  Crowdion defects are produced in body centred cubic metals under irradiation.
Their structure and diffusive dynamics play a governing role in microstructural
evolution, and hence the mechanical properties of nuclear materials. In this
paper we apply the analytical Frenkel-Kontorova model to crowdions and clusters
thereof (prismatic dislocation loops) and show that the Peierls potential in
which these defects diffuse is remarkably small (in the micro eV range as
compared to the eV range for other defects). We also develop a coarse-grained
statistical methodology for simulating these fast-diffusing objects in the
context of object kinetic Monte Carlo, which is less vulnerable to the low
barrier problem than na\"ive stochastic simulation.
