92+146=238), which constitutes 99.2742% of naturally occurring uranium. The next most common isotope is uranium 235 (U-235), with 143 neutrons (92+143=235), constituting only 0.7204% of the natural element.
Nuclear fission
One important concept here is that matter can never be destroyed but it can be converted into energy. A bomb based on nuclear fission operates on this principle.
While U-238 and U-235 have the same chemical properties, they have an important difference. The U-235 nucleus, if struck by a neutron, can be split. This produces, very briefly Uranium 236 (92 protons + 144 neutrons = 236) which then splits into Krypton 92 (92 neutrons), Barium 141 (141 neutrons) and three free neutrons. These free neutrons can then go on to split other U 235 atoms. If this process is sustained, it is called a nuclear chain reaction.
The power of an atomic weapon comes from release of the very strong binding force of the U-235 nucleus by the impact of a free neutron. On the atomic level this causes a very small amount of matter to be converted into energy. How much energy is produced is calculated by Einstein's famous equation, e=mc2. The power released by splitting even a small amount of fissile material such as U-235 becomes enormous, as c represents the speed of light.
Design of the nuclear fission weapon
The principles I've set out briefly above were discovered over the course of about 50 years running from about 1890 to 1940. By March 1940, the basic design of the atomic bomb was set out in the Firsch-Peierls memorandum (.PDF), (the "FP memo") written at Birmingham University in the United Kingdom.
The FP memo is mandatory reading for an understanding of atomic weaponry, as it explains the basic principles associated with construction of the weapon, as well as the problems, that, in March 1940, remained to be solved to produce a functional atomic weapon based on the use of U-235.
The FP memo identified a means of separation of U-235 from U-238 (mass use of centrifugal tubes containing uranium hexafluoride gas), the size and recommended shape of the U-235 material (spherical, from 1 to 5 kg, divided in half and driven together by high explosive to detonate), and the likely blast and radioactive effects of instituting a chain reaction in the fissile material.
The FP memo was written 71 years ago, even before Hitler had conquered France, and when it was not known how far along the Germans might be towards developing an atomic weapon. From the FP memo:
Any estimates of the effects of this radiation on humans must be rather uncertain because it is difficult to tell what will happen to the radioactive material after the explosion. Most of it will probably be blown into the air and carried away by the wind. This cloud of radioactive material will kill everybody within a strip estimated to be several miles long. If it rained the danger would be even worse because active material would be carried down to the ground and stick to it, and persons entering the contaminated area would be subjected to dangerous radiation even after days. If one percent of the active material sticks to the debris in the vicinity of the explosion and if the debris is spread over an area of, say, a square mile, any person entering this area would be in serious danger, even several days after the explosion.
The Iranian nuclear industry
All that I've said above is well known worldwide. There are numerous details and changes that have been produced by the huge production of nuclear armament during the Cold War. But the essentials remain the same. To construct a nuclear weapon, one needs a supply of fissile material, as well as a sufficiently technical and educationally sufficient pool of labor to construct and detonate the weapon.
Obtaining the fissile material
Furnace used in Brazil, 2010, to convert yellow cake
into uranium hexafluoride, a necessary prior step
before use of centrifuge enrichment. Wiki Commons /
Vladimir Platonow
All fissile material, whether it is U-235 or Pu-239, ultimately comes from naturally occurring uranium ores. Iran's nuclear weapon's program appears to be primarily based on that of Pakistan, that is, it focus on U-235 production, rather than of Plutonium-239, although there seem to have been some experiments in plutonium production as of 2006. (
source) (.PDF).
As proposed in the FP memo, and later developed in the Soviet Union and other nuclear powers, centrifuges are used to separate out the slightly lighter U-235 from U-238 in the gaseous compound uranium hexafluoride. The lighter U-235 from the inner portion of the centrifuge is bled off. Gradually with each round of separation, a higher and higher proportion of U-235 can be achieved. Centrifuges so linked are called "cascading".
Generally it is necessary to obtain a percentage of about 80% U-235, that is, over 100 times the natural concentration, to obtain a sufficiently fissile material to construct a bomb. (There have been a number of developments which have reduced this percentage, sometimes substantially, but this may be used as rough idea of the purity necessary to construct a crude weapon of the Hiroshima type.)
Gas centrifuge
Gas centrifuge battery, Piketon, Ohio, 1984. Used for
processing enriched uranium for atomic weapons
The name of the game then is to attempt to determine whether Iran has been using its existing civil nuclear program to produce 80% enriched uranium. The FP memo had thought that gas centrifuges could be used to separate sufficient U-235 to construct a weapon, but the Manhattan Project found that centrifuge production could not generate enough U-235 within the likely timeframe of the war, and therefore choose other methods, including something called "gaseous diffusion", to enrich uranium.
Modern U-235 production has shifted over to the use of gas centrifuges, again as envisioned in the FP memo. The critical point is to assemble a battery of centrifuges. One gets the image of a spinning top or a potter's wheel when speaking of a centrifuge, but the ones used for enrichment of uranium are very different looking. They are vertically mounted cylindrical devices which run continuously off a stream of uranium hexafluoride gas (called "hex" in the nuclear industry). At the end of the process, what emerges are two things: enriched U-235 hex (which is then refined back into U-235 metal), and a much greater amount of material known as "depleted" hex.
