I wondered this too. Urban legend has it that Geiger counters will go off over their graves, so I looked for any info on that. Apparently that’s not true. Their bones are most certainly radioactive, as Ra has a half-live of 1,600 years, but the six or so feet of earth above them keeps the radiation contained (as far as we know, anyway). https://roadtrippers.com/magazine/radium-girls-graves/