This informative article by NPR explains why the Fukushima incident is not nearly as severe as the Chernobyl accident of 1986, despite being reclassified as Level 7 on the International Nuclear and Radiological Event Scale. Level 7 is the highest possible value of the scale.
The term “terabecquerel” in the article is a measure of activity, a measure of the rate of radioactive decay of a substance. The metric prefix “tera” means one trillion and “becquerel” means one radioactive decay per second. One gram of iodine-131 results in approximately 4,600 TBq, where one gram of cesium-137 is about 3.215 TBq.
Note that this change has been made in retrospect, due to a re-examination of the events that occurred in mid-March, not due to any more recent developments.
