What kind of reactor? LWR? BWR or PWR? or HWR, or GCR? or test reactor? What temperature?
If Al has contact with water at 300C, then corrosion could be a problem. Al will become activated in a neutron flux. One might also get a chemical reaction with Cl and Al.
If the Al cladding were to breach then there would probably be a chemical reaction.
I would think that one would put the Al capsule in a hermetically sealed stainless steel capsule.