@CSmith327 while incredibly inhuman and cruel, you can indeed keep people with high levels of radiation damage alive for enough time to study them. She still had worth to be exploited. Offing her so quickly was simply too wasteful.
https://en.wikipedia.org/wiki/Tokaimura_nuclear_accident (TLDR...
They should have kept the girl alive. Radiation poisoning is prejudicial as fuck. For the girl to be only hallucinating she must have had some very good genes. What a waste.