China's main centers for designing, making and storing nuclear arms lie in the shattered earthquake zone, leading Western experts to look for signs of any damage that might allow radioactivity to escape. A senior federal official, who spoke on the condition of anonymity because of the delicacy of the issue, said the United States was using spy satellites and other means to try to monitor the sprawling nuclear plants. "There appear to be no immediate concerns," the official said. Nonetheless, "it's potentially a serious issue," Hans Kristensen, a nuclear arms expert at the Federation of American Scientists, a private group in Washington, said in an interview. "Radioactive materials could be released if there's damage." China began building the plants in the 1960s, calculating that their remote locations would make them less vulnerable to enemy attack. China's main complex for making nuclear warhead fuel, codenamed Plant 821, is beside a river in a hilly, forested part of the earthquake zone. It is some 15 miles northwest of Guangyuan in Sichuan Province. The vast site holds China's largest production reactor and factories that mine its spent fuel for plutonium