Just a week after Japan's nuclear regulator declared the leak of radioactive water at the Fukushima Daiichi nuclear plant big enough for a Level 3 incident rating, the regulator's head is wondering publicly if it really deserved that rating after all.

Based on the International Nuclear and Radiological Event Scale, a Level 3 rating is considered a "serious incident." For the sake of comparison, the 2005 leak of 83,000 liters of radioactive waste at the Sellafield nuclear reprocessing site in the U.K. was also given...