Nuclear Fuels using Lasers1. ENRICHED URANIUM

1

Nuclear Fuels using Lasers

Proportions of uranium-238 (blue) and uranium-235 (red) found naturally versus enriched grades

Enriched uranium is a kind of uranium in which the percent composition

of uranium-235 has been increased through the process of isotope separation.

Natural uranium is 99.284% 238 U isotope, with 235U only constituting about

0.711% of its weight. 235U is the only nuclide existing in nature (in any appreciable

amount) that is fissile with thermal neutrons.

Uranium-235 is an isotope of uranium making up about 0.72% of natural

uranium. Unlike the predominant isotope uranium-238 it is fissile, i.e., it can

sustain fission chain reaction. It is the only fissile isotope that is a primordial

nuclide or found in significant quantity in nature.

Uranium-235 has a half-life of 700 million years. It was discovered in 1935

by Arthur Jeffrey Dempster. Its nuclear cross section for slow thermal neutrons is

about 1000 barns. For fast neutrons it is on the order of 1 barn. Most but not

all neutron absorptions result in fission; a minority result in neutron

capture forming uranium-236. The fission of one atom of U-235 generates

202.5 MeV = 3.244 × 10−11 J, i.e. 19.54 TJ/mol= 83.14 TJ/kg. Heavy water

reactors, and some graphite moderated reactors can use unenriched uranium,

but light water reactors must use low enriched uranium because of light

water's neutron absorption. Uranium enrichment removes some of the uranium-

238 and increases the proportion of uranium-235. In nuclear weapon

design, highly enriched uranium containing 40% or greater U-235 is sometimes

used in the secondary stage in place of natural or depleted uranium. Primary

stages today most commonly use plutonium but when uranium is used, it is even

more highly enriched in U-235.

If at least one neutron from U-235 fission strikes another nucleus and

causes it to fission, then the chain reaction will continue. If the reaction will

sustain itself, it is said to be critical, and the mass of U-235 required to produce

2

Nuclear Fuels using Lasersthe critical condition is said to be a critical mass. A critical chain reaction can be

achieved at low concentrations of U-235 if the neutrons from fission

are moderated to lower their speed, since the probability for fission with slow

neutrons is greater.

A fission chain reaction produces intermediate mass fragments which are

highly radioactive and produce further energy by their radioactive decay. Some of

them produce neutrons, called delayed neutrons, which contribute to the fission

chain reaction. In nuclear reactors, the reaction is slowed down by the addition

of control rods which are made of elements such as boron, cadmium,

and hafnium which can absorb a large number of neutrons. In nuclear bombs, the

reaction is uncontrolled and the large amount of energy released creates a nuclear

explosion.

The Little Boy gun type atomic bomb dropped on Hiroshima on August 6,

1945 was fuelled by highly enriched uranium with a large tamper. The nominal

spherical critical mass for an unhampered 235U nuclear weapon is 56 kg, a sphere

17.32 cm (6.8") in diameter. The required material must be 85 percent or more

of 235U and is known as weapons grade Uranium, though for a crude, inefficient

weapon 20 percent is sufficient (called weapon(s)-usable).

Even lower enrichment can be used, but then the required critical

mass rapidly increases. Use of a large tamper, implosion geometries, trigger tubes,

Polonium triggers, Tritium enhancement, and neutron reflectors can enable a more

compact, economical weapon using one-fourth or less of the nominal critical

mass, though this would likely only be possible in a country that already had

extensive experience in engineering nuclear weapons. Most modern nuclear

weapon designs use plutonium as the fissile component of the primary

stage however HEU is often used in the secondary stage.

3

Nuclear Fuels using Lasers

Lighter:

Uranium-234

Uranium-235 is an

isotope of Uranium

Heavier:

Uranium-236

Decay product of:

Protactinium-235

Neptunium-235

Plutonium-239

Decay chain

of Uranium-235

Decays to:

Thorium-231

SourceAverage energy released

[MeV]Instantaneously released energyKinetic energy of fission fragments 169.10Kinetic energy of prompt neutrons 4.8

4

Nuclear Fuels using LasersEnergy carried by prompt γ-rays 7.0Energy from decaying fission productsEnergy of β—particles 6.5Energy of delayed γ-rays 6.3Energy released when those prompt neutrons which don't

(re)produce fission are captured8.8

Energy converted into heat in an operating thermal nuclear reactor 202.5Energy of anti-neutrinos 8.8Sum 211.3

3. ENRICHMENT METHODS

3.1 Diffusion techniques

3.1.1 Gaseous diffusion

Gaseous diffusion is a technology used to produce enriched uranium by

forcing gaseous uranium hexafluoride (hex) through semi-permeable membranes.

This produces a slight separation between the molecules containing 235U and 238U.

