  We revisit the assumption that reactors based on deuterium-deuterium (D-D)
fusion processes have to be necessarily developed after the successful
completion of experiments and demonstrations for deuterium-tritium (D-T) fusion
reactors. Two possible mechanisms for enhancing the reactivity are discussed.
Hard tails in the energy distribution of the nuclei, through the so-called
kappa-distribution, allow to boost the number of energetic nuclei available for
fusion reactions. At higher temperatures than usually considered in D-T
plasmas, vacuum polarization effects from real $e^+e^-$ and $\mu^+\mu^-$ pairs
may provide further speed-up due to their contribution to screening of the
Coulomb barrier. Furthermore, the energy collection system can benefit from the
absence of the lithium blanket, both in simplicity and compactness. The usual
thermal cycle can be bypassed with comparable efficiency levels using hadronic
calorimetry and third-generation photovoltaic cells, possibly allowing to
extend the use of fusion reactors to broader contexts, most notably maritime
transport.
