Particles / Circles moving random within a shape

Dear animators out there,

I want the blue (and in a second step the red) dots moving smoothly in the two "atomic shells". I already created a template with an expression that makes the blue dots moving randomly with the help of a slider expression. Unfortunately I don't know how I make them stay inside the grey circle. In the end there should be additionally some red dots (second screenshot) which should keep in the second "atomic shell". How do I do this? Does anyone maybe also have a template? It's really not so easy for me and I already spent several hours on this topic 😞

I doesn't have to be a complicated 3D animations but I would also be open for a particle filter or whatever you suggest...

Thanks for your support!!

Tom