Throughout the Cold War, gaseous diffusion played a major role as a uranium

enrichment technique, and as of 2008 accounted for about 33% of enriched

uranium production but is now an obsolete technology that is steadily being

replaced by the later generations of technology as the diffusion plants reach their

ends-of-life.

3.1.2 Thermal diffusion

5

Nuclear Fuels using LasersThermal diffusion utilizes the transfer of heat across a thin liquid or gas to

accomplish isotope separation. The process exploits the fact that the lighter 235U

gas molecules will diffuse toward a hot surface, and the heavier 238U gas

molecules will diffuse toward a cold surface. The S-50 plant at Oak Ridge,

Tennessee was used during World War II to prepare feed material for

the EMIS process. It was abandoned in favour of gaseous diffusion.

3.2 Centrifuge techniques

3.2.1 Gas centrifuge

A cascade of gas centrifuges at a U.S. enrichment plant

The gas centrifuge process uses a large number of rotating cylinders in

series and parallel formations. Each cylinder's rotation creates a strong centrifugal

force so that the heavier gas molecules containing 238U move toward the outside of

the cylinder and the lighter gas molecules rich in 235U collect closer to the centre.

It requires much less energy to achieve the same separation than the older gaseous

diffusion process, which it has largely replaced and so is the current method of

choice and is termed second generation. It has a separation factor per stage of 1.3

relative to gaseous diffusion of 1.005 which translates to about one-fiftieth of the

energy requirements. Gas centrifuge techniques produce about 54% of the world's

enriched uranium.

3.2.2 Zippe centrifuge

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Nuclear Fuels using Lasers

Diagram of the principles of a Zippe-type gas centrifuge with U-238 represented in dark blue and U-235 represented in light blue

The Zippe centrifuge is an improvement on the standard gas centrifuge, the

primary difference being the use of heat. The bottom of the rotating cylinder is

heated, producing convection currents that move the 235U up the cylinder, where it

can be collected by scoops. This improved centrifuge design is used commercially

by Urenco to produce nuclear fuel and was used by Pakistan in their nuclear

weapons program.

3.3 Laser techniques

Laser processes promise lower energy inputs, lower capital costs and

lower tails assays, hence significant economic advantages. Several laser processes

have been investigated or are under development. None of the laser processes

below are yet ready for commercial use, though SILEX is well advanced and

expected to begin commercial production in 2012. and May 2010 Investor

Presentation

3.3.1 Atomic vapour laser isotope separation (AVLIS)

Atomic vapour laser isotope separation employs specially tuned lasers to

separate isotopes of uranium using selective ionization of hyperfine transitions.

The technique uses lasers which are tuned to frequencies that ionize 235U atoms

and no others. The positively charged 235U ions are then attracted to a negatively

charged plate and collected.

3.3.2 Molecular laser isotope separation (MLIS)

7

Nuclear Fuels using LasersMolecular laser isotope separation uses an infrared laser directed at UF6,

exciting molecules that contain a 235U atom. A second laser frees a fluorine atom,

leaving uranium pentafluoride which then precipitates out of the gas.

3.3.3 Separation of Isotopes by Laser Excitation (SILEX)

Separation of isotopes by laser excitation is an Australian development that

also uses UF6. After a protracted development process involving U.S. Enrichment

Company USEC acquiring and then relinquishing commercialization rights to the

technology, GE Hitachi Nuclear Energy (GEH) signed a commercialization

agreement with Silex Systems in 2006 (see here).

GEH has since begun construction of a demonstration test loop and

announced plans to build an initial commercial facility. Details of the process are

restricted by intergovernmental agreements between USA and Australia and the

commercial entities. SILEX has been indicated to be an order of magnitude more

efficient than existing production techniques but again, the exact figure is

classified.

3.4 Other techniques

3.4.1 Aerodynamic processes

Schematic diagram of an aerodynamic nozzle. Many thousands of these small foils would be combined in an enrichment unit.

Aerodynamic enrichment processes include the Becker jet nozzle

techniques developed by E. W. Becker and associates using the LIGA process and

the vortex tube separation process. These aerodynamic separation processes

depend upon diffusion driven by pressure gradients, as does the gas centrifuge. In

effect, aerodynamic processes can be considered as non-rotating centrifuges.

Enhancement of the centrifugal forces is achieved by dilution of UF6 with

8

Nuclear Fuels using Lasershydrogen or helium as a carrier gas achieving a much higher flow velocity for the

gas than could be obtained using pure uranium hexafluoride. The Uranium

Enrichment Corporation of South Africa (UCOR) developed and deployed

the Helikon vortex separation process based on the vortex tube and a

demonstration plant was built in Brazil by NUCLEI, a consortium led by

Industries Nucleares do Brasil that used the separation nozzle process. However

both methods have high energy consumption and substantial requirements for

removal of waste heat; neither is currently in use.

