AVLIS-U Researches and Developments in the World

Gheorghe VǍSARU

National Institute for Isotopic and Molecular Technology (NIIMT), Cluj-Napoca

Aleea Tarniţa Nr. 7, Apt. 11

400659 CLUJ-NAPOCA , ROMANIA

e-mail: gvasaru@hotmail.com

Abstract:

After a short introduction to uranium isotope enrichment (especially by gaseous

diffusion and ultracentrifugation), a survey on researches and developments on AVLIS-U

method in Brazil, China, France, India, Italia, Japan, Romania, Russia, United Kingdom

and United States of America, are presented.

Key Words: Uranium Enrichment, Atomic Vapor Laser Isotope Separation.

INTRODUCTION

The various activities associated with the production of electricity from nuclear

reactions are referred to collectively as the nuclear fuel cycle. The nuclear fuel cycle

starts with the mining of uranium and ends with disposal of nuclear waste.

At the start of 2006, there were 352 nuclear units in operation, ten units were

under construction and 17 units were firmly committed for construction, almost in the

Pacific region [1]. All of these require uranium enriched in the 235

U isotope for their fuel.

Uranium is a slightly radioactive metal that occurs throughout the earth’s crust, of about

500 times more abundant than gold and about as common as tin. It is present in most

rocks and soils as well as in many rivers and in seawater, and also, in concentrations of

about four parts per million (ppm) in granite, which makes up 60% of the earth’s crust. In

fertilizers, uranium concentration can be as high as 400 ppm (0.04%), and some coal

deposits contain uranium at concentrations greater than 100 ppm (0.01%).

There are a number of areas around the world where the concentration of uranium

in the ground is sufficiently high that extraction for use as nuclear fuel is economically

feasible.

Uranium found in nature consists largely of two isotopes, 235

U and 238

U. The

production of energy in nuclear reactors is from the “fission” or splitting of the 235

U

atoms, a process which releases energy in the form of heat. 235

U is the main fissile isotope

of uranium.

Natural uranium contains 0.72 % of the 235

U isotope. The remaining 99.3 % is

mostly the 238

U isotope, which does not contribute directly to the fission process.

235

U and 238

U are chemical identical, but differ in their physical properties, particularly

their mass. The nucleus of the 235

U atom contains 92 protons and 143 neutrons, giving an

atomic mass of 235 units. The 238

U nucleus also has 92 protons and 146 neutrons - three

more than 235

U, and therefore has an atomic mass of 238 units.

The difference in mass between 235

U and 238

U allows the isotopes to be separated

and makes it possible to increase or “enrich” the percentage of 235

U. All enrichment

processes, directly or indirectly, make use of this small mass difference.

In the most common types of nuclear reactors, a higher concentration of 235

U than

natural is required. The enrichment process produces this higher concentration, typically

between 3.5% and 4.5% 235

U, by removing a large part of the 238

U (80% for enrichment

to 3.5%).

CONVERSION

The product of a uranium mill is not directly usable as a fuel for a nuclear reactor.

Additional processing, generally referred to as conversion, is required.

At a conversion facility, uranium is converted to either uranium dioxide, which

can be used as the fuel for those types of reactors that not require enriched uranium, or

into uranium hexafluoride (UF6), commonly referred to as “hex”, which can be enriched

to produce fuel for the majority of types of reactors.

After refining, uranium trioxide is reduced in a kiln by hydrogen or ammonia to

uranium dioxide (UO2). This is then reacted in another kiln with hydrogen fluoride (HF)

to form uranium tetrafluoride (UF4). The tetrafluoride is then fed into a fluidized bed

reactor with gaseous fluorine to produce UF6. Removal of impurities takes place at

several of these steps.

An alternative wet process involves making the UF4 from UO2 by reaction with

aqueous HF.

UF6 is a solid at room temperature, but becomes a gas when heated above 570C,

suitable for use in the enrichment process. Particularly if moist, is highly corrosive. At

lower temperature and under moderate pressure, the UF6 may be liquefied and the liquid

runs into special designed steel shipping cylinders, which are thick, walled and weigh up

to 15 tones when full. As it cools, the liquid UF6 within the cylinders becomes a white

crystalline solid and is shipped in this form.

The sitting and environmental management of a conversion plant requires no

special arrangements beyond those needed for any chemical processing plant involving

fluorine chemicals [2].

Conversion plants are operating commercially in USA, Canada, France, UK, and

Russia.

ENRICHMENT

Uranium enrichment is a critical step in transforming natural uranium into nuclear

fuel to produce energy. A number of enrichment processes have been demonstrated in the

laboratory but only two, the gaseous diffusion and gas centrifugation are operating on a

commercial scale. In both of these, UF6 gas is used as the feed material.

Molecules of UF6 with 235

U atoms are about one percent lighter than the rest, and

this difference in mass is the basis of both processes. Large commercial enrichment

plants are in operation in France, Germany, Netherlands, UK, USA and Russia, with

smaller plants elsewhere.

The capacity of enrichment plants is measured in terms of “separative work

units” or SWU. The SWU is a complex unit, which is a function of the amount of

uranium processed and the degree to which it is enriched, i.e. the extent of increase in the

concentration of the 235

U isotope relative to remainder. The unit is strictly: kilogram

Separative Work Unit (kg SWU), and it measure the quantity of separative work

(indicative of energy used in enrichment) when feed and product quantities are expressed

in kilograms. The unit “ton SWU” (t SWU) or “million SWU” (M SWU) is also used.

For instance, to produce one kilogram of uranium enriched to 3% 235

U requires

3.8 SWU if the plant is operated at a tails assay 0.25 %, or 5.0 SWU if the tails assay is

0.15% (thereby requiring only 5.1 kg instead of 6.0 kg of natural U feed.