Monitoring Iran's nuclear industry
The construction of the non-nuclear components of an atomic bomb, such as the high explosive trigger, while not simple, are within the capacity of a country such as Iran to construct, particularly if, as it appears, Iran has obtained the assistance of foreign experts in designing the triggers. (The actual delivery system may be more difficult -- more on that later.)
Hence the only real barrier to construction of an atomic bomb is the acquisition of sufficient fissile material. For a U-235-based atomic weapon is the assembly of the requisite numbers of cascading gas centrifuges. The solution to this problem by A.Q. Khan is what permitted Pakistan to construct its atomic weapons.
Iran's nuclear industry is monitored by the International Atomic Energy Agency (IAEA), and it has prepared a series of reports which constitute the principal public source of information for the Iranian nuclear program. The most recent report, from November 2011, can be found here (.PDF).
The IAEA's November report contains an Annex, which is longer than the report itself entitled "Possible Military Dimensions to Iran’s Nuclear Programme". This is probably the best short statement of the evidence regarding Iranian plans for a nuclear weapon. Certainly from the Annex one could readily conclude that Iran was actively engaged in the development of a nuclear weapon.
Much of the evidence that the IAEA relies upon was "provided by a Member State" which i take to mean the United States. I suppose it is possible that some "Bomb Iran" faction in the U.S. government is feeding the IAEA false information, but it seems like IAEA haa actively looked for corroboration of such information, and hasn't relied solely on information "from a member State".
The summary of the findings on the Iranian nuclear weapons development from the November IAEA report:
43. The information indicates that Iran has carried out the following activities that are relevant to the development of a nuclear explosive device:
• Efforts, some successful, to procure nuclear related and dual use equipment and materials by military related individuals and entities (Annex, Sections C.1 and C.2);
• Efforts to develop undeclared pathways for the production of nuclear material (Annex, Section C.3);
• The acquisition of nuclear weapons development information and documentation from a clandestine nuclear supply network (Annex, Section C.4); and
• Work on the development of an indigenous design of a nuclear weapon including the testing of components (Annex, Sections C.5–C.12).
44. While some of the activities identified in the Annex have civilian as well as military applications, others are specific to nuclear weapons.
How close is Iran to a nuclear weapon?
According to a paper by Gregory S. Jones (.pdf) dated 12/6/11, it takes about 20 kilograms of Highly Enhance Uranium ("HEU") to make a bomb. (Jones defines HEU as 90% U-235). Right now, Iran has about 2,810 kilograms of 3.5% enriched uranium and, as mentioned, 53.9 kilograms of 19.7%.
Jones believes that the simplest way for Iran to generate 20 kg of HEU would be simply to recycle its existing stock of 3.5% and 19.7% uranium through the centrifuge cascades. He calculates that it would take Iran 60 days to acquire the 20 kg required for a bomb from its existing stock of enhanced uranium, as follows:
In the first cycle, which would run for 47 days, 1,230 kg of the 3.5% enhanced material would be enhanced up to 104.3 kg of 19.7% enhanced material, giving Iran a total of 158.2 kg on hand of 19.7% enhanced uranium. Of this amount, 153.2 kg would be fed through in the second cycle, which would last only 13 days, and would result in 20 kg of 90% enhanced uranium.
In other words, Iran could be 60 days away from having sufficient fissile material to construct an atomic bomb. But this number depends on an assumption of efficiency in Iran's cascade centrifuge processing which may be unwarranted. Changing around some assumptions, Jones places Iran at 1.75 years away from 20kg of fissile material. To produce any of this material, Iran would either have to kick out the IAEA inspectors or find a way to do it behind their backs.
Iran's delivery systems -- are they capable?
Even assuming Iran could construct an atomic bomb, the question arises: could Iran deliver the weapon onto an enemy target? Now, the geo-political value of the atomic bomb is not so much that it
is used, but rather whether it can be employed as a credible effect, that is, it must be shown that it
can be used. This was at least part of the reason behind the approximately
two thousand nuclear weapons tests conducted over the years, the vast majority having been done by the United States and the Soviet Union.
In theory, Iran could deliver an atomic bomb by aircraft, as was done at Hiroshima and Nagasaki, with the likely targets being Israel, or, less likely, Saudi Arabia. This would require Iran to achieve air superiority over the target, something unlikely to happen given the current levels of armament in the region, as well as the constant presence of U.S. carrier-based naval aviation. It is known that in 2002 and 2003 Iran was investigating use of a medium range ballistic missile, called the Shahab ("Meteor") 3, but it doesn't appear that anything yet has come of this. (One can read a somewhat dated (10/14/08) but comprehensive report of Iranian missile program here.)
It has been speculated (for example here) that Iran could simple turn over a completed weapon to a terrorist group, with the delivery mechanism to be a truck or automobile mega-suicide bomb. Of course the same charge could be made against Pakistan, and perhaps with more factual basis. However, as this is the scariest scenario, I would expect it to be amped up the max should the Republicans win the 2012 election.
Conclusion.
I would say there is clear evidence that Iran has explored the possibilities of producing a nuclear weapon, and some of this exploration has been extensive. The actual construction of a workable and deliverable weapon really cannot be done, at least at this time, by the Iranians, unless they import extensive expertise from abroad. Production of sufficient fissile material even for a single weapon, while currently within the capability of Iran, would likely be something detectable well in advance. The case for war has not been met on this record.