The chlorella from Japan has been monitored for radiation contamination and so far there has not been any contamination.  In fact, herbs are tested multiple times not only by exporters, but also by the importers and manufacturers.  They have to because they need to make sure the proper species is being bought and sold and that there is not excessive contamination from metals, molds, bacteria, etc.  GMP manufacturers in particular have to test and retest their products by FDA rules.  Herbs really are more stringently regulated than pharmaceutical drugs.  So there really is not an issue.  

Still, if you are worried then I checked my Starwest catalog and they have chlorella from China instead of Japan.  Starwest can be found online.