About 100,000 - 120,000 SWU is required to enrich the annual fuel loading for a typical

1000 MWe light water reactor (LWR). Enrichment costs are related to electrical energy

used. The gaseous diffusion process consumes about 2,400 kWh (8,600 MJ) per SWU,

while modern gas centrifuge plants require only about 60 kWh (200 MJ) per SWU.

Enrichment accounts for approximately one third of the cost of nuclear fuel and

about 10% of the total cost of electricity generated. It can also account for the main

greenhouse impact from the nuclear fuel cycle, if the electricity used for enrichment is

generated from coal. However, it still only amounts to 0.1 % of the carbon dioxide from

equivalent coal-fired electricity generation, if modern gas centrifuge plants are used, or

up to 3% in a worst-case situation.

1. Gaseous Diffusion

At present the gaseous diffusion process is the most widely used method,

producing over 30 M SWU. The process separates the lighter 235

U isotope from the

heavier 238

U. The gas is forced through a series of porous membranes with microscopic

openings. Because the 235

U molecules are lighter than 238

U molecules they move faster

and have a slightly better chance of passing through the pores in the membrane. The UF6,

which diffuses through the membrane, is thus slightly enriched, while the gas, which did

not pass through, is depleted in 235

U.

This process is repeated many times in a series of diffusion stages called a

cascade. Each stage consists of a compressor, a diffuser and a heat exchanger to remove

the heat of compression. The enriched UF6 product is withdrawn from one end of the

cascade and the depleted UF6 is removed at the other end.

Commercial uranium enrichment was first carried out by diffusion process in the

USA. The two US Enrichment Corporation plants in that country have a capacity of some

19 M SWU per year. At Tricastin (France), a more modern diffusion plant, EURODIF,

with a capacity of 10.8 M SWU per year has been operating since 1979. This plant can

produce enough 3.7% enriched uranium per year to fuel some ninety 1,000 MWe nuclear

reactors.

2. Gas Centrifugation

A centrifuge comprises an evacuated casing containing a cylindrical rotor, 1 - 2 m

long, and 15 - 20 cm diameter, which rotate at high speed (50,000 - 70,000 rpm) in an

almost friction-free environment. The uranium is fed into rotor as gaseous UF6 where it

takes up the rotational motion. The centrifugal forces push the heavier 238

U closer to the

wall of the rotor than the lighter 235

U. The gas closer to the wall becomes depleted in 235

U

whereas the gas nearer the rotor axis is enriched in 235

U. The gas flowing within the rotor

can be produced by a temperature gradient over the length of the centrifuge. UF6 depleted

in 235

U flows upwards adjacent to the rotor wall, whilst UF6 enriched in 235

U flows

downwards close to the axis. The two gas streams are removed through small pipes.

To obtain efficient separation of the two isotopes, centrifuges rotate at very high

speeds, with the outer wall of the spinning cylinder moving at between 400 and 500 m/s

to give a million times the acceleration of gravity.

The enrichment effect of a single centrifuge is small, so they are linked in

cascades similarly to those for gaseous diffusion. In the centrifuge process, however, the

number of stages may be only 10 to 20, instead of a thousand or more for gaseous

diffusion. Passing through successive centrifuges of a cascade, the 235

U is gradually

enriched to the required assay - usually between 3 and 5% - and the depleted uranium is

reduced to 0.2 to 0.3% 235

U. Once started, a centrifuge runs for more than 10 years with

no maintenance.

Enrichment by centrifuge is energy efficient - consuming a tiny fraction of the

energy used by the older gaseous diffusion method.

The gas centrifuge process has been developed to a commercial level by

URENCO Group, developed from a joint Dutch, German and British initiative set up in

the 1970’s following the signing of the Treaty of Almelo. Since this time this Group has

been one of the leaders in the field of uranium enrichment by centrifuge. Today,

URENCO is a global supplier of uranium services, delivering more than 10% of the

worldwide enrichment requirements. On March 26, 1998, URENCO opened a new E23

enrichment plant at Capenhurst, UK. The centrifuge is more than twice as fast and an

order of magnitude longer than the early pilot plants and will have an output around 50

times as large as the earliest production machines [3].

Russia’s 10 M SWU per year enrichment capacity is also centrifuge-based. In

Japan, PNC and JNFL operate small centrifuge plants. The total production of Russia +

URENCO + JNFL is over 14 M SWU.

3. Laser Enrichment

Laser enrichment processes have been the focus of interest for some time. They

promise lower energy inputs, lower capital costs and lower tails assays, hence significant

economic advantages. Although they may become significant in time none of these

processes is yet ready for commercial use.

In 1985, the US Government chooses Atomic Vapor Laser Isotope Separation

(AVLIS) as a new technology to replace its gaseous diffusion plants as they reach the

end of their economic lives, as one of the most promising new technologies for

improving the economy of uranium enrichment.

Principle of the AVLIS-U Method:

In an AVLIS-U process a supersonic beam of atomic uranium vapor is produced.

The ground-state atoms are excited by a sequence of 3 visible photons from dye lasers

operating at wavelength λ1a, λ2, and λ3. The metastable atoms are first excited by a fourth

laser of wavelength λ1b, then by λ2 and λ3. The final photon absorbed by the uranium

atom produces an auto-ionizing state, which rapidly decays into a uranium ion and a free

electron. The auto-ionizing state is used because the optical excitation cross-section to

that state is much larger than the photo-ionization cross section to the continuum. The

narrow bandwidth in each of the selected transitions means that each of the laser

wavelengths must be accurate to within one part in 106 before enrichment appears.

