In this video, David Vitalis goes into detail about sauna and clay/zeolite for detoxifying radiation.
I have a couple of nitpicks, but overall I think it’s a very good video.
The two radionuclides of primary concern are iodine, cesium and strontium. Cesium and strontium are water soluble and stored primarily in muscle and bone respectively, so, yes, you can sweat out these radionuclides, but the point about sweating out fat-soluble toxins does not apply to these two in particular. However, other lypophillic (fat loving) toxins, such as mercury, must be cleansed as well. It is the overall toxic load that must be cleansed, not only radionuclides.
Anyway, check out the video, and there is a lot more on that website I haven’t had the chance to take a look at, link below the video.