3.4.2 Electromagnetic isotope separation

Schematic diagram of uranium isotope separation in a calutron shows how a strong magnetic field is used to redirect a stream of

uranium ions to a target, resulting in a higher concentration of uranium-235 (represented here in dark blue) in the inner fringes of

the stream.

In the electromagnetic isotope separation process (EMIS), metallic

uranium is first vaporized, and then ionized to positively charged ions. The cations

are then accelerated and subsequently deflected by magnetic fields onto their

respective collection targets.

A production-scale mass spectrometer named the Calutron was developed

during World War II that provided some of the 235U used for the Little Boy nuclear

bomb, which was dropped over Hiroshima in 1945. Properly the term 'Calutron'

applies to a multistage device arranged in a large oval around a powerful

electromagnet. Electromagnetic isotope separation has been largely abandoned in

favour of more effective methods.

3.4.3 Chemical methods

9

Nuclear Fuels using LasersOne chemical process has been demonstrated to pilot plant stage but not

used. The French CHEMEX process exploited a very slight difference in the two

isotopes' propensity to change valency in oxidation/reduction, utilising immiscible

aqueous and organic phases. An ion-exchange process was developed by the Asahi

Chemical Company in Japan which applies similar chemistry but effects

separation on a proprietary resin ion-exchange column.

3.4.4 Plasma separation

Plasma separation process (PSP) describes a technique that makes use

of superconducting magnets and plasma physics. In this process, the principle

ofion cyclotron resonance is used to selectively energize the 235U isotope in

a plasma containing a mix of ions. The French developed their own version of

PSP, which they called RCI. Funding for RCI was drastically reduced in 1986, and

the program was suspended around 1990, although RCI is still used for stable

isotope separation.

3.4.5 Seperative work unit

"Separative work"—the amount of separation done by an enrichment process

—is a function of the concentrations of the feedstock, the enriched output, and the

depleted tailings; and is expressed in units which are so calculated as to be

proportional to the total input (energy / machine operation time) and to the mass

processed. Separative work is not energy. The same amount of separative work

will require different amounts of energy depending on the efficiency of the

separation technology. Separative work is measured in Separative work

units SWU, kg SW, or kg UTA (from the GermanUrantrennarbeit

literally uranium separation work)

1 SWU = 1 kg SW = 1 kg UTA

1 kSWU = 1 tSW = 1 t UTA

1 MSWU = 1 ktSW = 1 kt UTA

The work WSWU necessary to separate a mass F of feed of assay xf into a

mass P of product assay xp, and tails of mass T and assay xt is given by the

expression

10

Nuclear Fuels using Laserswhere is the value function, defined as

The feed to product ratio is given by the expression

whereas the tails to product ratio is given by the expression

For example, beginning with 100 kilograms (220 lb) of NU, it takes about

61 SWU to produce 10 kilograms (22 lb) of LEU in 235U content to 4.5%, at a tails

assay of 0.3%.

4. LASER ENRICHMENT METHODS

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Nuclear Fuels using Lasers

5. ATOMIC VAPOUR LASER ISOTOPE SEPERATION

An atomic vapor laser isotope separation experiment at LLNL. Green light is from a copper vapor pump laser used

to pump a highly tuned dye laser which is producing the orange light.

AVLIS Is an acronym which stands for atomic vapour laser isotope

separation and is a method by which specially tuned lasers are used to

separate isotopes of uranium using selective ionization of hyperfine transitions.

In the largest technology transfer in U.S. government history, in 1994 the

AVLIS process was transferred to the United States Enrichment Corporation for

commercialization. However, on June 9, 1999 after a $100 million investment,

USEC cancelled its AVLIS program.

The AVLIS process provides high energy efficiency comparable with gas

centrifuges, high separation factor, and low volume of radioactive waste.

AVLIS continues to be developed by some countries and it presents some

specific challenges to international monitoring. Iran is now known to have had a

12

Nuclear Fuels using Laserssecret AVLIS program. However, since it was uncovered in 2003, Iran has claimed

to have dismantled it.

Similar technology, using molecules instead of atoms, is the molecular

laser isotope separation, MLIS.

The absorption lines of 235U and 238U differ slightly due to hyperfine

structure; for example, the 238U absorption peak shifts from 502.74 nanometers to

502.73 nm in 235U. AVLIS uses tunable dye lasers, which can be precisely tuned,

so that only 235U absorbs the photons and selectively undergoes excitation and

then photoionization. The ions are then electrostatically deflected to a collector,

while the neutral unwanted uranium-238 passes through.

The AVLIS system consists of a vaporizer and a collector, forming the

separation system, and the laser system. The vaporizer produces a stream of pure

gaseous uranium.

The laser commonly used is a two-stage tunable pulsed dye

laser usually pumped by a copper vapour laser; the master oscillator is low-power

but highly precise, and its power is increased by a dye laser amplifier acting

as optical amplifier. Three frequencies ("colours") of lasers are used for full

ionization of uranium-235.

