Laser Enrichment Process

What is Laser Enrichment Process?

In one of our science modules we have studied about the Uranium Enrichment Process by Centrifuges. The fundamental principle behind the Uranium Enrichment by centrifuges is that U-235 weighs slightly less than U-238. This difference in weight is used to separate them. First Uranium is made to react with hydrofluoric acid (HF), an extremely powerful acid and the result is Uranium Hexafluoride (UF6) or Hex. In Hex, we have the Uranium in Gaseous form. This gas is put in the centrifuge and is made to spin at a huge speed such as 1 lakh rpm. The centrifuge creates a very powerful centrifugal force and, as we read, that U-238 atoms are slightly heavier than the U-235 atoms, they tend to move out toward the walls of the centrifuge.

But the process is not as easy as it has been described here. There is a huge complex system of thousands of centrifuges is required for the process, making it costly, complex and not every country / company’s cup of tea.

The scientists have long sought easier ways to make enriched uranium. One of these ways is the Laser Enrichment Process. Recently, the General Electric was in news for successfully testing the laser enrichment for two years and is now seeking Government permission in United States to build a $1 billion plant that would make reactor fuel in bigger quantity.

This is a good news for the nuclear industry but the critics are saying that once the secrets of the new technique go out, the scoundrel states and terrorists would make bomb fuel in much smaller plants that are difficult to detect. We need to discuss the process in little detail:

What is Laser Enrichment?

Laser Enrichment or Laser isotope separation is a technology of isotope separation using selective ionization of atoms or molecules by the means of precisely tuned lasers. Accordingly, the processes are called Atomic vapor laser isotope separation (AVLIS) and Molecular laser isotope separation (MLIS) for atomic and molecular techniques respectively.

Atomic Vapour Laser Isotope Separation (AVLIS)

In 1905, Albert Einstein had laid the theoretical foundation for laser isotope separation (though laser was not invented then) and launched the quantum revolution in physics with his Nobel-Prize-winning insight that the energy of a ‘quanta of light’ above a certain threshold has the ability to eject a single electron from matter.

The Laser isotope separation technique is based fundamentally on this light and matter interaction. We all know that the Light is made up of the Photons and each photon consists of energy and travels in waves. The length of the waves depends upon their energy levels. The Human beings are able to see the wavelengths between the 380nm and 750nm as different colours of light.

The atoms are constituent units of the matter. Atoms as we know are made up of protons, electrons and neutrons. A particular type of isotope is a unique set of Protons and Neutrons in the nucleus of the Atom. An atom can absorb a photon only if it is of a precise energy level that will move that atom to one of these possible higher energy states. Now, the basic difference between the U235 and U238 is that U235 has three fewer neutrons than a U238 atom. The Proton-Neutron-Electron interaction makes it possible that the U235 atom will absorb a photon with different energy than the U238.

The invention of laser technology made it possible that a beam of photons with precise colour/ energy can be produced. This beam will be selectively absorbed by the U235 atoms or U238 atoms but not BOTH of them.

  • If the beam is precise enough that the photons are absorbed by the U235 atoms, then its electrons move in higher and higher energy levels.
  • Finally, the atom loses an electron, when it absorbs the Ionization Potential (amount of energy atom must absorb to release an electron) and thus becomes positively ionized. Now, the mixture has net positively charges U235 atoms and Non-charged U238 atoms.
  • When they are passed to an electromagnetic field, the ionized U235 atoms deflected onto an electro-negatively charged metallic collector plate.
  • U238 pass through the field unaffected. This is how it works and is also known as Photoionization approach.

Molecular Laser Isotope Separation (MLIS)

In the MLIS process, U235 Hexafluoride (Hex) Gaseous molecules selectively absorb Photons are made to absorb Photons of a particular level till the time that energy absorbed is enough to dissociate one of the Fluorine atom, and the UF6 turns into UF5. This UF5 is precipitated and collected.

To do it, UF6 gas is mixed with a suitable carrier gas (a noble gas including some hydrogen) which allows the molecules to remain in the gaseous phase after being cooled by expansion through a supersonic Laval nozzle.

A scavenger gas such as methane is also included in the mixture to bind with the fluorine atoms after they are dissociated from the UF6 and inhibit their recombination with the enriched UF5 product.

In the first stage the expanded and cooled stream of UF6 is irradiated with an infrared laser operating at the wavelength of 16 µm. The mix is then irradiated with another laser, either infrared or ultraviolet, whose photons are selectively absorbed by the excited 235UF6, causing its photolysis to 235UF5 and fluorine. The resultant enriched UF5 forms a solid which is then separated from the gas by filtration or a cyclone separator. The precipitated UF5 is relatively enriched with 235UF5 and after conversion back to UF6 it is fed to the next stage of the cascade to be further enriched.

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