A worker exposed to excessive radiation in Japan's worst nuclear accident died this morning, officials said. Masato Shinohara, 40, was the second plant worker to die since the Sept. 30 accident at a uranium processing facility in Tokaimura, about 80 miles northeast of Tokyo. Hisashi Ouchi, 35, died Dec. 21 after having been exposed to a massive amount of radiation. Ouchi was Japan's first nuclear industry casualty. A third worker, Yutaka Yokokawa, also was hospitalized for exposure to radiation but was discharged in December. The accident occurred when the three used too much uranium to make fuel, setting off an uncontrolled reaction. More than 400 residents are believed to have been exposed.