Radioactivity found in Japan seawater 22 Mar 2011 High levels of radioactive substances have been detected in seawater near a quake-crippled nuclear power plant in Japan, its operator says. The radioactive substances have been detected in seawater sampled Monday 100 meters south of the Fukushima No.1 plant. A Tokyo Electric Power Co. (TEPCO) official said the level of iodine-131 (a major radioactive hazard present in nuclear fission products) was 127 times higher and caesium-134 (extracts from waste produced by nuclear reactors) was 25 times higher than government-set standards.