5.1 ATOMIC HYPERFINE STRUCTURE

5.1.1 Magnetic dipole

The dominant term in the hyperfine Hamiltonian is typically the magnetic

dipole term. Atomic nuclei with a non-zero nuclear spin have a magnetic dipole

moment, given by:

.

There is an energy associated with a magnetic dipole moment in the

presence of a magnetic field. For a nuclear magnetic dipole moment, μI, placed in

a magnetic field, B, the relevant term in the Hamiltonian is given by:

.

13

Nuclear Fuels using LasersIn the absence of an externally applied field, the magnetic field

experienced by the nucleus is that associated with the orbital (l) and spin (s)

angular momentum of the electrons:

.

Electron orbital angular momentum results from the motion of the electron

about some fixed external point that we shall take to be the location of the

nucleus. The magnetic field at the nucleus due to the motion of a single electron,

with charge -e at a position r relative to the nucleus, is given by:

,

where -r gives the position of the nucleus relative to the electron. Written

in terms of the Bohr magneton, this gives:

.

Recognizing that mev is the electron momentum, p, and that r×p/ħ is the

orbital angular momentum in units of ħ, l, we can write:

.

For a many electron atom this expression is generally written in terms of

the total orbital angular momentum, , by summing over the electrons and using

the projection operator, , where . For states with a well defined

projection of the orbital angular momentum, Lz, we can write , giving:

.

The electron spin angular momentum is a fundamentally different property

that is intrinsic to the particle and therefore does not depend on the motion of the

electron. Nonetheless it is angular momentum and any angular momentum

14

Nuclear Fuels using Lasersassociated with a charged particle results in a magnetic dipole moment, which is

the source of a magnetic field. An electron with spin angular momentum, s, has a

magnetic moment, μs, given by:

,

where gs is the electron spin g -factor and the negative sign is because the

electron is negatively charged (consider that negatively and positively charged

particles with identical mass, travelling on equivalent paths, would have the same

angular momentum, but would result in currents in the opposite direction).

The magnetic field of a dipole moment, μs, is given by:

.

The complete magnetic dipole contribution to the hyperfine Hamiltonian is thus

given by:

The first term gives the energy of the nuclear dipole in the field due to the

electronic orbital angular momentum. The second term gives the energy of the

"finite distance" interaction of the nuclear dipole with the field due to the electron

spin magnetic moments. The final term, often known as the "Fermi contact" term

relates to the direct interaction of the nuclear dipole with the spin dipoles and is

only non-zero for states with a finite electron spin density at the position of the

nucleus (those with unpaired electrons in s-subshells). It has been argued that one

may get a different expression when taking into account the detailed nuclear

dipole moment distribution.

15

Nuclear Fuels using LasersFor states with l ≠ 0 this can be expressed in the form

,

Where .

If hyperfine structure is small compared with the fine structure (sometimes

called IJ-coupling by analogy with LS -coupling ), I and J are good quantum

numbers and matrix elements of can be approximated as diagonal in I and J.

In this case (generally true for light elements), we can

project N onto J (where J = L + S is the total electronic angular momentum) and

we have:

.

This is commonly written as

,

with determined by experiment. Since I.J = ½{F.F - I.I - J.J}

(where F = I + J is the total angular momentum), this gives an energy of

.

In this case the hyperfine interaction satisfies the Lande interval rule.

5.1.2 Electric quadrupole

Atomic nuclei with spin have an electric quadrupole moment. In the

general case this is represented by a rank-2 tensor, , with components given by:

,

where i and j are the tensor indices running from 1 to 3, xi and xj are the spatial

variables x, y and z depending on the values of i and j respectively, δij is

16

Nuclear Fuels using Lasersthe Kronecker delta and ρ(r) is the charge density. Being a 3-dimensional rank-2

tensor, the quadrupole moment has 32 = 9 components. From the definition of the

components it is clear that the quadrupole tensor is a symmetric matrix (Qij =Qji)

that is also traceless (ΣiQii = 0), giving only five components in the irreducible

representation. Expressed using the notation of irreducible spherical tensors we

have:

.

The energy associated with an electric quadrupole moment in an electric

field depends not on the field strength, but on the electric field gradient,

confusingly labelled , another rank-2 tensor given by the outer product of the del

operator with the electric field vector:

,

with components given by:

.

Again it is clear this is a symmetric matrix and, because the source of the

electric field at the nucleus is a charge distribution entirely outside the nucleus,

this can be expressed as a 5-component spherical tensor, , with:

,

where:

.

17

Nuclear Fuels using LasersThe quadrupolar term in the Hamiltonian is thus given by:

.

