My recommendation would be to get in touch with your supplier. They have to do testing of their teas (at least they had to last year) and are usually happy to give you information about it if you ask.
As far as I know, no increased radiation values were found last year in the area around Kyoto, where a majority of the high quality teas are grown (actually, the nearest area where concerning radiation values were found was about 200 km north of there).
Japan’s government was monitoring (and using mandatory testing) teas, especially those destined for export, and didn’t shy away from destroying large batches of contaminated tea.
I’ll have to look into the subject again later this year when the new harvest comes in.
Hope this helps a little.