Negligible excitation of 238

U atoms occurs when lasers are optimized to excite 235

U. Because of the high selectivity, enrichment from 0.2% to 3.2% can be achieved in a

single step; thus the atomic vapor process is particularly suitable for stripping 235

U from

depleted tails. Naturally, with appropriate plant design parameters, the AVLIS can also

enrich natural uranium [4].

The positively charged ions of 235

U are then attracted to a negatively charged plate

and collected as liquid metal. Guard plates are used to prevent unionized uranium atoms

of the beam from striking the product collector plates. The uranium vapor density must

be so high that appreciable charge exchange occurs during the acceleration of the 235

U to

the collecting plates. Care must also take to ensure that the sputtering of uranium atoms

from the collector plates does not seriously degrade the isotopic selectivity of the process.

The major building blocks of the AVLIS process are the separation chambers,

laser and optical systems, computer controls, and uranium handling system. Each process

is optimized to perform with low capital and operating costs.

The separation process uses finely tuned, high power lasers to tag the fissile

isotope of uranium, 235

U, by removing one of its electrons. Result a positive 235

U ion.

Collecting these ions as well as a portion of the feed material on charged plates forms the

product stream. Uranium depleted in 235

U forms the tails stream. This process takes place

in a vacuum chamber in which uranium is vaporized and exposed to the lasers. Both

streams are removed as small nuggets of solid uranium metal. Further chemical

processing and fabrication yields finished fuel for nuclear power reactors.

Ref.:

Nuclear Energy Data, OECD 2006, NEA No. 6100.

Uranium Enrichment, Nuclear Issues Briefing Paper 33, June 1999,

UIC Melbourne, Australia

P. Upson, CORE Issues Nr. 4, p., 5, Uranium Institute, Aug. Sept. 1998

Report on the Energy Research Advisory Board Study Group on Advanced

Isotope Separation, Report DOE/NMB-3012771, Nov, 1980.

ooo000ooo

THE STATUS OF R & D of AVLIS-U METHOD IN SOME COUNTRIES OF THE

WORLD.

1. AVLIS IN BRAZIL

The aim of the AVLIS Program at Instituto de Estudod Avançades, Centro

Tecnico Aeroespacial (IEAv/CTA), Sao Paolo, was to demonstrate the technical viability

of process using, as long as possible, resources available in Brazil. It implicates not only

on studying related processes but also on the development of critical associated

technology. The effort has been focused mainly on two actuation areas: copper vapor and

dye laser development and spectroscopy.

Laser development: The natural candidate that fills the requirements to deliver

tunable in the visible beams with high peak power at high repetition rates is CVL pumped

dye laser. In this case, both laser systems works in the Master Oscillator Power Amplifier

(MOPA) chain configuration.

The CVL development started in 1985, with a first prototype of a externally

heated copper bromide system that delivered about 100 mW at a repetition rate of 100

pps. From this system the work evolved to self heated true CVL’s, with maximum

average output power ranging from 5 W, from a compact air cooled system, to 40 W, for

conventional water cooled system.

Spectroscopy: In this field has been obtained the necessary experience to start the

investigation of the multi-frequency absorption in the atom of uranium, in order to get

convenient line sequences for the AVLIS process.

2. AVLIS IN CHINA

AVLIS in China started in 70’s and got rapid development since 1985. The

research ranges from uranium spectra, dynamic process of excitation and ionization of

uranium atoms under laser radiation field, extraction of ions from plasma, to R & D of

facilities such as copper vapor laser (CVL), tunable laser and electron beam heating and

separator.

Thanks to exploring in basic and development of facilities, a systematic scientific

data and some important experimental results have been reached. In three-step, three-

photon process, a high selective ionization was attained, and a macroscopic quantity of

enriched uranium sample has been collected which characterizes by its about 10%

concentration of 235

U and its collection rate of few mg/h. The experiments demonstrate

that an effective depletion down to 0.4% could be reached, together with a high

enrichment good extraction percentage of ions and excellent separation coefficient.

Basic research: The basic research activities relating to AVLIS are mainly

executed in some institute and universities and can be summarized as follow:

- Measurement and identification of energy levels, level life, branch ratio of

transition lines, cross section of absorption, isotope shift and hyperfine structure, Rydberg

state and auto-ionization state;

- Interaction between atomic system and strong laser radiation;

- Plasma of uranium atom induced by pulse laser and ion dynamics;

- Mathematical and physical model for ion extraction and collisions in AVLIS.

Facilities: The facilities necessary for AVLIS were prepared in two different

channels. A few were built through modifying the existed equipment, and the most were

specially designed, tested and manufactured. The facilities are mainly composed of lasers

and separator.

CVL: Two models, 20 W and 40 W CVL has been successfully made, the first

has been applied in separation experiments. The typical performance of CVL-20 is listed

below:

Output: 20 W; ratio of yellow/green: ~ 1:2; spot size: Φ = 30 mm; repetition rate:

6 kHz; pulse duration: ~ 30 ns; divergence: 1 mrad; tube length: 1.2 m; input power: 4

kW; conversion efficiency: 0.5%.

6 sets of CVL have been integrated to form oscillation-amplification chains: one

set is used for oscillator, and the rest 5 sets constitute two chains of amplification.

Separator: It is a special facility composed of 10 kW e-type electron beam gun,

uranium crucible, irradiation zone and collectors for thermal ions as well as laser induced

ions. All of these components are installed in one cylindrical vacuum chamber of 1x0.5

m. Uranium metal is put into one water-cooled crucible and heated through electron

beam. The atomic density at a distance of 10 cm over the crucible was of around 1012

/cm3.

