[sandworm]

It will take months, years, decades to scan each cask. They are basically doing a cat scan of a large object using natural muons, cosmic rays.

There aren't very many rays at the earth's surface. A cat scan will use millions, billions of particles to form an image. The flux of cosmic rays for this scanning method will be on the order of one or two rays per liter of object per second. Building up any image will, at that rate, take a loooong time.

These nuclear casks are probably the only targets willing stand still long enough. Scanning something small, like a lead safe, might take literally centuries.

[dmm]

The Wikipedia page for muon topography describes an experiment from 2011 where a reactor mockup was scanned in three weeks. That's a long time but seemingly reasonable for this application.