Adequate Calcium levels keep about 80% (iirc) of the strontium-90 from being absorbed into bone tissue. [Farmer's could feed calcium to their animals, for instance.] But, animal (& human) milk is the highest source of radioactivity exposure. So, yes, potentially, bones from an area highly contaminated by radioactive fallout could be an issue if making bone broth. We choose local, 100% grass-fed beef bones and organic chicken bones. [There are alternative sources of minerals, of course.]
For perspective: "In the 1986 Chernobyl disaster, radioactive material from a raging fire spread across much of Europe and was detected on the clothing of Swedish workers, which first alerted the world to the disaster that Soviet authorities tried to keep quiet.
That calamity, the world's worst nuclear disaster, resulted in the deaths of 32 workers who tried to put out the fire at the Ukrainian plant. A 20-year study released last year by the U.N. Scientific Committee on the Effects of Atomic Radiation found that there were about 6,000 cases of thyroid cancer in children from Ukraine and Belarus who drank milk with elevated levels of radioactive iodine from cows in the area. Treated correctly, thyroid cancer has a survival rate above 90 percent.
Radiation levels were not high enough to harm others in the former Soviet Union and Europe who lived in the area where the cesium-137, iodine-131 and other radioactive materials drifted for hundreds of miles, the study found."
Japan's Radiation Exposure: How Serious Is It?: http://ecocentric.blogs.time.com/2011/03/12/japans-radiation-exposure-how-serious-is-it/
"National Radiation Map, depicting environmental radiation levels across the USA, updated in real time every minute. This is the first web site where the average citizen (or anyone in the world) can see what radiation levels are anywhere in the USA at any time." http://www.radiationnetwork.com/
"Organic" sea vegetables have restrictions on proximity to pollutants such as industrial or metropolitan run off. Additionally, one can purchase "heavy metal tested" kelp, for instance. We buy it at Vitacost.com
Fish bones, in sardines for instance, might be an interesting thing to examine. I do not know if larger fish or fattier fish would absorb more radioactivity.