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Fission Target Design and Integration of Neutron Converter for Eurisol-Ds Project J

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Fission Target Design and Integration of Neutron Converter for Eurisol-Ds Project J FISSION TARGET DESIGN AND INTEGRATION OF NEUTRON CONVERTER FOR EURISOL-DS PROJECT J. Bermudez, O. Alyakrinskiy, M. Barbui, L.B. Tecchio Laboratori Nazionali di Legnaro I.N.F.N. Viale dell'Università 2, 35020 Legnaro (PADOVA) ITALY. F. Negoita, L. Serbina, E. Udup “Horia Hulubei” - National Institute for Physics and Nuclear Engineering (IFIN-HH) Str. Atomistilor 407, P.O. Box MG-6, 077125 Bucharest-Magurele, Romania Abstract A study of a new fission target for EURISOL-DS is presented with a detailed description of the target. Calculations of several configurations were done using Monte Carlo code FLUKA aimed to obtaining 1015 fissions/s on single target. In Eurisol, neutrons inducing the fission reactions are produced by a proton beam 1GeV- 4mA interacting with a mercury converter. The target configuration was customized to gain fission yield from the large amount of low energy neutrons produced by the Hg converter. To this purpose, the fissile material is composed by discs of 238- Uranium carbide enriched with 15 g of 235-U. Studies of several geometries were done in order to define the shape and composition of uranium target, taking into account the mechanical and space constraints. Furthermore different configurations of reflector and moderators materials were considered to increase the thermal neutrons confined around the target and so enhance the performance of the system. The final configuration consists in six modular target containers inclined respect to the vertical, containing the fission targets, ion sources with RIB extraction and steering elements, cryopanels and devices for handling. The cross sections of the modules are rectangular with 10 mm coating of water for cooling. The UCx fission targets are cylindrical dwelling in a graphite holder inside a tantalum container. The analysis includes also the shielding, reflectors, isolators, and connections. A detailed study of reflector materials, moderator materials, fission target geometry, fissile materials and containers were done. Neutronic calculations, fission rates, energy deposited on main parts of the system and neutron distribution results are reported. 1 I. INTRODUCTON It has been asserted that the increase of primary beam intensity does not necessarily lead to an increase to intensity of the secondary Radioactive Ion Beams (RIB) [1]. In order to profit of the production potential of a 1 GeV proton beam of MW power the concept of charged particle to neutron converter technique has been chosen. Here, the power of the primary proton beam is dissipated and disposed in a primary cooled target (i.e. mercury converter) and the resultant neutron flux induces fission products in a thick ISOL target (fission target) without destruction of the latter by overheating. Designing this converter and the surrounding fission targets is one of the main scope of this project. Converter and fission targets design is conceived in a modular way so that its individual parts can be rapidly replaced and serviced by means of remote handling. Referring to the concept previously adopted by the PIAFE [2] and MAFF [3] planned facilities, profiting of the fact that several technological aspects were developed within such projects a possibly solution based on the same concept has been proposed for the EURISOL fission target assembly [4]. Conceptually, a target filled with 235U is inserted, through a channel created in the shielding, close to the neutron converter in a position where the neutron flux shown has the maximum intensity. The neutron flux is moderated in energy to optimize the number of fissions induced in the target. Six fission targets are foreseen to be installed around the converter: 2 above the neutron converter, 2 on the right side and 2 to the left side. The spatial mapping of the neutron flux gives the placement of the fission target elements. The vertical placement of these assemblies (see Fig. 1) has several advantages, one of them is that the alignment of ion beam optics elements is better preserved in operation, which is essential for good extraction and transport of secondary beam. The proton beam parameters correspond to 4 MW power delivered to the mercury converter, with a maximum current of 4 mA. Combined iron+concrete shielding with a total thickness of about 6 meters has to be considered. Fig. 1. Schematically arrangement of the six fission targets around the neutron converter. Each target is placed inside a vacuum tube. In the right figure, only one of the rectangular vacuum tube in which the targets are placed is shown. 