  We investigate the prompt emission of few intermediate-mass fragments in
spallation reactions induced by protons and deuterons in the 1 GeV range. Such
emission has a minor contribution to the total reaction cross section, but it
may overcome evaporation and fission channels in the formation of light
nuclides. The role of mean-field dynamics and phase-space fluctuations in these
reactions is investigated through the Boltzmann-Langevin transport equation. We
found that a process of frustrated fragmentation and re-aggregation is a
prominent mechanism of production of IMFs which can not be assimilated to the
statistical decay of a compound nucleus. Very interestingly, this process may
yield a small number of IMF in the exit channel, which may even reduce to two,
and be wrongly confused with ordinary asymmetric fission. This interpretation,
inspired by nuclear-spallation experiments, can be generalised to heavy-ion
collisions approaching the fragmentation threshold.
