|No 54 Em Português||
Ministério do Desnvolvimanto, indústria e Comércio Exterior
Ministério da ciência e Tecnologia
e&e : the Organization and the Periodical
The e&e Organization
Ttext for Discussion
URANIUM ENRICHMENT IN BRAZIL
Development of ultra-centrifugation technology
Othon Luiz Pinheiro da Silva
André Luis Ferreira Marques
After the initiatives of the Admiral Álvaro Alberto in the 1950s
concerning scientific research in Brazil in the nuclear sector, the Brazilian government had decided to invest resources at the beginning of the 1970s so that the country would have full capacity in the nuclear fuel cycle, fabrication of research and power reactors and finally in the reprocessing of the nuclear fuel used in the reactors. Such line of action aimed at providing the means necessary for strengthening our energy matrix using the existing natural resources (uranium and thorium mines whose reserves are among the largest of the world) for generating electric energy, within the particular approach of the developing program at that time.
In this context, it was foreseen the construction of several nuclear plants, around 56 units of the PWR (pressurized water reactor) type. Besides that, there was the strategic need of raising the Brazilian nuclear sector to the same level as that of other countries of the same size. Therefore, several measures were taken such as personnel training abroad and creation and strengthening of research institutes.
In this context, the Brazil-Germany Agreement in the nuclear sector was signed in 1975, within which German technology and other means would be transferred regarding the objectives mentioned above. Some items from this Agreement are the NUCLEP (Nuclebrás Equipamentos Pesados, in Itaguaí) reactor factory and Angra 2 Nuclear Power Plant itself, both built in the Rio de Janeiro state. In the nuclear fuel cycle, the technology transfer initially foreseen for uranium enrichment was ultra-centrifugation, which was mastered by the Germans for some years. However, due to international pressure, transfer of this technology to Brazil was denied and the jet-nozzle alternative, which was at the laboratory development stage, was offered. The reason for enriching uranium (increase its U235 content (relative to the natural occurrence) is because the probability of fission of this chemical element is much larger than in other chemical elements (about a thousand times).
At that time (end of the seventies), the Brazilian Navy (BN) had identified the need of having nuclear-propelled submarines, since these have a large dissuasive power and tactical advantage due to their discretion (they can remain submerged for a long time, making it difficult to be detected by surface ships). Because of the large energy density of its nuclear reactor, besides generating oxygen for its crew, the nuclear submarine can maintain high velocities during a long time allowing for extended territorial seacoast to be patrolled.
It is important to mention that a country that intends to operate nuclear submarines must provide all the necessary infrastructure (support base, fabrication and maintenance of the main components, among other things), because depending on the supply of vital components such as the naval reactor core is a discarded hypothesis due to the logistic vulnerability inherent to this type of dependency. In a not very long past we have observed this situation in the English veto regarding supplies to Argentina during the Malvinas (Falkland) War which has much reduced the efficiency of its Navy.
Among the different uranium enrichment methods (isotopic separation of U235,higher than the natural 0.7% natural value) only two processes are attractive from the industrial scale point of view: gaseous diffusion and ultra-centrifugation.
In the first one, uranium hexafluorine (UF6) gas is compressed through micro-porous membranes associated in series so that U238 will be separated from U235, the latter being more important regarding fission by neutrons. In ultra-centrifugation, the separation is carried out by the centrifuge force that is exerted on the UF6 particles whose principle is identical to that we are familiar with, concentrating U238 in a more external region that of U235, because the former is only about 1% heavier than the latter. That is the reason why the term “ultra” centrifugation (it operates at very high tangential velocities) is used to separate the two elements whose masses are very close to each other.
All other processes (e.g. electromagnetic, thermal columns) do not come close in terms of efficiency (at least 100 times) to ultra-centrifugation both concerning high energy consumption and/or large quantity of chemical effluents. Furthermore, there are processes that still are at the laboratorial phase as the laser enrichment. The jet-nozzle followed the same path: it is not industrially efficient.
