  We develop a fully three dimensional numerical code describing the diffusion
of cosmic rays in the Milky Way. It includes the nuclear spallation chain up to
Oxygen, and allows the study of various cosmic ray properties, such as the CR
age, grammage traversed, and the ratio between secondary and primary particles.
This code enables us to explore a model in which a large fraction of the cosmic
ray acceleration takes place in the vicinity of galactic spiral arms and that
these spiral arms are dynamic. We show that the effect of having dynamic spiral
arms is to limit the age of cosmic rays at low energies. This is because at low
energies the time since the last spiral arm passage governs the Cosmic Ray (CR)
age, and not diffusion. Using the model, the observed spectral dependence of
the secondary to primary ratio is recovered without requiring any further
assumptions such as a galactic wind, re-acceleration or various assumptions on
the diffusivity. In particular, we obtain a secondary to primary ratio which
increases with energy below about 1 GeV.
