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When you look at a regular plant, and you can see the building that houses the reactor.... how big is the uranium ball that is actively in use?

And how long does that last before it has to be changed?

My memory is 2 years between refueling, and they use rods, not balls. It's not super large... a few inches thick by few feet long? guess i should google that, but that's all from when i worked at the local utility.