Considering the denial regarding ultra-centrifuge technology import and the German transfer of a method that is not efficient at the industrial scale, it was decided that uranium enrichment in Brazil would be carried out by the Navy, using ultra-centrifugation, and by the Air Force (FAB), using laser, also at the end of the 1970s. It is important to mention that concerning the fuel cycle activities, uranium enrichment is the most technologically complex because it deals with very strict technical requirements regarding material selection and development, dimensional quality control, several electromechanical fabrication methods, among other aspects.
Ultra-centrifugation technology was developed in Germany during the Second World War by the staff of Prof. Zippe. Subsequently, the Russians have improved it with the help of Prof. Zippe himself and some of his collaborators. Presently less than 10 countries master this technology and among them, Brazil.
Figure 1 –Ultra-centrifuge scheme
In Figure 1, based on open sources and in general lines, it is presented a scheme of an ultra-centrifuge with the following parts :
d) Distribuidor e coletores de hexafluoreto de Urânio
In order to obtain the above mentioned isotopic separation it is necessary to operate with the highest possible rotation because the centrifuge force is proportional to the square of the angular velocity. However, one must respect the limit of RESISTÊNCIA DOS MATERIAIS (due to high mechanical strains or ESFORÇOS SOLICITANTES) and decrease energy consumption as much as possible. From this point of view, the lower the friction among the different parts the better the centrifuge yield.
In order to decrease friction vacuum is made between the case and the rotor, attenuating at the same time friction in the bearings. Once a centrifuge model is developed and approved, some of them are fabricated and assembled in series and parallel which are then called “uranium enrichment cascade” as a function of the boundary conditions of the project (quantity of mass and enrichment content). For Angra 1 or 2 reactors type it is necessary tons of UF6 with enrichment between 3 and 5%.
For example, parallel assembling aims at producing large mass but with low enrichment. On the other hand, the series assembling yields a small mass quantity but high enrichment content. Figure 2 presents a cascade assembling where the product of a certain stage feeds the following one while its “waste” returns to feed the previous one. It is observed that along the process UF6 is recycled: the yield of the enrichment stage is directed to the following one while the “waste” of the initial stage returns to feed the previous one.
Like in high performance applications (e.g. aerospace, biomedicine), materials that are potentially applicable in isotope separation systems must have high mechanical resistance, low density and resistance to corrosive medium (as that of UF6). Concerning mechanical resistance, for didactical effects, the associated mechanical strain is considered to be proportional to the square of the elasticity module (N/m2) divided by the density (kg/m3). Examining the open references, it is evident in Table 1 that polymer materials are extremely desirable in spite of the fact that it is hard to design and fabricate them since they have large “anisotropy”, i.e., large properties variation according to the direction. Metallic materials such as maraging steels or titanium alloys are also attractive but their density considerably decreases the performance when compared with composites.
Table 1 – Comparison of the materials properties 
Mechanical Resistance Aluminum alloy = strain resistance = 500 MPa;
Aluminum alloy density= 2800 kg/m3
Maximum peripheral velocity (Al) = 425 m/s
Figure 2 – Cascade scheme
F – Feed (with natural uranium)
P – Product (or enriched part)
W – Waste (or depleted part)
The separation power of a centrifuge is measured in kg of SWU or Separative Work Unit per year (kg SWU/year). This unit is defined in the theory of cascade medium operation where it is used the mathematical concept of value function. In general lines, Equation 1 defines this power .
Separative Power ~ L x rotation n x D x (DM) 2 x Temp –2 (1)
L – vertical length
D – UF6 diffusion coefficient
DM –mass difference of isotopes (238U – 235U)
Temp – UF6 temperature
n – coefficient between 4 and 5.
As can be noticed, the larger the rotation the larger the separative power, as well as the larger rotor’s length and the lower the UF6 temperature. However, the triple point of this gas is very close to the normal temperature and pressure conditions and therefore it can easily de-sublimate (change from the gaseous state to the solid one) in these conditions and this may cause a blockage in the piping. Furthermore, uranium hexafluoride reacts with the humidity when it is in contact with air, producing fluoridic acid (HF) which is hazardous.
