Physics: new technology makes building nuclear bombs easier

Norbert Lossau

T he subject of uranium enrichment is a highly political one these days. Ultimately, the question is whether the relevant facilities in Iran only produce uranium for use in nuclear power plants – or whether they also produce highly enriched uranium that is suitable for building nuclear weapons.

Natural uranium consists mainly of uranium-238. The isotope uranium-235 accounts for only a little over 0.7 percent. The fuel rods for a nuclear power plant must contain around five percent uranium-235 in order to maintain the desired chain reaction.

To build an atomic bomb, however, you need about 85 percent uranium-235. For both applications, an increase in the concentration of uranium-235 is therefore required, although the effort required for military applications is significantly greater.

All uranium-235 enrichment processes are based on the tiny difference in mass between the two isotopes of uranium. Uranium-238 is a touch heavier than uranium-235 because there are three more neutrons in its nucleus.

At the beginning of the enrichment process, the heavy metal uranium is given wings: After the reaction with fluorine, uranium hexafluoride (UF6) is produced, which is gaseous. A UF6 molecule is naturally somewhat heavier if it contains a uranium-238 atom.

Then either the centrifuge or diffusion technique is used to separate the two types of UF6 from each other. In one case, it is exploited that the diffusion of the two UF6 molecules through porous membranes occurs at different rates.

With the other method, centrifugal forces provide for the separation. Since centrifuge technology is about ten times more efficient than gaseous diffusion, it is now the method of choice for enriching uranium. As is well known, the enrichment plants in Iran also operate with ultra-fast gas centrifuges.

New technique uses laser radiation

However, largely unnoticed by the public, a new, third technique for enriching uranium has been developed in recent years. The method called "silex," devised by Australian scientists, does not work mechanically but uses radiation from a laser.

Silex stands for "Separation of Isotopes by Laser Excitation". Although most of the details of the Silex process are kept strictly secret for good reason, it is known that the uranium hexafluoride gas is irradiated with pulsed infrared light from a CO2 laser (wavelength 16 micrometers).

This light excites the UF6 molecules with uranium-235 to vibrate. On the other hand, UF6 molecules containing uranium-238 are not affected by. This pre-sorting can be used in subsequent process steps to separate the different UF 6 molecules – probably by selectively ionizing the vibrating molecules and then separating them using electric fields.

Experts estimate that uranium enrichment using the silex technique is likely to be up to 16 times more efficient than the centrifuge process.

Construction of the world’s first commercial plant planned

Global Laser Enrichment, a subsidiary of GE Hitachi Nuclear Energy, plans to build the world’s first commercial silex facility in Wilmington, North Carolina, USA. Approval from the responsible U.S. authorities is still pending, however. A decision is expected later this year.

Laser enrichment is controversial among experts because it could lower the technical hurdle for building nuclear weapons worldwide and encourage the proliferation of nuclear weapons-grade material. This is because the new enrichment technology is less costly than the centrifuge method, and the facilities and buildings needed can therefore be smaller.

According to the operating company, a silex plant takes up about 75 percent less volume than a comparable centrifuge plant. This makes detecting a clandestine enrichment facility using spy satellites an even greater challenge than with centrifuge facilities.

Since laser enrichment also requires much less energy, it is correspondingly more difficult to detect the corresponding waste heat, which can also be used to reveal the existence of a secret facility.

Use of silex technology also drives down costs

What speaks for the commercial use of the flint technique is the lower cost of enriching uranium for fuel rods. U.S. physicist Francis Slakey of Georgetown University in Washington calculates that assuming a 50 percent reduction in the cost of enrichment, the average U.S. electricity customer would save 66 cents a month – assuming the savings were fully passed on to the end consumer.

The professional associations of American and German physicists warn of the dangers that could arise from uncontrolled use of the flint technique.

Professor Wolfgang Sander, President of the German Physical Society, states: "The risks associated with the Silex process must be carefully examined. Nuclear proliferation must be avoided at all costs." Technical progress has brought a new problem to politics.

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