6. MOLECULAR LASER ISOTOPE SEPERATION

Molecular laser isotope separation (MLIS) is a method of isotope

separation, where specially tuned lasers are used to

separate isotopes of uranium using selective ionization of hyperfine transitions

of uranium hexafluoride molecules. It is similar to AVLIS. Its main advantage

over AVLIS is low energy consumption and use of uranium hexafluoride instead

of vaporized uranium.MLIS was conceived in 1961 at the Los Alamos National

Laboratory.

MLIS operates in cascade setup, like the gaseous diffusion process. Instead

of vaporized uranium as in AVLIS the working medium of the MLIS is uranium

hexafluoride which requires a much lower temperature to vaporize. The 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 (e.g. 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. The laser for

the excitation is usually a carbon dioxide laser with output wavelength shifted

from 10.6 µm to 16 µm; the photolysis laser may be a Xe Cl excimer laser

operating at 308 nm, however infrared lasers are mostly used in existing

implementations.

18

Nuclear Fuels using Lasers

The process is considerably complex, with many technical difficulties. United

States, France, United Kingdom, Germany and South Africa reported termination

of their MLIS programs, however Japan has a small scale program in operation.

The Commonwealth Scientific and Industrial Research Organisation in

Australia has developed the SILEX pulsed laser separation process. GE, Cameco

and Hitachi are currently involved in developing it for commercial use. Silex

information

6.1 MOLECULAR HYPERFINE STRUCTURE

The molecular hyperfine Hamiltonian includes those terms already derived

for the atomic case with a magnetic dipole term for each nucleus with and an

electric quadrupole term for each nucleus with . The magnetic dipole terms

19

Nuclear Fuels using Laserswere first derived for diatomic molecules by Frosch and Foley and the resulting

hyperfine parameters are often called the Frosch and Foley parameters.

In addition to the effects described above there are a number of effects specific to

the molecular case.

6.1.1 Direct nuclear spin-spin

Each nucleus with has a non-zero magnetic moment that is both the

source of a magnetic field and has an associated energy due to the presence of the

combined field of all of the other nuclear magnetic moments. A summation over

each magnetic moment dotted with the field due to each other magnetic moment

gives the direct nuclear spin-spin term in the hyperfine Hamiltonian, .

,

where α and α‘ are indices representing the nucleus contributing to the energy and

the nucleus that is the source of the field respectively. Substituting in the

expressions for the dipole moment in terms of the nuclear angular momentum and

the magnetic field of a dipole, both given above, we have:

.

6.1.2 Nuclear spin-rotation

The nuclear magnetic moments in a molecule exist in a magnetic field due

to the angular momentum, T (R is the inter nuclear displacement vector),

associated with the bulk rotation of the molecule.

20

Nuclear Fuels using Lasers6.2 IR MULTIPLE PHOTON DISSOCIATION

Infrared multiphoton dissociation (irmpd) is a technique used in mass

spectrometry to fragment molecules in the gas phase usually for structural analysis

of the original (parent) molecule.

An infrared laser is directed through a window into the vacuum of the mass

spectrometer where the ions are. The mechanism of fragmentation involves the

absorption by a given ion of multiple infrared photons. The parent ion becomes

excited into more energetic vibrational states until a bond(s) is broken resulting in

gas phase fragments of the parent ion.

By applying intense tuneable ir lasers, like ir-opos or ir free electron lasers, the

wavelength dependence of the irmpd yield can be studied. This irmpd

spectroscopy allows for the measurement of vibrational spectra of (unstable)

species that can only be prepared in the gas phase. Such species include molecular

ions but also neutral species like metal clusters that can be gently ionized after

interaction with the ir light for their mass spectrometric detection.

21

Nuclear Fuels using Lasers

7. SILEX METHOD

SILEX is an acronym for Separation of Isotopes by Laser Excitation,a

technology developed in the 1990s for isotope separation to produce enriched

uranium using lasers.

The SILEX process was developed in Australia by Silex Systems Limited, a

publicly listed high technology innovation company founded in 1988, and was

invented by Dr Michael Goldsworthy and Dr Horst Struve.The process builds on

earlier work in laser enrichment that began in the 1970s, such as AVLIS (atomic

vapour laser isotope separation) and MLIS (molecular laser isotope separation.)

In November 1996 Silex Systems Limited signed a license and development

agreement for the application of SILEX technology exclusively to uranium

enrichment with the United States Enrichment Corporation (USEC) avoiding any

problems for Australia under the Nuclear Non-Proliferation Treaty.

Silex Systems Limited concluded the second stage of testing in 2005 and

began enacting its Test Loop Program. In 2007, an exclusive commercialization

and licensing agreement was signed with General Electric Corporation. The Test

Loop Program was transferred to GE's facility in Wilmington, North Carolina.

Also in 2007, GE-Hitachi signed Letters of Intent for uranium enrichment services

with Exelon and Entergy - the two largest nuclear power utilities in the USA.

