Technical Note: Self-Fuelling Fusion Hybrid Propulsion for Interstellar Missions

T. Kammash (2015), JBIS, 68, pp.217-219

Refcode: 2015.68.217

Abstract:
A fusion hybrid reactor where the fusion component is the Gasdynamic Mirror (GDM) is presented as a potential energy source for utilization in interstellar space propulsion. Such a reactor consists of a fusion component whose primary function is to supply neutrons to a surrounding blanket containing fertile material where they will breed fissile material and simultaneously burn it to produce power. Since the primary function of the fusion component is to supply high energy neutrons, it can operate at or near “breakeven” condition, a much less stringent condition than that required for a pure fusion reactor. Since the GDM is linear, cylindrically symmetric, and can operate in the steady state, it is particularly suitable for this application. In the proposed system we employ the thorium fuel cycle, namely a blanket made of thorium-232 which upon interacting with the fusion-produced neutrons, breed uranium-233 which in turn undergoes fission upon absorption of these neutrons. By design, the reactor will be “safe” since it will be “subcritical,” and produce little radioactive waste since the “actinide” isotopes generated by other neutron-induced reactions will be burned in situ. Preliminary investigation of such a propulsion system reveals that about 500 Megawatts of thermal power are produced per cm in a system whose length is 18 m and a blanket radius of 4m, 25% of which is devoted to the helium coolant ducts. In steady state, one tenth of the thorium density is converted to uranium and under these conditions the reactor will be self-fuelling for about 800 days. The total mass of the system including 500 mT of tankage and payload is estimated at 7300 mT and with such a propulsion system a round trip to Pluto at the edge of the solar system will take about 182 days, and a one-way mission to the Sun’s gravitational lens at 500-1000 AU’s will take about 558 days.