The tsunami-flooded reactors overheated and released radioactive materials.
Residents were evacuated from areas with > 20 mSv/y exposure.
This limiting doze (20 mSv/y) is well below what thousands of people have been exposed to every year through their lifespan.
To highlight the “over-safety” I mention that this limit is some ten times less than the recommended limit set by IAEA

A UN panel of expert scientists concluded that radiation caused no attributable health effects and likely none in the future.
Radiation killed no one.
But the evacuation stress did kill over a thousand.
Most refugees could have safely returned home.

Not only that: Children from areas that were not evacuated got their childhood broken because of an agitated fear.
See Mor er der radioaktivitet i maden?
In my opinion, a heavy burden of guilt rests on the shoulders of Greenpeace and other scare-mongores.

Therapeutic doses are high.
A rotating X-ray beam focused on cancer tissue delivers up to 80,000 mSv.
To minimize the risk of causing cancer in nearby tissue, radiologists divide the radiation dose into fractions, administered daily rather than all at once, giving healthy tissue time to recover.
If the Linier No Treshold hypoteses was valid, this fractionated radiation therapy wouldn’t work.