By WILLIAM J. BROAD An American expert on China's nuclear arms establishment plans to sue the United States government today to end its 18-month delay on publication of his 500-page memoir, which the government says contains federal secrets. The case is attracting attention because the expert, Danny B. Stillman, concludes that China made its nuclear breakthroughs on its own, contrary to accusations by the United States that China used stolen American secrets to make its advances. Mr. Stillman contends that the government may be blocking publication of the book, "Inside China's Nuclear Weapons Program," for reasons of politics rather than national security. Mr. Stillman once directed intelligence at the Los Alamos nuclear laboratory in New Mexico and worked there for 28 years before retiring in 1993. He made nine trips to China between 1990 and 1999 as a federal analyst and as a private citizen. He met top officials, toured nuclear institutes, and compiled a detailed history of China's program. His book includes information on its 46 nuclear tests and more than 2,000 Chinese scientists who have had major involvement in the program. In January 2000, Mr. Stillman gave the government his manuscript, which he was required to do as a condition of his having obtained a secret clearance, and it has languished at federal agencies ever since. Yesterday, Mr. Stillman's lawyer said he would file a suit in federal district court in Washington today, charging that the government is wrongfully blocking the book's publication and Mr. Stillman's freedom of speech. The suit names the Department of Defense, the Defense Intelligence Agency, the Department of Energy and the Central Intelligence Agency as the parties involved in blocking the book. "Danny's got a white hat on," said Mark S. Zaid, his lawyer. "He's not threatening to go public with classified information. He's willing to work with them, whatever the concerns. But from Day 1, the government has refused to allow the manuscript to be published." Glenn E. Flood, a spokesman for the Defense Department, said it was still reviewing the manuscript to see if it contained secrets. "We haven't reached a decision," he said. "There's a lot of staff work involved in this." He said that any lawsuit would be handled by the Justice Department and that he had no idea when the manuscript review might be completed. Among other things, Mr. Stillman said in an interview, the book describes a tirade delivered to American visitors in May 1999 by Hu Side, a weapons designer and former chief scientist director of China's bomb- makers. "He chastised us for calling them spies," Mr. Stillman said. Dr. Hu described how Chinese arms builders had been hampered not by a shortage of insights that necessitated espionage but by a lack of powerful computers for design computations. After acquiring the computers, Mr. Stillman said, China was able to make a breakthrough in miniaturizing their hydrogen bombs. It occurred on Sept. 25, 1992, in a nuclear test in China's western desert. Dr. Hu said the Chinese had come up with the breakthrough idea on their own in the 1970's. But it was only in that 1992 test, Mr. Stillman said, recalling Dr. Hu's claims, "that it went out and worked perfectly." Experts say the book, if it sheds light on that explosive test, could be central to resolving a major controversy over whether China made its scientific advances as a result of stealing nuclear secrets from the United States. Mr. Stillman said his long investigations had led him to concur with the Chinese claim. "I think they did it on their own," he said of China's scientific advances. "I don't think it was espionage." Monitored by the United States, the 1992 Chinese advance led American intelligence officials to charge that China had stolen secrets for the W-88, America's most advanced nuclear warhead, which Los Alamos had designed. By 1995, suspicions of espionage centered on Wen Ho Lee, a Taiwan-born scientist at Los Alamos who had a history of contact with Chinese scientists. After a limited investigation, Dr. Lee was jailed in December 1999, shortly before Mr. Stillman submitted his manuscript for review. But prosecutors never proved the charge. They discovered that Dr. Lee had downloaded much weapons information, but he was freed in September 2000 after pleading guilty to one felony count of mishandling secrets. Dr. Lee's backers say racial bias lay behind the case. Weapons experts say Mr. Stillman's book promises to give the most authoritative view of China's side of the W-88 story. Copyright 2001 The New York Times Company