Linear electron beam facility: Is composed of a vacuum chamber, linear gun,

magnetic coil and HV power supply: Typical performances: beam power: 20 kW; beam

size: 10x0.2 cm; power density: 10 kW/cm2; deflection radius: 5-10 cm; deflection angle:

180-2700.

The electron beam set up of power 50 kW has been put into operation from 1995.

Separation experiment: Experimental demonstration of three-color three-photon

process. Mass spectrometers measurement shows separation coefficients of 1,000-2,000

which demonstrates the high selectivity of AVLIS.

Applying four-color three-photon process instead of three-color three-photon

process, the total ionization probability can be increased by 30%.

Thanks to various measures, the concentration of samples reaches 30%, and the

depletion reduces to 0.4%. The total ionization probability is high up to 42% with

separation coefficient of 100.

Prospects: The development of a project of AVLIS and to establish one

comprehensive set-up with higher separation capacity.

Ref.:

1. Xu Pinfang et al.: Proc. of Fourth Workshop “Separation Phenomena in Liquids

and Gases”, Beijing, 1994, p.15.

2. Min Yan et al.: Proc. of Fourth Workshop “Separation Phenomena in Liquids

and Gases”, Beijing, 1994, p. 45.

3. AVLIS (SILVA) IN FRANCE

Objectives:

- Long term goal with a priority for a high performance process, available when

world stocks of enriched uranium are exhausted and aging enrichment plants have to be

shut down. In reach this goal, the French Atomic Energy Commission has focused since

1985 on AVLIS (SILVA), in agreement with the industrial operator, COGEMA.

- A network of co-operation supports the program with advanced technology

companies, particularly in the field of lasers, optical components, and materials, power

supplies.

Technical program:

- Basic research in each field with models developments adjusted through specific

and integrated experiments;

- A progressive development of components with specific facilities;

- Integrated experiments, especially with the pilot facility for separation

experiments;

- A general process model including operational and economical data.

Basic research:

- Uranium spectroscopy:

The multi-step photo ionization of uranium atoms implies to choose an irradiation

scheme and this choice is only possible if the following spectroscopic parameters and

specific effects are known: oscillator strength, isotope shift, hyperfine structure, lifetime,

auto-ionization spectrum, effect of electric and magnetic fields, effect of laser

polarizations, effect of multiphase processes upon selectivity.

Since the oscillator strengths determine the laser fluencies needed to efficient

atomic photo-ionization this parameter has been accurately measured. It enables to

choose the best wavelengths, selected by appropriate criterions.

- Light matter interaction: Several computing codes have been set up for coherent

interaction calculations (Bloch equations) in order to compute ionization yield and its

variation with the pulse energy density.

- Evaporation: Optimization of the uranium vaporization by an electron beam is

one of the keys of the SILVA process. Experiments are made on several benches of

different sizes including process scale seize (HORUS)

- Vapor flow: All vapor properties must be known in all regions where laser-

vapor interactions take place, as they take part in the process optimization. Monte-Carlo

computing codes have been developed in order to interpret the vapor measurements.

- Extraction: In order to choose the best extraction system, beside experimental

set-up, a Monte Carlo computation code applied to charged particles has been performed.

- Collecting flows: High temperature liquid metal collecting of enriched product

and waste tail was one of the most difficult task of this process from material and

technology points of view. It is also connected with various fundamental problems linked

to material and liquid material interaction (adhesion energy, wetting angles, chemical

interactions), and hydrodynamics (film, drop, stability under various orientations).

Technological development:

Uranium vaporization and management facilities:

CEA has quite numerous specific facilities, each of them devoted to one process

function. Most of them are located at Pierrelatte:

- HORUS - devoted to vaporization process optimization;

- Material test and behavior in conditions similar to those found in a separator was

the main aim of CORDY facility. This facility includes an evaporation apparatus and a

thermal control system. The experimentation concerns two kinds of tests: short duration

tests for checking solutions during their development (6-30 hours) and long duration tests

for high performance solutions (time higher than 100 hours);

- Uranium flow handling outside vapor deposition areas was studied on a special

facility called IRIS, which was used to generate uranium flows and generates drops and

films flows. Various shapes and slopes of guiding components was tested with this

facility;

- Technological studies for ions extraction and collection was especially

undertaken in ISABEL laboratory (Saclay) with two evaporation facilities. They are also

one of the pilot facility targets;

- Complete metal-liquid flows management systems are experimented in the

MAEVA facility, which was the higher sized SILVA evaporating facility;

- Facilities for material processing are associated with the previous facilities.

Laser development:

Nominal optical pumping systems utilize copper vapor lasers (CVL) developed by

CILAS Company. The first lasers produced (MNT 40) constitute the pumping system of

the pilot facility A2. The next (ASTER) include 100 W laser modules. For 100 W CVL,

individual running times are several thousand hours;

- Dye oscillation and amplifiers (developed by CEA) and studies on pumping

schemes using solid lasers;

- Laser chain, optical components and associated automations (developed by

CEA).

Pilot process facility A2:

The facility includes two main parts: the laser system named HERA and the

separator named ANDROMEDE. More than 90 test runs have been achieved, each run

corresponding to evaporation duration between 2 and 20 hours. The main test was the

followings ones:

- Production test: Production rate between 1 to 10 g/h of enriched uranium, with

uranium enrichment assay up to 5.5%;

- Design optimization for: extraction systems; matter-light interaction areas;

photon management.

The pilot extension to a higher size facility named ASTER is going on. It will

include a laser system with a power output about ten times higher than the present one

and a new separator named ALDEBARAN.

