An endodontic reamer is least likely to fracture when inserted in a:
Irrigants perform physical and biologic functions. Dentin shavings get removed from canals by irrigation.
Thus, they do not get packed at the apex of root canal.
Instruments do not work properly in dry canals. 
Their efficiency increases by use in wet canals. 
Instruments are less likely to break when canal walls are lubricated with irrigation.
Dry, clean canal
Wet, clean canal
Wet, debris laden canal
Dry, debris laden canal