Jack Haley and co-workers report in Acta Materialia an investigation of the formation of helical dislocations in Fe-9Cr during neutron irradiation. For the first time the origin of these helical dislocations has been identified as being from a vacancy-biased mechanism. Interaction of vacancy defects with pre-existing screw dislocations causes the formation of mixed screw-edge helical dislocations. The screw dislocations act as vacancy-biased sinks thus leading to a higher proportion of visible interstitial clusters in the vicinity of helical dislocations and a heterogeneous spatial distribution of dislocation loops in neutron-irradiated ferritic materials. An improved understanding of dislocation structures in such alloys is critically important to a wide range of radiation effects models of material evolution during irradiation.