SILVA general schedule:

The SILVA program was periodically assessed from both scientific and the industrial

point of view. The general assessment includes several demonstrations related to each of

the main process functions (“DEMO”) as well as an evaluation of the economics.

General schedule: Basic research: 1985 – 1998; Process demonstration: 1985 –

1999; Technological development: 1987 – 2001; General assessment: 1996 – 1997;

Production demonstration: 1994 – 2005; Industrial deployment: 1997 – 2015. But, in

2000, SILVA Program has been abandoned.

Ref:

1. P.Rigny, Nuclear Europe, 3-4, 1990, p. 11

2. N. Camarcat et al., Proc. Third Workshop on Separation Phenomena in Liquid

and Gases (SPLG’92), Charlottesville, Virginia, 1992, p. 151.

4. AVLIS IN INDIA

Researches on uranium spectroscopy, at Bhabha Atomic Research Centre,

Bombay, as follows:

- Spectroscopic and thermal properties of uranium relevant to atomic schemes for

laser isotope separation (S.A. Ahmad et al., Report BARC 1091, 1980);

- Two-color three-step photo-ionization of uranium (V.K. Mago et al., 1987);

- Single color photo-ionization in uranium I, (V.K. Mago et al., 1987);

- New high-lying odd levels of U I in a two-color multi-photon ionization

spectrum (B.M. Suri et al., 1987);

- Two color multi-photon ionization spectroscopy of uranium from a meta-stable

state (P.N. Bajaj et al., 1988);

- Study of high-lying odd levels in U I by two-color photo ionization (V.K. Mago

et al., 1988);

- Energy levels, isotope shifts, hyperfine structures, lifetimes, transition

probabilities and other spectroscopic parameters of neutral uranium atom - update 1987

(S. A. Ahmad et al., Report BARC-1413, 1988);

- New odd-parity Rydberg and auto ionization levels in U I (A.K. Ray et al.,

1990);

- Resonantly enhanced single-color multiphase ionization of uranium atom (A.K.

Ray et al., 1992);

- Two-step single color photo ionization spectroscopy of uranium atom (V.K.

Mago et al., 1993).

5. AVLIS IN ITALY

Studies on the basic principles of AVLIS along with some spectroscopic aspects

of this method, some experimental data, and the uranium photo ionization process

(CNEN-CSN Casaccio Centre):

- AVLIS (P.Benetti et al., Report CNEN-RT/FI(80)16, 1980);

- The isotopic separation of uranium by laser method: spectroscopic aspects

(P.Benetti et al., Report CNEN-RT/FI(80)19, 1980).

6. AVLIS IN JAPAN

The R & D of AVLIS started when the ad-hoc committee of Japan Atomic Energy

Committee (AEC) stated that this process might have great potential as leading process

for future uranium enrichment, and issued a directive, which mandated that feasibility

study of this process can be completed as soon as possible. Since domestic demand for

enrichment is so limited, the process must be shown to be profitable on a moderate scale,

such as 1000 t SWU per year [1].

AVLIS study in JAERI [2].

JAERI has been working in this field for many years aiming at the basic data

acquisition for most adequate separation process especially for uranium isotopes. In 1984

the technological assessment program has been initiated based on data, which had been

obtained by that time.

For the accomplishment of AVLIS technology, both the development of tunable light

source and the development of separation process are inevitable. Tunable light source

with a broad tuning, narrow line width, high stability and high efficiency has been

developed. From point of view of industrial application, performances such as high

repetition rate, high average power and high efficiency are added to the quality above

mentioned. For these purposes, copper vapor lasers or excimer lasers has been used as a

pumping source, and high repetition rate dye laser has been developed as a tunable

source.

For the development of the AVLIS, energy levels, isotope shifts, hyperfine

structure and photo-absorption cross-section are the basic parameters, which would

determine the laser specification.

An AVLIS Test Plant has been studied by Laser Atomic Separation Research

Association of Japan (LASER-J), founded in 1987. The main objectives of LASER-J

was:

- To develop engineering-scale components;

- To construct the test plant;

- To conduct enrichment tests in the test plant.

The JAERI, which has been developed the AVLIS process since 1976, was in

charge of obtaining fundamental data regarding this process, in close cooperation with

the LASER-J.

When LASER-J started, components of the AVLIS system available in Japan

were limited to small scale ones, and the performance was not sufficient for an

engineering-scale test. It was first necessary to develop the hardware components, such as

lasers and electron beam guns, before to start the construction of the test plant. By now,

these hardware components of the AVLIS process were developed and the original goal

has been achieved. In addition, R & D on physical processes, such as uranium

vaporization, photo-ionization and ion recovery have also been undertaken. Construction

of the test plant was completed in May 1990 and the first stage test was to obtain physical

data for processes such as photo-ionization and ion recovery. For this purpose a small

separator with an array of instruments has been used.

A single unit of a CVL could produce a power of 122 W powers, an improved

CVL, 211 W, in case of MOPA, the system used in the test plant produced 318 W, and

with an improved CVL MOPA, 488 W. Repetition frequency of a CVL was of 5 kHz.

For the electron beam gun has been developed a 300 kW power linear type gun.

Acceleration voltage was as high as 50 kV and the width of the electron beam, less than 5

mm.

The separator system has been used at a temperature higher than the uranium

melting temperature of approximately 1,400 K. The uranium vapor density was of

1013

/cm3 at the stand distance of 50 cm, vaporization efficiency being of 3 - 4% [3].

The next stage will be the enrichment test, which will employ a larger separator.

The purpose of this test is to demonstrate the enriching capability of approximately 1 t

SWU/year, and also to obtain engineering data of the AVLIS components.

