Radioactivity rises in sea off Japan nuclear plant 16 Apr 2011 Levels of radioactivity have risen sharply in seawater near a tsunami-crippled nuclear plant in northern Japan, signaling the possibility of new leaks at the facility, the government said Saturday. The government said Saturday that radioactivity in the seawater has risen again in recent days. The level of radioactive iodine-131 spiked to 6,500 times the legal limit, according to samples taken Friday, up from 1,100 times the limit in samples taken the day before. Levels of cesium-134 and cesium-137 rose nearly fourfold.