2 Loading and unloading the fission targets is accomplished by a mobile tube mounted on the top of a fix tube; the fission target and all other elements in the fixed tube are pull into the mobile one and moved into a hot cell where mounting/dismounting of the fission targets (as well as of other elements) can be performed under visual control. All components are placed inside a vacuum tube embedded in heavy concrete shielding. A double wall tube is proposed to provide a coupling water flow in order to evacuate the heat produced by fission in target and keep at least the upper part of tube at normal temperatures. Large surface cryogenic-panels are distributed inside the vacuum tube to maintain a good quality vacuum and trapping the radioactivity in a confined region. Two geometries of the vacuum tube, cylindrical and rectangular, were considered in calculations. The rectangular shape was adopted, since the fission targets can be placed closer to the converter in higher neutron flux. It is considered more suitable than cylindrical shape also from the safety point of view allowing taking advantage of the available space to separate the services (water cooled high voltage and high current bars) from the target and the RIB line. The modification of the tube cross section provides the space for the inclusion of a moderator material in the nearest area around the target and could represent a profit for increasing the fission rate. Modifications were done keeping the UC targets closer to the converter in the most intense neutron flux zone. Different fission target materials were chosen to get a in-target production rate of 1015 fission per second : MKLN (special graphite), POCO foam (graphite foam) and high density UC pellets. In the first two cases, the fissile material is highly enriched 235U uranium dispersed in a graphite matrix with an apparent density is about 2 g/cm3. For the high density UC pellets the fissile material consists mainly of 238U enriched with about 2% of 235U with a density of 12 g/cm3. The target assembly is shown in Figure 2. Main insulator Cryopanel Water cooled, high voltage, RIB line high current bars Extraction electrodes Flexbile Ion source connectors Transfer tube UCx material Vacuum tube External container Fig. 2. Fission target assembly (left); detail of the finned target and thermal calculation (right). 3 The fuel is housed in a graphite primary container, 200 mm long with 35 mm of diameter surrounded by a tantalum container acting as protection and as heat radiator. The fuel consists of 86 discs, 1 mm thick and in between 1,3 mm thick grafoil discs with the aim to keep a high thermal conductivity for a better heat dissipation. The target presents a central hole of 8 mm diameter. Different kind of ion sources (laser ion source, plasma ion source, ECR,) are planned to be installed close to the target to ionize (charge state 1+) the selected fission products. The fission target has been designed to operate at 100 kV respect to the ground potential. To dissipate the heat power (30 kW) released by the fission reaction in the fuel the target container has a finned construction which increase the emissivity and gives a good power dissipation. This allows the target to remain at the required high temperature of around 2000 °C and to have large temperature drop across the fins. Transport of secondary beam through the shielding inside the about 6 meters long tube is assured by several electrostatic quadrupole lenses, one doublet each 2 meters, with apertures of 60 mm equal to secondary beam line diameter. At upper end of this vertical beam line a 90° electrostatic deviation turns the secondary beam in a horizontal plane toward the beam purification and acceleration area. In order to extract all the 6 beams in the same direction, the vertical tubes have to be inclined. Increasing the distance between the tubes at upper end is also necessary to allow installation of gate valves, connectors, etc. Resulting geometry can be seen in Fig 3. Fig. 3: Geometry of fission target modules. Incident proton beam line is red. The 6 radioactive beams are delivered on 6 parallel lines (green) at about 7 meters above. Around neutron converter (not visible) a neutron reflector (dark gray) can be used to increase neutron flux at fission targets positions. In the present report the fission target design optimized for the such geometry is discussed mainly from the point of view of fission yields. Results on neutron fluxes as weel as power deposition are also presented, while other important issues associated to fission targets such as release properties or handling are treated is other reports. 4 II. FISSION TARGET AND MODULE DESING. ONE TARGET SIMULATIONS The design study of the target and the optimization of its geometry, using different configurations were done with the Monte Carlo code FLUKA [5, 6, 7] in order to achieve improvements on the performances. The performance of targets in the configuration shown in Fig. 3 was analyzed simulating one target. Cross section views of the geometrical model used for Monte Carlo simulations, including container module with the UC fission target, the beam line and the services starting above the top of the UCx target, is presented on Fig.
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