A detailed quantitative assessment of the energy multiplication capacity of a
hybrid requires a specific blanket design followed by a detailed neutronic analysis. However, an indication of this important energy gain can be obtained by employing a simplified lumped parameter characterization of the several processes occurring in the blanket and associated reactors.

We define Cb to be the average number of neutrons produced in the blanket by all neutron multiplication processes, expressed on a per fusion reaction basis. One of the neutrons so produced must be used for breeding tritium while the remaining Cb-1 neutrons could be used for fissile fuel breeding. Allowing for parasitic neutron captures and losses such as neutron leakage gives finally £b(Cb- 1) as the total number of fissile nuclei produced. Each of the bred fissile nuclei is able to eventually generate Qf, units of energy, which will take place-if the bred fuel is extracted and transported to client power plants-in associated fission reactors. Then, the total nuclear energy generated by the hybrid fusion-fission system must also include the breeding capacity of the medium in which these eventual fissions take place. Letting Cfi be the conversion ratio of these associated fission reactors, a total fission energy of Qn/(1-Cfl) is therefore eventually generated and attributed to each initial fissile breeding reaction in the fusion reactor blanket. The total energy generated per unit initial fusion reaction energy release Qft, defines an energy multiplication and is therefore given by

Here ME may well be called the energy multiplication capacity and is displayed in Fig.15.3. Clearly, the energy multiplication of a fusion reactor operating in tandem with a fission reactor can be substantial compared to that for a stand­alone fusion reactor (ME =1).

The fundamental reason for the remarkable energy multiplication of a fusion hybrid reactor operating in tandem with a fission system lies in the complementary nature of fusion and fission reactions. A d-t fusion reaction results in one neutron and a total energy release of 17.6 MeV while a fission reaction results in 2 to 3 neutrons and -200 MeV. Consequently, fission reactions can be viewed as energy "rich" and fusion reactions, by this yardstick, as energy
"poor". Further, a hybrid fusion reactor blanket may well regenerate the fuel it consumes, i. e. breed tritium and have some neutrons left over. In contrast, improving the performance of a fission breeder reactor-which produces abundant amounts of energy-demands an enhanced neutron population. Hence, a "neutron — for-energy" and "energy-for-neutron" exchange in the hybrid provides mutual advantages.

Another way of viewing fusion-fission energetics is to note that a neutron used for tritium breeding in support of a fusion reaction contributes basically

17.6 MeV of energy while a neutron used to support fission energy by fissile fuel breeding adds more than 10 times as much energy to the entire yield of the system. Thus, the high energy neutrons from a fusion reaction possess a greater capacity for neutron multiplication than thermal neutrons in a thermal reactor, reinforcing the "neutron rich / energy poor" view of fusion. This contrasts to the recognition that on a basis of "energy per initial fuel mass" involved, fusion is exceedingly more "energy rich".