Nuclear fuel pellets wrapping. The cladding usually has the form of a long tube, closed at the ends, inside which pellets are stacked. The cladding protects fuel from corrosion, holds pellets inside, and prevents fission products from being released into the primary circuit or environment. The gap between fuel and cladding is usually filled with helium for heat removal and for possible pellet expansion caused by heat and gases produced during fission. In water-cooled reactors, the most commonly used cladding material is zircalloy (atomically clean zirconium alloyed with titanium, niobium, iron, chromium, or nickel). Highly durable stainless-steel cladding is used in the aggressive environment of reactors cooled by molten salts.