IRAN’S NUCLEAR ACTIVITIES (2)

Dr. Frank Barnaby ^(*)

Uranium enrichment using gas centrifuges

The capacity of a gas centrifuge is measures in separative work units (SWUs). A reasonable estimate is that each centrifuge of the type that Iran is likely to produce (most likely made from carbon fibre) would have a capacity of about 2.5 SWU per year. That this is likely is indicated by the example of Iraq. In 1991, Iraq was a prototype centrifuge with a carbon-fibre rotor spun at up to 60,000 rpm (a wall speed of roughly 450 meters per second). The enrichment capacity during the best test run reached 1.9 SWU per year. IAEA inspectors estimated that an output of 2.7 SWU per year could have eventually been achieved.

A reasonable estimate is that each centrifuge of the type that Iran is likely to produce, the P-1 type, would have a capacity of about 2.5 SWU per year. Iran is experimenting with the P-2 type gas centrifuge (operated by Brazil, Pakistan and India) that may be about twice as efficient, with a capacity of about 5 SWU per year.

An Iranian facility containing, say, 3,000 P-1 centrifuges could produce 7,500 SWU per year or about 40 kilograms of highly enriched uranium per year. It would take this facility at least 5 years to produce enough highly enriched uranium for the nuclear force of six nuclear weapons. With sufficient expertise in HEU-based nuclear weapons 40kg per year could provide 2 nuclear weapons.

Assuming that about 60 per cent of the centrifuges have to be rejected as sub-standard, a reasonable assumption, Iran would need to produce about 5,000 centrifuges for the facility. Moreover, gas centrifuges break down frequently because of the mechanical stresses they are under. A steady supply of replacement machines must, therefore, be produced.

A facility operating a cascade of 3,000 centrifuges would use as much energy, electrical power, as a largish city – approximately 200 kilowatt-hours per SWU or roughly 1,000 kilowatt-hours per gram of highly enriched uranium. It would, therefore, be impossible to operate such a facility clandestinely. Building and operating effectively a gas centrifuge facility of a useful size is not a trivial task – it is an industrial undertaking. It would probably take Iran at least four or five years to build such a facility and begin producing significant amounts of highly enriched uranium.

Iran will need to produce many thousands of gas centrifuges to produce enough highly enriched uranium to make a strategically significant number of nuclear weapons – say 5 or 6 weapons (comparable to the South African arsenal). They are unlikely even to begin producing a significant amount of highly enriched uranium for 5 years or longer. If Iran does produce highly enriched uranium suitable for use in nuclear weapons, it is unlikely to have significant amounts until between about 2012 and 2015 or later. (For use in nuclear weapons, uranium should be enriched to at least 90 per cent in uranium-235; for use as fuel in nuclear-power reactors, uranium should be enriched to about 3.5 per cent in uranium-235.)

Laser isotope separation (LIS)

Uranium can be enriched using a laser method called laser isotope separation (LIS). LIS separates uranium isotopes more efficiently than gas centrifuges because it is based on the fact that each isotope of an element has a unique set of electronic energy states. Consequently, electrons of atoms of each isotope will absorb light of a specific colour (i.e., of a specific energy level). If illuminated by a laser beam containing light of this precise colour, electrons of atoms of the selected isotope will absorb photons and
become excited. An atom may give up its excited electron, and become a positively charged ion. The atoms of the other isotopes will not absorb photons, because they do not have the “right” energy, and will not be ionised. The ionised atoms can be separated from the neutral ones by an electromagnetic field.

The Iranians have experimented with an Atomic Vapor Laser Isotope Separation (ALVIS) system that consists of two main units - a separator and a laser. When used to separate uranium isotopes, natural uranium metal is vaporised in the separator, using an intense electron beam that creates a uranium vapour stream in a vacuum chamber that rapidly moves away from the uranium metal. The vapour contains atoms of U-235 and U-238.