Once the main technical aspects of ultra-centrifuges development had been identified, the BN nuclear program has built all the laboratorial and industrial means necessary for developing and implementing this technology in Brazil. As the BN always does, the best talents and means available in Brazil were mobilized and teams in several sectors, namely mechanical, electronic, processes, material engineering, among others, were gathered. For the management of the material and human resources, the BN has created the Coordination of Special Projects (COPESP in Portuguese) in 1986, later on renamed Navy Technological Center (CTMSP in Portuguese) in São Paulo, at the campus of the São Paulo University (USP) and the ARAMAR Experimental Center (CEA) at the municipality of Iperó/SP. The state of São Paulo was chosen because it has the best industrial park as well as first rate engineering schools and research centers.
As is well known, transport phenomena (gas dynamics) and the heat transfers associated with fluids that flow at very high velocity are non linear and it is very difficult to simulate them using pure analytical and numeric means, even with the large capacity computational resources now available.
Therefore, several laboratories were built to develop ultra-centrifuge technology, and they were associated with many research institutes, enterprises and universities all over the country. Briefly, such development needs experiments and simulations in several scales (including the natural one or 1:1) so that a component or system aiming at industrial production and assembly must be approved. In the several partnerships made, the continuous optimization of the different expertise was aimed for, and it was incorporated to the machines the most advanced knowledge in the different technological areas concerning advanced materials, fabrication techniques, control mesh and electronics.
The result of this national effort can be seen in the two BN enrichment installations at CEA: the Uranium Enrichment Laboratory (LEI) and the Industrial Enrichment Demonstration Plant (USIDE) that operate since the end of the 1980s and beginning of the 1990s. More recently, the effort regarding ultra-centrifuge technology development is materialized in the contract between the BN and the Industrias Nucleares do Brasil (INB) relative to the installation of uranium enrichment cascades at the Resende/RJ unit for the fabrication of nuclear fuel for the Angra 1 and 2 nuclear plants. Other information about the uranium enrichment process in the world can be found in references 4 and 5.
The development of uranium ultra-centrifugation technology is a milestone in the Brazilian technological history. From the initial interest of the Admiral Álvaro Alberto, who tried to bring centrifuges from the post-war Germany, meeting strong external resistance, it was possible, with the effort, dedication, creativity and obstinacy of the Brazilian technicians and engineers along 15 years, to conceive and improve a series of machines for the production of material to be used in nuclear fuel, the peaceful use of nuclear energy, as established in our Federal Constitution.
The decision made at the end of 1970s regarding the ultra-centrifuge choice was right since it is a very efficient method, in terms of energy consumption, and modular, operating with standardized units and organized in series and parallel assembling that guaranties good operational flexibility. This is corroborated by the fact that the United States and France that used gaseous diffusion have changed to the ultra-centrifugation process.
As a result of the ultra-centrifugation technology development, the production of high-resistant steels has been developed in Brazil as well as special valves that operate with corrosive substances. Equally important, several satellite and missile components have been fabricated using CEA’s laboratorial and industrial resources, originally created for the nuclear program development conducted by the Navy. Recently another expressive result of the co-operative effort of the BN, FAB, universities and research institutes is the production of high performance carbon fiber in the country through an agreement with the Project Financing (FINEP), an organ of the Ministry of Science and Technology (MCT).
The success achieved by the enrichment project, under the CTMSP’s motto “Technology of one’s own is Independence”, is an example of the path that gives credit to the potential of Brazilians and using it to overcome the hindrances to our development is, in some cases, the only viable way, and in many others, the way that will guarantee a larger autonomy and independence to future generations.
1 – Villani, S. – “Uranium Enrichment” – Topics in Applied Physics – Volume 35 – Springer Verlag – 1979.
2 – Green, R. – “Back to the future” – Nuclear Engineering International – Sept 2002.
3 - Upson, P – “Centrifuge Technology: the future for enrichment” – World Nuclear Association Annual Symposium – London – 2001.
4 – www.usec.com
5 – www.urenco.com
 Vice-Admiral (Naval Engineer – Ref.), was the formulator and coordinator of the Navy’s Nuclear Program 1979-1994. Presently he is the Director-President of ELETRONUCLEAR.
 Commander (Naval Engineer), is the Coordinator of the Isotopic Separation Program of the Navy’s Technological Center in São Paulo - CTMSP
Graphic Edition/Edição Gráfica:
Tuesday, 11 November 2008.