Design Optimization of Toroidal Fusion Shield

1. n n n Li Reflecting Material Multiplying Layer j Fusion Theory [BLAHBLAHBLAH] Fusion energy production is based on the collision nuclei in a deuterium and a tritium plasma. The resulting interaction produces a 3.5 MeVα-particle and a 14.1 MeVneutron. The high-energy neutrons deposit their energy as they scatter through a shield covering a toroidal shaped fusion reactor. Design Optimization of Toroidal Fusion Shield Matthew Franzi, Andrew Haefner, Ian Rittersdorf, Andrew Stach n Vacuum Wall [Thickness Selection] l MCNP [Slab Model] Plotting the MCNP data illustrates relative current at various positions through the fusion shield. A simple slab design was modeled in MCNP to determine vacuum wall thickness, lithium layer thickness, and the number and position of neutron multiplier layers. l Future Works The neutron-Li6 interaction is a highly exothermic reaction that generates approximately 4.86 MeV per absorption. Cross sections of Li6 show that only thermal neutrons are probable to interact. Increasing the population of thermal neutrons in lithium is essential in maximizing this excess energy. In order to further enhance the amount of thermal neutrons within the lithium blanket, it is important to understand the distribution of neutron energies within the fusion shield. 1 4 3 2 1 - Neutron source Emits 14.1 MeV neutrons 2 - Vacuum wall Must use materials that are structurally sound, inert, and pass a sufficient, low-energy flux to the blanket 3 - Lithium blanket Energy gained through neutron interactions 4 - Biological shield Stops virtually all neutrons from exiting fusion shield Neutrons entering the tungsten vacuum wall undergo (n,2n) multiplication reactions which significantly increase current. The following reactions take place inside the fusion shield: Li7 + n = He4 + T + n - 2.5 MeVLi6 + n = He4 + T + 4.86 MeV A neutron reacting with Li6 produces more tritium for fuel and also releases 4.86 MeV of energy. That energy, along with any kinetic energy from the interacting neutron, is captured in the form of heat and creates power by a conventional steam turbine system. The plots above display neutron currents for different energies in the shield. This data will be utilized to insert neutron multiplication layers in the shield. mVacuum Wall [Material Selection] Multiplication Module • For the vacuum wall, materials with the following properties were selected: • Melting point above 950 K • High tensile strength, hardness • Inert to plasma interactions • Penetrable by source neutrons • The current into the lithium blanket increases with decreasing vacuum wall thickness. • Conclusions from Data: • 6 cm of tungsten produces highest current into lithium • 6 cm also allows for the highest percent of neutrons to be absorbed in lithium k Motivation [Power and Efficiency] Each material has a unique energy dependent cross section for the (n,2n) reaction, the reaction responsible for neutron multiplication. As a result, these materials will be placed where they can most effectively multiply neutrons. Beryllium and lead multiply neutrons effectively in the energy ranges present in the lithium blanket. • The scope of this project was to design the toroidal shield to be as efficient as possible in order to maximize power output. To achieve such a design a concentration will be placed on varying the thicknesses and types of materials used in the shield. • Goals • Minimize neutron flux out of the fusion shield. • Maximize neutron interaction inside the lithium layer • Take into account physical / structural limitations • Using MCNP simulations, an idealized fusion shield will be investigated and an optimized configuration will be the end result. The seemingly low relative currents at the start of the vacuum wall are indicative of neutrons scattering back out of the slab. These numbers are promising as the actual geometry is toroidal. Based on these cross sections and the energy currents, beryllium and lead will be utilized in areas where they can most effectively multiply neutrons within the fusion shield to produce more favorable energy yields. The next MCNP model is cylindrical to better represent the radial geometry present in the toroidal fusion reactor. Tungsten was chosen on the basis that it showed the largest relative current after 10 cm.