03.09.2008

Iran: Nuclear Q&A

This set of questions and answers has been compiled for Hands Off the People of Iran with the help of nuclear and process engineers, including a former International Atomic Energy Agency inspector

What is the relationship between the enrichment of uranium and nuclear military capability? What is the level of highly enriched uranium (HEU) used in nuclear bombs?

Mined uranium ore is purified and reconstituted into a solid form known as yellowcake, which is converted into a gas by heating. Gas is fed through centrifuges, where its isotopes separate, and the process is repeated until uranium is enriched. Low-level enriched uranium (LEU) is used for nuclear fuel. HEU can be used in nuclear weapons.

Enriched uranium can be used for commercial reactors (as can plutonium). But countries that have signed the non-proliferation treaty are not supposed to enrich uranium without mandatory International Atomic Energy Agency (IAEA) inspections because of its possible ‘dual use’. To make a bomb that will be carried in a missile 25kg of highly enriched uranium or 8kg of plutonium is needed.

Natural uranium contains less than 0.1% of fissile material. This fraction needs to be increased to 20-90% to make a weapon. This ‘enrichment’ is what the centrifuges do. The other route to a weapon is to produce or buy plutonium.

What is the current level of uranium enrichment in Iran’s nuclear plants?

The last IAEA report of May 2008 estimates that Iran’s current level of enrichment is near 4%, as opposed to the 40%-90% necessary for achieving nuclear weapons capability. The full IAEA report can be seen at www.iaea.org/Publications/Documents/Board/2008/gov2008-15.pdf.

There is confusion about the number of Iran’s centrifuges, which play an essential role in enriching uranium. The IAEA May report specifies 3,000, while IAEA inspectors making regular visits to Iran confirm the presence of 4,000 and in late July Iran itself claimed to have 6,000. However, both US intelligence organisations and IAEA inspectors consider this to be an exaggerated number, used mainly for propaganda. Even the hard-line Israeli, Shaul Mofaz, who is in favour of bombing Iran’s nuclear installation, does not claim that Iran will reach uranium production at military levels before 2010.

Is uranium enrichment in industrial nuclear reactors the cheapest way to achieve nuclear weapons capability?

No, it is cheaper and faster to buy plutonium on the black market.

An example of a uranium bomb deployed in war was the one dropped over Hiroshima. The bomb consisted of 60kg of U-235. However, 99% of all mined uranium has an atomic weight of 238 and only 1% is 235. There are many separation processes available to enrich uranium - or extract 235 from its depleted 238 counterpart. They include gas diffusion, centrifugal diffusion and mass separation via a magnetic field.

Unlike its plutonium counterpart, uranium 235 and 238 react similarly, thus making this enrichment process much more energy-intensive. Both gas diffusion processes involve superheating UF6 until it vaporises. Since U-235 is slightly smaller and lighter than its counterpart, it can diffuse faster through membranes. Thus, forcing this gas via either back pressure or a centrifuge through multiple layers of semi-permeable membranes slowly filters out the UF6 formed with U-235. This is then condensed and reacted to extract the U-235.

The mass separation method relies on ejecting a stream of uranium atoms through a very strong magnetic field. The mass differences causes the U-235, the lighter atoms, to deflect into a collecting tray. Though slow and also more energy-intensive than gas diffusion, this process produces the least nuclear waste.

Depending on how thoroughly these processes are carried out, the product carries different names. Highly enriched uranium is a product containing over 20% uranium235. This is sufficient for a weapon, but efficiency and portability typically require bombs to have weapons-grade plutonium with 90% enriched uranium. Anything lower can be considered reactor-grade.

Because reactor-grade uranium has such a low concentration of U-235 and the final spent rod has even less, extracting enriched uranium from spent fuel rods is not considered a feasible method of creating weapons-grade uranium.

In what way is plutonium used to produce nuclear weapons?

The destructive power of the plutonium bomb in war was first demonstrated on Nagasaki. The bomb consisted of 10kg of Pu-239 at its core.

Unlike uranium, many combinations of plutonium isotopes can be used to make a nuclear weapon. However, plutonium Pu-238 and Pu-239 are the most effective. Plutonium is typically harvested from a reactor running off of uranium fuel rods. When U-235 decays and releases neutrons in a reactor, stray neutrons fuse with U-238 making the final U-239 product decay to produce Pu-239.

The maximum Pu-239 production occurs before the fuel rod is entirely spent, meaning plutonium enrichment does not also have the by-product of creating usable energy. Weapons-grade plutonium is said to contain greater than 93% Pu-239 (80-93% Pu-239 is considered fuel-grade and below is considered reactor-grade). However, since any plutonium can be used to create a bomb, no matter how unstable, plutonium is considered the material most used in the proliferation of nuclear weapons. Its production as a by-product of uranium reactors means that harvesting it requires much less energy than creating enriched uranium.

So far no-one is claiming that Iran is producing plutonium Pu-238/Pu-239. However, clearly one can buy this type of plutonium on the black market and surely that would be a faster, cheaper and more secret way of obtaining nuclear weapons capability than enriching uranium in plants inspected regularly by the IAEA.

In the last century Israel, sharing its nuclear know-how with the apartheid regime in South Africa, managed to develop nuclear military capability using both enriched uranium and plutonium. However, Israel chose not to join the IAEA or sign the non-proliferation treaty, and its entire nuclear programme was developed without inspections (or sanctions). Israel was illegally supported by its allies in the existing nuclear club, the USA and Britain.

In addition to the enrichment of uranium and plutonium, what are the prerequisites of becoming a nuclear military power?

It is true that plutonium production and uranium enrichment present a serious proliferation issue. However, they are only one of several hurdles a country has to overcome before it becomes nuclear-weapons-capable.

The next hurdle is the electronic trigger, whose split-second timing is essential for unleashing the chain reaction necessary for a military nuclear device. Another is what is sometimes called weaponisation - this refers to putting the device into a missile or bomb that can be delivered to a target. Currently Iran is in no position to deliver a missile or a bomb by a land-to-land missile warplane equipped with a warhead and in fact no-one claims that it is.

For that reason claims that somehow Israel or Europe are threatened by an Iranian nuclear bomb are fictitious. Once it achieved uranium enrichment, Iran could deliver without weaponisation a large-sized nuclear bomb (a barge of tens of square metres) on land or in the Gulf. But such a large nuclear device would presumably be detected by US satellites and destroyed before reaching its destination.