Nuclear properties:

The nuclear properties of fuel cladding material must also be satisfactory.  For thermal reactors, it is important which the material have a reasonably small absorption cross section for neutrons. Just  four  components  and  their  alloys  have  low thermal-neutron  absorption  cross  sections  and reasonably  high melting  points:  beryllium,  magnesium,  aluminum,  and zirconium. Of these, magnesium, aluminum, and zirconium are or have been utilize in fuel-element cladding.

Aluminum, such as the 1100 categories, that is associatively pure (greater than 99 percent), has been used in low power, training, water-cooled research, and materials testing reactors in that the operating temperatures are below 100ºC.  Magnesium, in the form of the alloy magnox, serves as cladding for the uranium metal fuel in carbon-dioxide cooled and graphite-moderated power reactors in the United Kingdom. The alloy zircaloy, whose main constituent is zirconium, is hugely used as the fuel-rod cladding in water-cooled power reactors. A alloys in general use as cladding material are zircaloy-2, zircaloy-4  both of that have mechanical properties and corrosion resistance  superior  to  those  of  zirconium  itself. While beryllium is appropriate for use as cladding, it is not used because of its high cost and poor mechanical properties.