A return to in-person classes at a Navajo Nation school will be on hold indefinitely because of unknown radiation levels, likely caused by decades of uranium mining. A Navajo Nation education official said Monday the presence of radioactive hotspots inside …A return to in-person classes at a Navajo Nation school will be on hold indefinitely because of unknown radiation levels, likely caused by decades of uranium mining. A Navajo Nation education official said Monday the presence of radioactive hotspots inside …