In 2008, GE Hitachi Nuclear Energy (GEH) spun off Global Laser Enrichment

(GLE) to commercialise the SILEX Technology and announced the first potential

commercial uranium enrichment facility using the Silex process. The USA's

Nuclear Regulatory Commission (NRC) approved a license amendment to operate

the Test Loop. Also in 2008, Cameco Corporation, the world’s largest uranium

producer, had joined GE and Hitachi as owners of their laser enrichment venture

GLE.

22

Nuclear Fuels using LasersIn 2010, concerns were raised the process poses a threat to global nuclear

security; the process requires up to 75% less space and consumes considerably

less energy than current enrichment technologies, it is reportedly almost

undetectable from orbit potentially allowing rogue governments activities to go

undetected by the international community.

In August, 2011 The New York Times reported that Global Laser Enrichment, a

subsidiary of GE Hitachi Nuclear Energy, had applied to the Nuclear Regulatory

Commission for a permit to build a commercial plant at Wilmington. Details of

the process are secret.

23

Nuclear Fuels using Lasers

7.1 Raman spectroscopy

It is a spectroscopic technique used to study vibrational, rotational, and

other low-frequency modes in a system. It relies on inelastic scattering, or Raman

scattering, of monochromatic light, usually from a laser in the visible, near

infrared, or near ultraviolet range. The laser light interacts with molecular

vibrations, phonons or other excitations in the system, resulting in the energy of

the laser photons being shifted up or down. The shift in energy gives information

about the vibrational modes in the system. Infrared spectroscopy yields similar,

but complementary, information.

Typically, a sample is illuminated with a laser beam. Light from the

illuminated spot is collected with a lens and sent through a monochromator.

Wavelengths close to the laser line, due to elastic Rayleigh scattering, are filtered

out while the rest of the collected light is dispersed onto a detector.

Spontaneous Raman scattering is typically very weak, and as a result the

main difficulty of Raman spectroscopy is separating the weak inelastically

scattered light from the intense Rayleigh scattered laser light. Historically,

Raman spectrometers used holographic gratings and multiple dispersion stages to

achieve a high degree of laser rejection. In the past, photomultipliers were the

detectors of choice for dispersive Raman setups, which resulted in long

acquisition times. However, modern instrumentation almost universally

employs notch or edge filters for laser rejection and spectrographs (either axial

transmissive (AT), Czerny-Turner (CT) monochromator) or FT (Fourier transform

spectroscopy based), and CCD detectors.

There are a number of advanced types of Raman spectroscopy,

including surface-enhanced Raman, resonance Raman, tip-enhanced Raman,

24

Nuclear Fuels using Laserspolarised Raman, stimulated Raman (analogous to stimulated emission),

transmission Raman, spatially-offset Raman, and hyper Raman.

8. LASERS USED

8.1 Gas lasers

Following the invention of the HeNe gas laser, many other gas discharges

have been found to amplify light coherently. Gas lasers using many

different gases have been built and used for many purposes. The helium-neon

laser (HeNe) is able to operate at a number of different wavelengths, however

the vast majority are engineered to lase at 633 nm; these relatively low cost

but highly coherent lasers are extremely common in optical research and

educational laboratories. Commercial carbon dioxide (CO2) lasers can emit

many hundreds of watts in a single spatial mode which can be concentrated

into a tiny spot. This emission is in the thermal infrared at 10.6 µm; such

lasers are regularly used in industry for cutting and welding. The efficiency of

a CO2 laser is unusually high: over 10%. Argon-ion lasers can operate at a

number of lasing transitions between 351 and 528.7 nm. Depending on the

optical design one or more of these transitions can be lasing simultaneously;

the most commonly used lines are 458 nm, 488 nm and 514.5 nm. A

nitrogen transverse electrical discharge in gas at atmospheric pressure (TEA)

laser is an inexpensive gas laser, often home-built by hobbyists, which

produces rather incoherent UV light at 337.1 nmMetal ion lasers are gas lasers

that generate deep ultraviolet wavelengths. Helium-silver (HeAg) 224 nm

andneon-copper (NeCu) 248 nm are two examples. Like all low-pressure gas

lasers, the gain media of these lasers have quite narrow oscillation linewidths,

less than 3 GHz (0.5 picometers), making them candidates for use

in fluorescencesuppressed Raman spectroscopy.

25

Nuclear Fuels using Lasers8.2 Chemical lasers

Chemical lasers are powered by a chemical reaction permitting a large

amount of energy to be released quickly. Such very high power lasers are

especially of interest to the military, however continuous wave chemical lasers

at very high power levels, fed by streams of gasses, have been developed and

have some industrial applications. As examples, in the Hydrogen fluoride

laser (2700-2900 nm) and the Deuterium fluoride laser (3800 nm) the reaction

is the combination of hydrogen or deuterium gas with combustion products

of ethylene in nitrogen trifluoride.

