Effects of Neutron Radiation and Shielding Recommendations for the PFRC4 Kevin P. Griffin and Matthew T. Walsh, Samuel Cohen, Russell Feder, Jonathan Klabacha, Quan Zhou Abstract Shielding is required in a fusion reactor to protect both the sensitive parts of the reactor and operators from energetic neutrons produced by the reaction being burned. To determine how much shielding is necessary to maintain superconductivity at liquid nitrogen temperatures, a variety of shielding materials and configurations were tested and the fluence delivered to the conductors was recorded. Alternate configurations included varying the thickness of shielding between and beneath the conductors. Alternate materials included isotopically enriched varieties of B4C, and the presence or absence of a tungsten layer between the B4C shielding and the plasma. Also considered were the possible effects of the radiation on humans in close proximity to the reactor. In addition to determining how these design changes affect the neutron fluence to the superconductors, we also investigated the effects of these changes on the helium production, nuclear heating, and displacements per atom (DPA) in the inner vacuum vessel wall, the B4C shielding, and the RF antenna. Conductor fluence was the most significant determinant for the minimum amount of shielding needed. Heating due to the neutron radiation was not a major factor in reactor design because a very small amount of energy of the reactor comes out in neutrons. Heating due to bremsstrahlung and synchrotron radiation is much more significant than heating from neutron radiation, and thus would dominate design choices based on cooling requirements. Displacements per atom could have a significant effect on reactor materials, but further studies on the rate of DPA caused by the reactor neutron flux would be required to determine these effects precisely. Additional shielding could even keep human operators near the reactor safe during testing, but would not be recommended for a space-travelling reactor due to the additional mass. 32:85 cm of shielding was sufficient to ensure a lifetime of at least 30 years for the superconductors. A detailed study on the precise effects of neutron irradiation on high-temperature superconductor materials would be desirable, since it is currently unknown exactly how the materials respond to such irradiation. Data on fluence levels for changes in critical current and temperature exist, but understanding the microscopic effects on the materials would yield a better understanding of the mechanisms that cause these effects. It is also of interest to study the effects of the small number of 14:1 MeV neutrons produced by secondary deuterium-tritium reactions. As the reactor design is further developed, a study of different shielding designs would be helpful in determining the optimal configuration for the final reactor. Introduction The field-reversed configuration (FRC) reactor is a fusion reactor that burns deuterium and Helium-3 2D + 3He ! 4He +1 p + 18:3 MeV This reaction is aneutronic, which has several advantages in reactor design. Neutron bombardment causes many harmful effects on materials. Energetic neutrons can collide with atoms in materials, knocking them out of their lattice sites, inducing radioactivity, and causing heating. This damage can cause embrittlement, swelling, and helium production on a macroscopic scale [1]. Additionally, neutron radiation is considered the most dangerous form of radiation to humans due to the high kinetic energy of neutrons and the secondary beta and gamma radiation they can cause [2]. Because they have no charge, neutrons cannot be contained in a magnetic field, requiring shielding to protect against these effects. An aneutronic fusion reactor thus will require less shielding than one that burns D-T. − A reactor burning D 3He will produce some neutrons through the small number of D-D and secondary D-T reactions that will occur. These reactions and the resulting wall load of neutrons from these reactions can be reduced in a number of ways, including making the reactor smaller, removing the produced tritons, using a 3He-rich fuel mixture, and operating at a non-optimal temperature for D-T and D-D fusion, and it is theoretically possible to reduce the wall load of neutrons by a factor of ten thousand compared to a D-T Tokomak of similar power output [3]. However, some neutrons will inevitably be produced, so some shielding will be required in a FRC reactor. 