Fusion Driven Fission System for Space Surface Power Application

Abstract

It is well known that fission reactors are energy rich and neutron poor, while fusion reactors are neutron rich and energy poor. As a result, it may be desirable to use the neutrons produced by a fusion reactor operating at a multiplication factor Q ~ 1 to breed fissile material in a blanket, which can also undergo fission reactions with the fast neutrons to produce power. We examine this approach in conjunction with a bi-modal fusion propulsion system which, upon reaching destination, is operated primarily as a neutron source. The neutrons produced by the DT reactions are allowed to impinge on a blanket containing thorium-232, thereby producing uranium-233. This uranium isotope has a fast neutron fission cross reaction of about 2 barns, which we employ to calculate the power density produced in a steady-state operating system. We consider a cylindrical system whose length is significantly larger than its diameter, thereby invoking a one-dimensional approximation for the neutron flux equation which we solve in conjunction with the U 233 production rate equation. For reasonable blanket dimensions consistent with the fusion plasma parameters, we calculate the power density in the fission system and find that it is about 70 times larger than would have been the case in a pure fusion reactor. Hence, such a hybrid system may be viewed as equivalent to a fusion reactor with a Q ~ 70 that can be utilized for space surface power production with no proliferation risk. Nomenclature