Again, absence of evidence does not mean much. Nobody is expecting the workers to fall down after minutes of work there, the effects of low level radiation are subtle but given the known mechanism or damage there is no basis for calling such levels safe. Workers working with radioactive tailings may suffer harmful long-term effects. But it is hard to causally connect old age health issues with presence in these areas decades ago. You cannot conclude "likely safe" from absence of information or because the workers seem fine. You would have to do the studies, which as you say, probably were not made.