From a recent economical evaluation and optimization on an AVLIS plant of

1,500 t SWU/year, it would be sufficient a CVL unit power output of about 500 W [4].

If LASER-J could clear the process, it would go to the last step of building a set

of Demo Facility and making Enriching Demonstration Test thereof.

Studies on AVLIS are also performed at Institute of Laser Engineering (ILE),

Osaka University and Institute for Laser Technology (ILT), Osaka. These studies are

concentrated on developments of high-power CVL and dye laser, fundamental studies on

high-resolution spectroscopy and coherent dynamics of excitation and ionization of

atoms, resonant and near-resonant effects in laser beam propagation and studies on atom-

ion collision in the atomic vapor beam [5].

Ref.:

1. Y. Takashima, Proc. Intl. Symposium on Isotope Separation, Oct. 29 - Nov. 1,

Tokyo Japan, 1990, p. 127.

2. T. Arisawa, Proc. Intl. Symposium on Isotope Separation, Oct. 29 - Nov. 1,

Tokyo, Japan, 1990, p. 147.

3. Hamada, Proc. Intl. Symposium on Isotope Separation, Oct. 29-Nov.1, Tokyo,

Japan, 1990, p. 153.

4. N. Morioka, SPIE Vol. 1859, Laser Isotope Separation, 1993, p. 2.

5. Y. Izawa et al., Proc. Intl. Symposium on Isotope Separation, Oct. 29 - Nov. 1,

Tokyo, Japan, 1990, p. 233.

7. AVLIS IN ROMANIA

At NIIMT Cluj-Napoca has been performed a database on AVLIS. It contains 20

internal reports (in Romanian), as follows:

- Program Project for uranium enrichment by laser methods (G.Văsaru, M.Pascu,

Report ITIM-AVLIS-1, 15 Dec. 1987, 87 pp);

- Revised edition of the Program (G. Văsaru, M. Pascu, Report ITIM-AVLIS-2,

26 May 1988, 42 pp);

- Project for a laboratory scale plant for the study of selective photo-ionization of

the uranium vapor (G. Văsaru, I. Deac, Report ITIM-AVLIS-3, 20 Oct. 1988, 129 pp);

- Isotope separation by AVLIS method (G. Văsaru, Report ITIM-AVLIS-4, 1

March 1989, 79 pp);

- The components and the characteristics of a laser spectroscopy plant for the

study of selective photo-ionization of atomic vapor (I. Deac, G. Văsaru, A. Romanţan, I.

Trişcă, Report ITIM-AVLIS-5, 15 April 1989, 45 pp);

- Thermodynamic of the vaporization of the metallic uranium (G. Văsaru, Report

ITIM-AVLIS-6, 15 December 1989, 128 pp);

- High-lying odd levels of U I in the range 34000-43000 cm-1

identified by a

single-color three-photon ionization technique (G. Văsaru, Report ITIM-AVLIS-7, 15

March 1990, 14 pp);

- High lying odd levels in U I by two-color three-photon photo-ionization in the

range 34000 - 37000 cm-1

and 39900 - 41600 cm-1

respectively (G. Văsaru, Report ITIM-

AVLIS-8, 15 November 1990, 56 pp);

- Energy levels of neutral atomic uranium (U I) (G. Văsaru, Report ITIM-AVLIS-

9, 5 August 1991, 164 pp);

- Isotope shifts and hyperfine structure of neutral uranium atom (U I) (G. Văsaru,

Report ITIM-AVLIS-10, 10 November 1991, 98 pp);

- Transition probabilities, oscillator strengths, branching ratio, and absorption

cross-sections of neutral uranium atom (U I). Lifetimes of the odd and even levels of U I

(G.Văsaru, Report ITIM-AVLIS-11, 5 November 1992, 106 pp);

- Thermal properties of uranium, (G. Văsaru, Report ITIM-AVLIS-12, 15 May

1993, 48 pp);

- Ionization processes of uranium atom (G. Văsaru, Report ITIM-AVLIS-13, 15

October 1993, 46 pp);

- Copper vapor lasers, (G.Văsaru, Report ITIM-AVLIS-14, 1 April 1994, 98 pp);

- Dye for lasers. Photo-physical and photo-chemical properties (G. Văsaru, Report

AVLIS-ITIM-15, 1 October 1994, 63 pp);

- Laser systems for the uranium enrichment (G. Văsaru, Report ITIM-AVLIS-16,

15 December 1994, 91 pp);

- Physics of the vaporization process of metallic uranium (G. Văsaru, Report

ITIM-AVLIS-17, 10 May 1995, 67 pp);

- Laser-atomic uranium vapor interaction. The selective resonant multi-photon

photo-ionization process (G. Văsaru, Report ITIM-AVLIS-18, 10 October 1995, 43 pp);

- Laser systems for AVLIS-U. I. Kinetics of CVL. II. Physical and technological

conditions for laser systems of AVLIS-U (G. Văsaru, Report ITIM-AVLIS-19, 30

November 1995, 65 pp);

- Uranium vaporization system for AVLIS-U (G. Văsaru, Report ITIM-AVLIS-

20, 15 December 1995, 13 pp).

8. AVLIS IN RUSSIA

The scientific activity of the Institute of Molecular Physics (Moscow) included

researches on laser methods for isotope separation.

AVLIS needs about 6 eV to ionize the uranium atom. The IMP laboratory

separation facility is based on using copper vapors lasers (CVL) pumped dye lasers (DL)

to generate radiation with needed wavelengths. The investigated process scheme involves

two steps of successive photo-excitation and photo-ionization. Recent experiments have

demonstrated rather promising results on laser equipment improvement, optical scheme

optimization, evaporating set-up and collection method development.

