That was caused by water leakage into the J-rod annulus, the space between the reactor vessel itself, the heavy-water filled reactor vessel, and the light-water filled reflector. There was a gas space in between, and because of the presence of water in that gas space, as a result of chronic light-water leakage, and as a result of the presence of air in that space, in the presence also of radiation, that allowed the formation of nitric acid. Nitric acid corrodes aluminum. The vessel's aluminum.
On October 19th, 2009. See this statement in context.