I had the same thought briefly, but then thought more about it. These are high energy particles. You might remember the article about cosmic rays that sometimes cause a single bit flip in a videogame, and that there is no easy way to shield against it. That makes satellite imaging difficult, because how do you focus some sort of lens, if the particles go right thru everything? You might detect that a cosmic ray or ionizing particle hit the satellite, but not where it came from. The sun? Australia? Space? Plus, think about particle accelerator images and cloud chambers. They are individual streaks. They aren't gradients the way normal light would create an image. So, the farther away you are, the less likely these particles will hit you. If you attached a geiger counter to a hot air balloon, starting from this radioactive capsule, you'd hear rapid clicks, then infrequent clicks, then no clicks, then the clicks would increase due to cosmic rays. If directional geiger counters existed, I feel like people would have been using those instead of what they currently use. So, I doubt satellite nuclear radiation imaging exists.

I looked up Fukushima satellite maps, and they all say they are fluid simulations based on ground sensor data.

That technique doesn't work on Earth; the atmosphere's too thick to pass gamma rays (in either direction). An atmospheric column has the mass of a ~10 meter column of water.

It's pretty effective on other planetoids, though:

https://en.wikipedia.org/wiki/Lunar_Prospector?useskin=vecto...