Avlis u r&d in the world


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Avlis u r&d in the world

  1. 1. 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 235U 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, 235U and 238U. The production of energy in nuclear reactors is from the “fission” or splitting of the 235U atoms, a process which releases energy in the form of heat. 235U is the main fissile isotope of uranium. Natural uranium contains 0.72 % of the 235U isotope. The remaining 99.3 % is mostly the 238U isotope, which does not contribute directly to the fission process. 235 U and 238U are chemical identical, but differ in their physical properties, particularly their mass. The nucleus of the 235U atom contains 92 protons and 143 neutrons, giving an atomic mass of 235 units. The 238U nucleus also has 92 protons and 146 neutrons - three more than 235U, and therefore has an atomic mass of 238 units.
  2. 2. The difference in mass between 235U and 238U allows the isotopes to be separated and makes it possible to increase or “enrich” the percentage of 235U. 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 235U than natural is required. The enrichment process produces this higher concentration, typically between 3.5% and 4.5% 235U, by removing a large part of the 238U (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 57 0C, 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 235U 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.
  3. 3. 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 235U 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% 235U 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 235U isotope from the heavier 238U. The gas is forced through a series of porous membranes with microscopic openings. Because the 235U molecules are lighter than 238U molecules they move faster and have a slightly better chance of passing through the pores in the membrane. The UF 6, which diffuses through the membrane, is thus slightly enriched, while the gas, which did not pass through, is depleted in 235U. 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 238U closer to the wall of the rotor than the lighter 235U. The gas closer to the wall becomes depleted in 235U
  4. 4. whereas the gas nearer the rotor axis is enriched in 235U. The gas flowing within the rotor can be produced by a temperature gradient over the length of the centrifuge. UF6 depleted in 235U flows upwards adjacent to the rotor wall, whilst UF6 enriched in 235U 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 235U is gradually enriched to the required assay - usually between 3 and 5% - and the depleted uranium is reduced to 0.2 to 0.3% 235U. 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
  5. 5. 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 238U 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 235U from depleted tails. Naturally, with appropriate plant design parameters, the AVLIS can also enrich natural uranium [4]. The positively charged ions of 235U 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 235U 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, 235U, by removing one of its electrons. Result a positive 235U ion. Collecting these ions as well as a portion of the feed material on charged plates forms the product stream. Uranium depleted in 235U 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.
  6. 6. 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, threephoton 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 235U 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%.
  7. 7. 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;
  8. 8. - 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 laservapor 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;
  9. 9. - 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);
  10. 10. - 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.
  11. 11. 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 atomion collision in the atomic vapor beam [5].
  12. 12. 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 ITIMAVLIS-8, 15 November 1990, 56 pp); - Energy levels of neutral atomic uranium (U I) (G. Văsaru, Report ITIM-AVLIS9, 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);
  13. 13. - 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-AVLIS20, 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.
  14. 14. 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 235U; - 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 235U 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 hardwarerelated 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
  15. 15. 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 235U from 238U 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
  16. 16. economic method of enriching uranium fuel for commercial nuclear power plants. A fullscale 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 235U isotope of uranium. The ionized 235U 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 235U atoms in the vaporized uranium, giving them a positive electrical charge while leaving the undesired 238U 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 235U isotope has a slightly different absorption spectrum than the 238 U, it will absorb the laser light while the 238U will not. The absorbed energy from the laser light will excite, or energize, the 235U 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 hightech 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.
  17. 17. 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…