A model of deuterium-deuterium (D-D) fusion in metal lattices is presented based on two phenomena: (a) reactions between virtual-state pairs of deuterons “bound” by electrons of high effective mass m* and (b) deuterium energy upscattering by fast ions from fusion or tritium reactions with virtual-state nuclear structure groups in palladium nuclei. Since m* is a decreasing function of deuterium ion bulk density n0, the exponential barrier tunneling factor decreases rapidly with m*. As a result, the fusion rate reaches a maximum at a loading density above zero but less than saturation. This can explain observations of transient neutron output from the (3He,n) branch of D-D fusion. At low energy, D-D reactions favor the (T,p) branch. Fast product tritium may be captured by palladium isotopes to form excited-state Ag*, removing tritium from the system and preventing deuterium-tritium fusion. This may decay by alpha or proton emission, yielding fast ions and excited state Rh* or Pd*. Fast ion collisional “trapping” may occur at Fermi electron speeds, enhancing in situ upscattering and yielding increased D-D reaction rates. Analysis of the dynamics of these processes suggests conditions for exponential growth.