A key issue for the fusion–fission concept is the number and lifetime of the neutrons in the various processes, the so-called neutron economy.
In a pure fusion design, the neutrons are used for breeding tritium in a lithium blanket. Natural lithium consists of about 92% Li and the rest is mostly Li. Li breeding requires neutron energies even higher than those released by fission, around 5 MeV, well within the range of energies provided by fusion. This reaction produces tritium and helium-4, and another slow neutron. Li can react with high or low energy neutrons, including those released by the Li reaction. This means that a single fusion reaction can produce several tritiums, which is a requirement if the reactor is going to make up for natural decay and losses in the fusion processes.
When the lithium blanket is replaced, or supplanted, by fission fuel in the hybrid design, neutrons that do react with the fissile material are no longer available for tritium breeding. The new neutrons released from the fission reactions can be used for this purpose, but only in Li. One could process the lithium to increase the amount of Li in the blanket, making up for these losses, but the downside to this process is that the Li reaction only produces one tritium atom. Only the high-energy reaction between the fusion neutron and Li can create more than one tritium, and this is essential for keeping the reactor running.
To address this issue, at least some of the fission neutrons must also be used for tritium breeding in Li. Every one that does is no longer available for fission, reducing the reactor output. This requires a very careful balance if one wants the reactor to be able to produce enough tritium to keep itself running, while also producing enough fission events to keep the fission side energy positive. If these cannot be accomplished simultaneously, there is no reason to build a hybrid. Even if this balance can be maintained, it might only occur at a level that is economically infeasible.