According to a guideline issued by the International Atomic Energy Agency (IAEA), an accident is classified as INES 7 if there has been a release of more than 50,000 terabecquerels of radioactivity. This value was exceeded at Fukushima. The becquerel (Bq) is the unit used to measure the activity of a radioactive material. 1 Bq is equal to one radioactive decay per second.