Won’t the canisters and casings quickly rust?

With Deep Isolation’s solution, the spent fuel canisters will be in contact with a “reducing environment.” Basically, this means very low oxygen in either gas or chemical form. Oxygen is a key element for many kinds of rust, and the low oxygen means that the corrosion-resistant alloys will last for a very long time. They will in time erode and that is why the geologic barrier is so robust and remains the primary barrier.