We know that ingesting some radioactive particles is particularly bad because your body considers them nutrients.
Your body doesn't differentiate between radioactive and stable iodine.
Moreover, radioactive cesium is a potassium analog; strontium is a calcium analog.
Consequently, your body absorbs these radionuclides and "stores" them as if they were nutrients.
(Here is a link to some research I did on cesium absorption http://majiasblog.blogspot.com/2012/04/bio-accumulation-of-cesium-cesium-is.html )
Paul Langely has a very good post today describing research conducted by the US government on the serious effects of ingestion of radionuclides, even of minute amounts