Practical engineering designs must first take into account safety as the primary goal.  All designs should incorporate passive cooling in combination with refractory materials to prevent melting and reconfiguration of fissionables into geometries capable of un-intentional criticality.  Blanket layers of Lithium bearing compounds will generally be included as part of the design to generate Tritium to allow the system to be self-supporting for one of the key fuel element components.  Tritium, because of its relatively short half-life and extremely high radioactivity, is best generated on-site to obviate the necessity of transportation from a remote location.  D-T fuel can be manufactured on-site using Deuterium derived from heavy water production and Tritium generated in the hybrid reactor itself.  Nuclear spallation to generate additional neutrons can be used to enhance the fission output, with the caveat that this is a tradeoff between the number of neutrons (typically 20-30 neutrons per spallation event) against a reduction of the individual energy of each neutron.  This is a consideration if the reactor is to use natural Thorium as a fuel.  While high energy (0.17c) neutrons produced from fusion events are capable of directly causing fission in both Thorium and U, the lower energy neutrons produced by spallation generally cannot.  This is a tradeoff that affects the mixture of fuels against the degree of spallation used in the design.