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Definition of Strontium 90
1. Noun. A radioactive isotope of strontium (with the mass number 90) that is present in the fallout from nuclear explosions; can be assimilated like calcium into bones. 
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1. Noun. A radioactive isotope of strontium (with the mass number 90) that is present in the fallout from nuclear explosions; can be assimilated like calcium into bones.