Generalities:

The damage that radiation can cause to an organism depends on the amount of radiation emitted by the source, the distance and the time of exposure. When the radiation is very intense, it ionizes the atoms or molecules with which it comes into contact, removing electrons, which is why it is called ionizing radiation.
In the case of leaks in nuclear reactors, it only takes a few minutes for organisms to be damaged. Symptoms of radiation poisoning usually appear almost immediately, but can come on over time, or come and go. Symptoms include nausea and vomiting, diarrhea, headache, dizziness, fatigue, bleeding, hair loss and swelling, skin irritation and/or burns. Very large doses of radiation can cause death.
It is also called acute radiation sickness, radiation sickness, acute radiation syndrome, and radiation sickness syndrome.