8.3 Excimer lasers

Excimer lasers are a special sort of gas laser powered by an electric

discharge in which the lasing medium is an excimer, or more precisely

an exciplex in existing designs. These are molecules which can only exist with

one atom in an excited electronic state. Once the molecule transfers its

excitation energy to a photon, therefore, its atoms are no longer bound to each

other and the molecule disintegrates. This drastically reduces the population of

the lower energy state thus greatly facilitating a population inversion.

Excimers currently used are all noble gas compounds; noble gasses are

chemically inert and can only form compounds while in an excited state.

Excimer lasers typically operate at ultraviolet wavelengths with major

applications including semiconductor photolithography and LASIK eye

surgery. Commonly used excimer molecules include ArF (emission at

193 nm), KrCl (222 nm), KrF (248 nm), XeCl (308 nm), and XeF

(351 nm). The molecular fluorine laser, emitting at 157 nm in the vacuum

ultraviolet is sometimes referred to as an excimer laser, however this appears

to be a misnomer inasmuch as F2 is a stable compound.

8.4 Solid-state lasers

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Nuclear Fuels using Lasers

Solid-state lasers use a crystalline or glass rod which is "doped" with ions

that provide the required energy states. For example, the first working laser

was aruby laser, made from ruby (chromium-doped corundum).

The population inversion is actually maintained in the "dopant", such

as chromium or neodymium. These materials are pumped optically using a

shorter wavelength than the lasing wavelength, often from a flashtube or from

another laser.

It should be noted that "solid-state" in this sense refers to a crystal or glass,

but this usage is distinct from the designation of "solid-state electronics" in

referring to semiconductors. Semiconductor lasers (laser diodes) are pumped

electrically and are thus not referred to as solid-state lasers. The class of solid-

state lasers would, however, properly include fibre lasers in which dopants in

the glass lase under optical pumping. But in practice these are simply referred

to as "fibre" with "solid-state" reserved for lasers using a solid rod of such a

material.

Laser spots (650, 532, 405 nm)

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Nuclear Fuels using Lasers

Neodymium is a common "dopant" in various solid-state laser crystals,

including yttrium orthovanadate(Nd:YVO4), yttrium lithium

fluoride (Nd:YLF) and yttrium aluminium garnet (Nd:YAG). All these lasers

can produce high powers in the infrared spectrum at 1064 nm. They are used

for cutting, welding and marking of metals and other materials, and also

in spectroscopy and for pumping dye lasers.

These lasers are also commonly frequency doubled, tripled or quadrupled,

in so-called "diode pumped solid state" or DPSS lasers. Under second, third,

or fourth harmonic generation these produce 532 nm (green, visible), 355 nm

and 266 nm (Ultraviolet|UV]) beams. This is the technology behind the

bright laser pointers particularly at green (532 nm) and other short visible

wavelengths.

Ytterbium, holmium, thulium, and erbium are other common "dopants" in

solid-state lasers. Ytterbium is used in crystals such as Yb:YAG, Yb:KGW,

Yb:KYW, Yb:SYS, Yb:BOYS, Yb:CaF2, typically operating around 1020-

1050 nm. They are potentially very efficient and high powered due to a small

quantum defect. Extremely high powers in ultra short pulses can be achieved

with Yb:YAG. Holmium-doped YAG crystals emit at 2097 nm and form an

efficient laser operating at infrared wavelengths strongly absorbed by water-

bearing tissues. The Ho-YAG is usually operated in a pulsed mode, and passed

through optical fiber surgical devices to resurface joints, remove rot from

teeth, vaporize cancers, and pulverize kidney and gall stones.

Thermal limitations in solid-state lasers arise from unconverted pump

power that manifests itself as heat. This heat, when coupled with a high

thermo-optic coefficient (dn/dT) can give rise to thermal lensing as well as

reduced quantum efficiency. These types of issues can be overcome by

another novel diode-pumped solid-state laser, the diode-pumped thin disk

laser. The thermal limitations in this laser type are mitigated by using a laser

28

Nuclear Fuels using Lasersmedium geometry in which the thickness is much smaller than the diameter of

the pump beam. This allows for a more even thermal gradient in the material.

Thin disk lasers have been shown to produce up to kilowatt levels of power.

8.5 Fibre lasers

Solid-state lasers or laser amplifiers where the light is guided due to

the total internal reflection in a single mode optical fibre are instead

called fibre. Guiding of light allows extremely long gain regions providing

good cooling conditions; fibres have high surface area to volume ratio which

allows efficient cooling. In addition, the fibre’s wave guiding properties tend

to reduce thermal distortion of the beam. Erbium and ytterbium ions are

common active species in such lasers.

