  From a recent re-analysis of the available data, Be and B Galactic evolution
appears to show evidence for a two-slope behaviour with respect to O. The
inferred Be/O abundance ratio in halo stars is constant at very low metallicity
(primary behaviour) and increases proportionally to O/H at high metallicity
(secondary behaviour). We show that this can be explained in the framework of
one single model, the `superbubble model', in which Li, Be and B are produced
by spallation reactions induced by energetic particles accelerated out of a
mixture of supernova (SN) ejecta and ambient interstellar medium inside
superbubbles (SBs). All the qualitative and quantitative constraints, including
the energetics, the value of the transition metallicity and the 6Li/9Be
isotopic ratio, are satisfied provided that the energetic particles have a
spectrum flattened at low energy (in E^-1) and that the proportion of the SN
ejecta inside SBs is of the order of a few percent. This lends support to
Bykov's acceleration mechanism inside SBs and to the SB dynamical evolution
model of Mac Low & McCray (1988).
