I'm not a nuclear scientist, but I can't think of any way pure water may become "toxic" due to exposure to radiation (instead of dissolving radioactive material

I'm not a nuclear scientist, but I can't think of any way pure water may become "toxic" due to exposure to radiation (instead of dissolving radioactive materials, which would definitely be a problem). Maybe excessive neutron radiation could build up tritium, but that already sounds like a stretch. (Most radioactive elements don't emit neutron spontaneously.)