Radioactive iodine 131 gas at a concentration double that set by the Science and Technology Agency was found to be escaping from exhaust ducts at the JCO Co. Uranium reconversion plant in Tokaimura, Ibaraki Prefecture, it was learned Monday. The plant was the site of the nation's worst nuclear accident on Sept. 30, when radiation leaked after an accidential criticality occurred during a part of processing. According to JCO officials, the leaked radioactivity was too low to be a health hazard outside the compound, but they have sealed the plant's windows as a precaution.