Kyshtym disaster

Some 5000 people in the first 72 hours were irradiated with doses ranging from 460 to nearly 1000 mSv.

An excerpt from the article 42 facts about Kyshtym disaster

At the same time as external radiation, the process of irradiation continued continuously from within as well. Isotopes of cesium and strontium were deposited in the soft tissues and skeletal system, causing numerous survivors to develop malignant tumors over the following years.