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Study on the potentiality of using the energy of alpha particles (4He), produced during fusion reactions

Daponta Chrysovalanti

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Summary

Fusion of relatively light nuclei is the method of clean energy production, that is aimed for the purpose of large scale power generation. European activity is mainly demonstrated by Euratom and more recently, EUROfusion. EUROfusion presents international innovation in both scientific and technological sectors, due to the manufacture and operation of large magnetic fusion devices (Tokamaks), including JET and ITER in France, which is currently under construction, won’t be completed until 2050 and costs a few tens of billions of Euros [1]. One of the most interesting advantages of fusion is the consumption of small quantities of fuel, which is abundant in nature. Furthermore, fusion products do not impact on the environment, through the emission of greenhouse gases (GHGs) [2]. Fusion devices operate mainly with the use of the two heaviest isotopes of hydrogen (D, T), while other fuels, like hydrogen - 11Boron, can also be used. The same facts, as mentioned below, are applied to inertial confinement fusion, with high laser energies (laser beams of 1 – 2 MJ) [3].For the achievement of a fusion reaction, two nuclei that tend to repel each other, due to their positive charge, have to come close enough. For this purpose and in order to overcome the barrier of the developing repulsive forces, a large amount of energy has to be provided to the reaction. The probability of a nuclear fusion reaction and as a result the production of beneficial clean energy is quantified by the parameter «cross section». The fusion cross section is the main criterion for the selection of the fusion’s fuel materials. The biggest challenges for fusion are: 1) The heating of the fusion fuel, at temperatures of a few tens of millions degrees (keV). The exact temperature depends on the fusion reaction. 2) The confinement method (magnetic or inertial) of the high temperature fusion fuel. Internationally, two types of fusion fuels are used in research. These are the heaviest isotopes of hydrogen (Deuterium and Tritium and hydrogen with 11Boron [4].Between the so called «advanced fusion fuels», the nuclear fusion reaction of protons with 11Boron is the one that produces no neutrons, but only charged particles. These charged particles are three nuclei of 4Helium, with total kinetic energy of 8.7 MeV, per fusion reaction. The maximum cross section of this fusion reaction corresponds to a kinetic energy of the reactants’ center of mass between, which is between 450 – 500 keV [5]. Charged particles can be converted into electrical current with appropriate devices that are suggested in literature. The fusion reaction of protons with 11Boron is very attractive for two reasons: 1) There is no neutron production and as a result, there doesn’t exist the activation problem of the fusion chamber due to neutrons, 2) In each proton 11Boron fusion reaction, three alpha particles are produced. In contrast, in the case of D – T fusion nuclear reaction, clean energy production is conducted through neutrons in specifically shaped blankets, that surround the fusion chamber. Meanwhile, D- T fusion reaction produces four times more radiation compared to uranium fission reactors [6]. Each fusion reaction coming from hydrogen’s isotopes, deuterium and tritium, produces an alpha particle of 4Helium and one neutron. At this point, the following observation has to be done: Both D - T and p - 11B fusion reactions produce alpha particles of 4Helium. The above observation is the purpose of the study, which is going to be based on the behavior of charged alpha (4Helium) particles, in the MeV range, that can be used and be added to the final clean energy of fusion reactors. In this master thesis, our interest focuses on the alpha particle production with MeV kinetic energy and on their interaction with materials, used in fusion reactors. Since the use of hydrogen isotopes is not easy in experiments because of technical and economical reasons, the use of p - 11B fusion reaction is of interest and is suggested. The hydrogen boron fusion reaction can be conducted, if proton beams are produced by laser pulses or accelerators. The time evolution of the produced alpha particles and their final energy at the time of their extraction from the solid target of 11Boron, depend on the initial values of the physical parameters of both the proton beam and the depth of alpha particles production in the Boron target [7]. Plenty of experiments focused on the alpha particles production from the proton (beam) - 11Boron (solid target) interactions, are mentioned in international literature. In these experiments, the proton beams are produced by laser pulses of short width [5,6,7,8,9].In the context of this master thesis, literature research is going to be carried out for: 1) The production of alpha particles during fusion reactions and 2) the interest of using their energy for the aim of clean energy production. In a next stage, the interaction of a proton beam with a solid target of 11Boron is going to be studied. Also, known physical and mathematical models are going to be used for: 3) the penetration depth of protons in the solid target of 11Boron and the production of alpha particles, as a function of the penetration depth, 4) The extraction of the produced alpha particles from the target, with their corresponding energy, 5) The energy deposition of alpha particles on materials is going to be evaluated. The results of this study are going to allow the description of the needed experimental variables for a possible experimental study in the field of alpha particles interaction with materials.

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