caged plutonium which is a transuranic material that has a half-life of 24,065 years, making it decay (into different elements) inside a zinc-aluminum hull (the aluminum delicates due to the radiation and thus might crumble depending on the weight of plutonium-239) but doesn't rust as easily due to the zinc coating
"Our team have created a simulation of the half-life decay of plutonium-239 in a zinc-aluminum material lined caging scenario."