1 The Princeton FRC reactor (PFRC) is an experimental reactor built by Sam Cohen and currently being studied at Princeton Plasma Physics Laboratory. Currently, the second version of the reactor, the PFRC2, has been built and is in the process of being tested. The PFRC4 reactor, currently being designed at Princeton Plasma Physics Laboratory, will produce more power than it consumes, but will still be considerably smaller than a traditional breakeven fusion device. This ellipsoidal reactor will burn a plasma that produces approximately 1MW per meter of reactor length. The reactor will have a semimajor axis of approximately 1.2m and a semiminor axis of approximately 0.3m without shielding, and will thus produce about 2.4 MW of power. Traditional magnetic confinement fusion reactors require strong magnetic fields to maintain confinement, so they must use low temperature superconductors like Nb3Sn because low temperature superconductors have higher critical field values. Because the PFRC4 will have a magnetic field of only 60-70 kG, it will be able to use high temperature superconductors like BiSCCO or YBCO without the concern of the critical field of the conductors being exceeded [4]. This means that instead of being cooled by liquid helium, they can be cooled with liquid nitrogen, as long as the conductors maintain a critical temperature below the evaporation point of liquid nitrogen, 77K. Using liquid nitrogen instead of liquid neon or helium will mean that far less power will need to be supplied to the conductor cooling system, so the reactor will be more efficient. However, high temperature superconductors like BiSCCO and YBCO will not maintain a critical temperature below 77K, if they are subject to large neutron fluences [5]. Although small fluences of neutrons can improve the performance of a superconduc- tor, large neutron fluences cause the critical current and the critical temperature to drop. Thus it is crucial that the superconductors are adequately shielded so that the conductors can maintain a high critical current and temperature. Methods The PFRC is still being developed and tested, so the PFRC4 is still in the preliminary design stage. In order to determine the effects of neutron radiation on the reactor, simulations using a model of the reactor must be used. The neutron radiation in the reactor and the effects on the materials of the reactor were simulated using computer model and particle simulation software. An initial model of the reactor was developed in SolidWorks to use for the simulations. This model was further developed and refined according to the results of these simulations. In order to reduce the complexity of the calculation, geometric symmetries were used so that only a portion of the reactor had to be modeled. The initial model was a 90-degree cylindrical sector (see figure 1). This model assumed that the reactor was an infinitely long cylinder, since it used reflecting boundary conditions on the two end faces. For this study, the neutron radiation will be assumed to be uniform throughout a cylindrical region within the plasma with a radius of 25cm and spanning the length of the reactor. In reality, the neutrons will not be emitted uniformly, with a greater intensity coming from the center of the reactor, but this model will provide a good approximation because the reactor length is several times its diameter. Figure 1: Initial solid model of reactor used for simulations. Model uses 90-degree symmetry of and model reactor as uniform, infinitely long cylinder. 2 Equation Energy % 2D + 3HE ! 4HE + p 18.3 MeV 98 2D + 2D ! 3T + p 4.03 MeV 1 2D + 2D ! 3He + n 3.27 MeV 1 Table 1: Energy and percentage of total fusion events for each reaction that will occur in the PFRC The particle simulation software Attila was used to analyze the effects of neutron radiation on the materials of the reactor as well as potential hazards to humans in the vicinity of the reactor. Attila uses Chebyshev-Legendre quadrature to solve a particle transport problem in space, angle, and energy. The model geometry is given to the program and divided into a finite element mesh. The precision of the solution can be adjusted by refining the mesh as well as controlling the order of the polynomials used in both angular quadrature and the scattering of particles and the fineness of the energy groups of particles that are used. In order to run the simulation in Attila, some parameters of the reactor’s operation must be determined. The reactor produces 2.4MW of power, resulting in a small number of 2.45MeV neutrons. The number of neutrons produced can be estimated as either 1% of the reactor or produced by 1% of the fusion events in the reactor. If 1% of the reactor power is released as 2.45MeV neutrons, there are 24000W of power released in the neutrons. Assuming the power is released as kinetic energy, there is 24000W, or 3:8452 · 1017 MeV of kinetic energy released in the neutrons per second. With 2.45MeV of kinetic energy per neutron, this works out to 6:11413 · 1016 neutrons released per second. If 1% of the fusion events produce a neutron, 1% of the fusion events in the reactor are deuterium-deuterium reactions that produce a 2.45MeV neutron. Since 50% of deuterium-deuterium fusion reactions follow this chain, the profile for the fusion events taking place in the reactor is: Therefore for every 100 fusion events, and thus every 2.45MeV neutron released, 824.62MeV of energy is produced by the reactor.