The laser unit uses powerful copper-vapour lasers that emit beams of green-yellow light. This light energizes (excites) ‘dye’ lasers that emit beams of red-orange light of precisely the right colour (i.e., frequency) to photoionise preferentially U-235 atoms. The red-orange beams are passed through the vapour of uranium atoms.

U-235 atoms absorb photons of the red-orange light whereas U-238 atoms do not. The excited U-235 atoms eject the excited electrons, becoming ionised; the U-238 atoms remain untouched. An electromagnetic field moves the positively charged U-235 atoms to a collecting plate where they condense. The enriched U-235 can then be removed. The remaining uranium vapour, containing a much greater proportion of U-238 than natural uranium, flows on through the separator chamber and is removed.

The ALVIS photoionisation process has an atomic selectivity of more than 10,000 - only one ion of U-238 is produced for every 10,000 ions of U-235. This high enrichment efficiency, combined with the fact that relatively little energy is needed to operate the separator and laser systems, makes the operating and capital costs of the ALVIS process relatively low. This makes laser-isotope separation appear more attractive than other enrichment technologies.

Iranian laser enrichment research and development and the manufacture of copper vapour lasers have been undertaken in a laboratory located at Lashkar Ab’ad. A pilot plant for laser enrichment was established at Lashkar Ab’ad in 2000 and, the Iranians claim, dismantled in 2003.

Experiments with plutonium

The Iranian government has acknowledged to the IAEA that it has irradiated uranium dioxide targets with neutrons in the Tehran Research Reactor and subsequently chemically separated the plutonium produced in the targets. According to the Iranians, only a small amount of plutonium was separated.

If the heavy water reactor planned at Arak is used to produce plutonium for use in nuclear weapons, it will be necessary to chemically separate the plutonium from the irradiated reactor fuel elements. The experiments performed by the Iranians in plutonium separation are, therefore, significant.⁽¹⁾

Iran’s Ballistic Missiles

If Iran does develop nuclear weapons it will need a delivery system for them. It is likely to use missiles rather than bombers for this purpose. Iran has acquired ballistic missiles and the technology to produce them from China, North Korea and Russia. However, the missiles now deployed by Iran (the CSS-8, Musak-120, and SCUD-B and SCUD-C missiles) have ranges of less than 600 kilometres and are not suitable for the delivery of nuclear warheads.

Iran reportedly has three types of ballistic missiles under development – the Shahab-3, -4, and -5. The Shahab-3, that has reportedly been tested and deployed (by the Revolutionary Guards), has a range of about 1,300 kilometres. The Shahab-4, apparently under development and based on the Russian SS-4 missile (some say it is based on the North Korean Nodong-2 missile), may have a range of about 2,000 kilometres. The Shahab-5, said to be in early development and perhaps based on the Russian SS-5, may have a range of about 4,000 kilometres. The Shahab-3 and Shahab-5 may have payloads of about 750 kilograms and could deliver nuclear warheads, as could the Sahab-4, with a payload of about 1,000 kilograms. These missiles are, however, inaccurate. The Shahab-3, -4, and –5 missiles reportedly have circular error probabilities of about 190 metres, 50 metres and 190 metres respectively. They are suitable for attacks on large urban areas, like cities, but not ones on military forces. The circular error probability is the radius of the circle centred on a target within which one half of a large number of missiles, fired at the target, will fall.

⁽¹⁾ Removing plutonium from spent reactor fuel elements (known as reprocessing) is straightforward chemistry. The elements are very radioactive and adequate shielding against radiation is required. The purex (an acronym standing for plutonium and uranium recovery by extraction) process is the standard chemical method for reprocessing. Unused uranium, plutonium, and fission products are separated from each other and from the fission products. The spent (irradiated) fuel is first dissolved into concentrated nitric acid. An organic solvent composed of 30% tributyl phosphate (TBP) in odourless kerosene is used to recover the uranium and plutonium; the fission products remain in the aqueous nitric phase. Once separated from the fission products further processing allows the separation of the heavier plutonium from the uranium. The PUREX extraction process uses a liquid-liquid extraction process in which a complex is formed between the tributyl phosphate and the extracted plutonium and uranium.
 

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