It has been shown that production of low-enriched (3-5 %) or highly enriched

(90%) uranium-235 is industrially feasible. The pilot version of industrial AVLIS module

for uranium isotope separation is now under development. The experiments on the

module will give the information for evaluation commercial potential for the industrial

application of AVLIS technology. It's generally supposed that this technology is

preferable in case of using low-enriched starting raw materials.

It seems reasonable to use the AVLIS method for separation and commercial

production some expensive stable isotopes, which cannot be separated by the centrifuge

method. The Institute has achieved considerable progress in development of the AVLIS

method for isotopes separation of Nd, Gd, Zr, Yt, and some other elements. Another

interesting field of AVLIS method application is production of isotope mixture depleted

with definite undesirable isotope [1].

Ref.:

1. V.A. Mishin, General Physics Institute of RAS, 38 Vavilova Str., Moscow,

Russia.

9. AVLIS IN UK

Work on methods of enriching isotopes using laser techniques started in 1974

within UKAEA. Both the molecular and atomic route was studied. In 1983 a decision

was taken to concentrate on the atomic route (AVLIS) as offering the greater economic

potential. In 1986 a collaborative agreement on AVLIS was entered into by BNFL and

the UKAEA.

The program of work has included:

- Theoretical considerations of photon-atom interaction, including the effects of

HFS and magnetic field (Zeeman effect) and cross sections for excitation transfer and

charge exchange;

- Experimental work to find theoretically favorable transitions between the levels

in the atom and to measure relevant transition parameters using initially, low density

uranium vapor;

- Development of techniques for the precision tuning and stabilization of suitable

lasers, obtaining the required bandwidth, and amplifying light to required power;

- Materials and technology related to high-density vapor production;

- Theoretical and experimental work on efficient separation of selectively

generated ions from a vapor stream;

- A watching brief on laser development, with active initiation of development for

specific purposes.

Later, the UKAEA and BNFL moving towards integrated development. It was

envisaged there would be five main areas for development:

- Vapor production using electron beam guns;

- Selective ionization of 235

U;

- Separation and collection of product and tails;

- Engineering of laser facility.

BNFL has installed a test facility for evaporating uranium, which, together with

other equipment will be used for studying uranium vapor properties, electron beam gun

development and feed system development. The CVL, which provide the light power

needed for the process, was planned to be developed for higher power and longer life.

Spectroscopic work will be continued by UKAEA with the objective of finding

energy levels, which would enable 235

U to be ionized more efficiently. In addition, work

will be carried out on the light transmission characteristics of the envisaged systems.

Techniques for the separation of the ions from uncharged atoms are being

explored by the UKAEA. Application of these techniques to uranium vapor, and

subsequent problems of product and tails collection and handling are also being

investigated.

In addition to the theoretical and experimental work re-estimation of the plant

costs of AVLIS which take into account of improving knowledge of key parameters such

as transition rates, process and geometric efficiencies, process modeling and hardware-

related costs are taken into consideration. Comparison is then made with the URENCO

future centrifuge costs (BNFL being one of the partners). At present the result of this

comparison is that BNFL continue this AVLIS research and development program. The

overall target is that BNFL should achieve technical competence in this area such that

consideration of the construction of a laser enrichment plant. This target is entirely

consistent with BNFL and URENCO aim of progressive technological advance. The plan

to exploit AVLIS methods jointly with the partners of URENCO remains; discussions of

the economic exploitation advantages have been held and a collaborative program is

being pursued.

Ref.:

1. V.S. Krocker, P.F.P. Roberts, Atom, 363, p.1, 1987

2. Whitehead, Preprint, USCEA Intl. Enrichment Conf, Monterey, Cf., June 18 -

21, 1989.

10. AVLIS IN USA

The USEC - AVLIS Program:

One of the key aspects in assuring that nuclear energy option remains

economically competitive for the future is the provision of an economic, reliable supply

of fuel. Nuclear fuel costs for a power plant include natural uranium, conversion services,

enrichment services, fuel fabrication and transportation. Of these fuel cost components,

one of the largest is enrichment service. The AVLIS technology option for enriching

uranium has been considered to provide the potential for stable or declining nuclear fuel

costs in the decades ahead.

The US has developed the AVLIS process, both to assure availability of the

nuclear option and as a key element of a strategy to ensure US competitiveness in the

uranium enrichment business in the twenty-first century. This technology, which uses the

selective laser excitation and ionization to separate the isotopes of uranium, has rapidly

advanced by US AVLIS team providing a database to support deployment of the

technology as required by market conditions.

AVLIS has high potential for achieving mature production costs that are $20 to

$50 per SWU, lower than production costs from gaseous diffusion, and that are lower

than any other process known today. As a result of this significant economic promise, all

major participants in the international enrichment business are developing AVLIS [1].

In the mid-1970s, DOE began R & D of a new generation of technology to produce

enriched uranium for civilian energy production. One technology involved the use of

high-energy lasers to separate vaporized 235

U from 238

U and process it into fuel.

The AVLIS technology was designed to operate on a smaller scale than existing

gaseous diffusion plants and produce a cheaper product. In 1988, DOE began running

“commercial scale” uranium enrichment tests using AVLIS facilities built at LLNL in

California.

AVLIS was a program of the largely self-financing US Enrichment Corporation

(USEC), created by Congress in 1992. In 1994, the US Enrichment Corporation

announced it would proceed with commercial development of a $2 billion AVLIS

program, despite debate over whether the Livermore AVLIS experiments had proven its

commercial viability.