Quite often, the fibre laser is designed as a double-clad fibre. This type of

fibre consists of a fibre core, an inner cladding and an outer cladding. The

index of the three concentric layers is chosen so that the fibre core acts as a

single-mode fibre for the laser emission while the outer cladding acts as a

highly multimode core for the pump laser. This lets the pump propagate a

large amount of power into and through the active inner core region, while

still having a high numerical aperture (NA) to have easy launching

conditions.Pump light can be used more efficiently by creating a fibre disk

laser, or a stack of such lasers.

Fibre lasers have a fundamental limit in that the intensity of the light in the

fibre cannot be so high that optical nonlinearities induced by the local electric

field strength can become dominant and prevent laser operation and/or lead to

the material destruction of the fibre. This effect is called photo darkening. In

bulk laser materials, the cooling is not so efficient, and it is difficult to

separate the effects of photo darkening from the thermal effects, but the

experiments in fibres show that the photo darkening can be attributed to the

formation of long-living colour.

8.6 Photonic crystal lasers

Photonic crystal lasers are lasers based on nano-structures that provide the

mode confinement and the density of optical states (DOS) structure required

29

Nuclear Fuels using Lasersfor the feedback to take place They are typical micrometre-sized and tunable

on the bands of the photonic crystals.

8.7 Semiconductor lasers

A 5.6 mm 'closed can' commercial laser diode, probably from a CD or DVD player

Semiconductor lasers are diodes which are electrically pumped.

Recombination of electrons and holes created by the applied current

introduces optical gain. Reflection from the ends of the crystal form an optical

resonator, although the resonator can be external to the semiconductor in some

designs.

Commercial laser diodes emit at wavelengths from 375 nm to 1800 nm,

and wavelengths of over 3 µm have been demonstrated. Low to medium

power laser diodes are used in laser printers and CD/DVD players. Laser

diodes are also frequently used to optically pump other lasers with high

efficiency. The highest power industrial laser diodes, with power up to 10 kW

(70dBm), are used in industry for cutting and welding. External-cavity

semiconductor lasers have a semiconductor active medium in a larger cavity.

30

Nuclear Fuels using LasersThese devices can generate high power outputs with good beam quality,

wavelength-tunable narrow-line width radiation, or ultra short laser pulses.

Vertical cavity surface-emitting lasers (VCSELs) are semiconductor lasers

whose emission direction is perpendicular to the surface of the wafer. VCSEL

devices typically have a more circular output beam than conventional laser

diodes, and potentially could be much cheaper to manufacture.

The development of a silicon laser is important in the field of optical

computing. Silicon is the material of choice for integrated circuits, and so

electronic and silicon photonic components (such as optical interconnects)

could be fabricated on the same chip.

Unfortunately, silicon is a difficult lasing material to deal with, since it has

certain properties which block lasing. However, recently teams have produced

silicon lasers through methods such as fabricating the lasing material from

silicon and other semiconductor materials, such as indium (III)

phosphide or gallium (III) arsenide, materials which allow coherent light to be

produced from silicon. These are called hybrid silicon laser. Another type is

a Raman laser, which takes advantage of Raman scattering to produce a laser

from materials such as silicon.

8.8 Dye lasers

Dye lasers use an organic dye as the gain medium. The wide gain spectrum

of available dyes, or mixtures of dyes, allows these lasers to be highly tunable,

or to produce very short-duration pulses (on the order of a few femto

seconds).

31

Nuclear Fuels using Lasers10. REFERENCES

a.i. Thomas B. Cochran (Natural Resources Defense Council) (12 June 1997). "Safeguarding Nuclear Weapon-Usable Materials in Russia". Proceedings of international forum on illegal nuclear traffic.

a.ii. Forsberg, C. W.; Hopper, C. M.; Richter, J. L.; Vantine, H. C. (March 1998). "Definition of Weapons-Usable Uranium 233". ORNL/TM-13517. Oak Ridge National Laboratories. Retrieved 2 October 2010.

a.iii. Sublette, Carey (4 October 1996). "Nuclear Weapons FAQ, Section 4.1.7.1: Nuclear Design Principles - Highly Enriched Uranium". Nuclear Weapons FAQ. Retrieved 2 October 2010.

a.iv. IEEE Spectrum 11.10.

a.v. C. Schwab, A.J. Damiao, C.A.B. Silveira, J.W. Neri, M.G. Destro, N.A.S. Rodrigues, R. Riva, “Laser Techniques

a.vi. Applied to Isotope Separation of Uranium,” Progress in Nuclear Energy, Vol. 33, No. 1-2, pp. 217-264, 1998 provides

a.vii. a good overview of laser-based isotope separation. On Silex see John L. Lyman, “Enrichment Separative Capacity for SILEX,” Los Alamos National Laboratory, LA-UR-05-3786, 2005.

a.viii. Elaine M. Grossman, "New Technology Offers Detectable 'Signatures,' Advocate Says,"Global Security Newswire,

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