AVLIS team: AVLIS is an advanced uranium isotope separation process under

development by USEC. The laser-based technology has the potential to be the most

economic method of enriching uranium fuel for commercial nuclear power plants. A full-

scale system has been tested at the LLNL.

USEC planned to initiate commercialization of AVLIS in the near future.

Working with USEC on team AVLIS are: Allied Signal Corp., BWX Technologies,

Bechtel National Inc., Cameco Corp., Duke Engineering Inc., GE Nuclear Energy,

Lockheed Martin Inc., Parsons Engineering and LLNL.

AVLIS development: The basic AVLIS concept development of the laser-based

AVLIS enrichment technology has been under way at LLNL since the mid - 70s. In 1990

the DOE transferred the proprietary rights to AVLIS to USEC in the largest transfer of

technology ever in the US by the DOE.

AVLIS used a system of high-powered lasers, tuned for a specific wavelength, to

ionize only 235

U isotope of uranium. The ionized 235

U atoms are positively charged and

are attracted to negatively charge collecting plates. The recovered enriched uranium alloy

is sent to a conversion facility to be charged into uranium oxide pellets which are loaded

into metal fuel assemblies for be used as fuel at nuclear power plants.

Separator process: The AVLIS process begins with uranium alloy being fed into a

large separator vessel in the form of solid rods. The separator is a vacuum chamber in

which a high-energy electron beam vaporizes the uranium rods. Light from a precisely

tuned laser selectively ionizes the 235

U atoms in the vaporized uranium, giving them a

positive electrical charge while leaving the undesired 238

U isotopes neutral. As the

isotopic mixture moves through the separator, the positively charged ions are attracted to

a negatively charged plate. The ions collect on the plate as enriched uranium and then

flow into a collector. Now are three full-scale separators one which is capable of

operation. Six production lines would be used in a commercial AVLIS plant with each

line consisting of 14 separators.

Pump lasers: Solid-state lasers are used to convert electricity into light energy for

the process lasers. The solid-state lasers convert electricity into green light, which is used

to energize (pump) the process lasers. The light is routed to process through fiber optics.

The small bore CVL is at plant size today (40 W) and the dye system can achieve

plant requirements by joining existing units. Optimally, the maximum power for large

bore CVL’s will be scaled from 300 W to 700-1000 W.

The AVLIS CVL’s are joined together in master oscillator power amplifier

(MOPA) chains that supply laser power to the dye lasers. Six of these chains are

organized into a corridor that has grown capacity from a few hundred W to 2500 W [1].

Process lasers: A process laser provides the precise frequencies of light to ionize

uranium vapor. Since 235

U isotope has a slightly different absorption spectrum than the 238

U, it will absorb the laser light while the 238

U will not. The absorbed energy from the

laser light will excite, or energize, the 235

U atom knocking off an electron and giving it a

positive charge. The process laser is often referred to, as a dye laser because it uses a dye

to produce the specific wavelengths required ionizing the uranium vapor.

The control room: The AVLIS process operated from the control room. This high-

tech facility continuously monitors key characteristics of the AVLIS process such as laser

beam shape, wavelength and pulse frequency, separator operating temperatures and

pressures, and other essential components. One control room in the plant can be used to

run both the laser and separator systems for two production lines. A commercial-size

enrichment plant would require six production lines with three control rooms.

Feedstock: The AVLIS feed material consists of metal rods of uranium-iron alloy.

USEC is working with the Cameco Company, a leading uranium producer based in

Canada, to develop a cost-effective process for converting uranium ore concentrate to

uranium-iron feed rods for use by a commercial AVLIS plant.

Product: The enriched uranium metal produced by AVLIS is in the form of small

nuggets. The AVLIS product is sent to a conversion facility, which purifies and converts

the uranium alloy into uranium dioxide, which fuels most of the world’s commercial

nuclear power plants. The purified uranium dioxide can then be made into pellets and

inserted into metal rods for use as reactor fuel. This conversion process is being

developed by USEC jointly with GE Nuclear Energy.

AVLIS enrichment plant: Was an architectural concept of a commercial-size

AVLIS uranium enrichment plant, capable of producing up to 8.7 M SWU per year. The

facility would occupy one-tenth the space today’s uranium enrichment plants employing

gaseous diffusion technology and consume only 5-10% of the electricity of a gaseous

diffusion plant.

The modular design of AVLIS allows a flexible deployment, enabling capacity to

be added in enrichments to meet market demands.

Commercialization: USEC was expected to deploy commercial-size AVLIS feed

production, enrichment and product conversion plants early in the 21st century [2]

Ref.:

1. J.S. Longenecker, N. Haberman, Nuclear Tech. Int., p. 99, 1987.

2.USEC.WWW, 1998 .

Final remarques:

The possible introduction of AVLIS-U as a competitive industry constitutes, at

this time, an open problem.

In June 10, 1999, USEC Inc. announced that it is suspending further development

of its AVLIS enrichment technology. USEC’s Board of Directors and management

reached this decision after a comprehensive review of operating and economic factors. In

making the announcement, William H. Timbers, Jr., President and Chief Executive

Officer of USEC Inc., said, "We commend Lawrence Livermore National Laboratory

(LLNL) for their concerted research and development efforts on AVLIS. However, we

have reexamined the AVLIS technology, performance, prospects, risks and growing

financial requirements as well as the economic impact of competitive marketplace

dynamics. We now have enough data to conclude that the returns are not sufficient to

outweigh the risks and ongoing capital expenditures necessary to develop and construct

an AVLIS plant."

Later, CEA France stopped also the SILVA R&D Program. And very possible

other countries…