A forthcoming study of government radiation experiments on hundreds of Americans in the 1950s has found no evidence of significant physical harm. The surprising findings by a panel of experts convened by the Administration are to be released on Sept. 22. Despite a lack of proven injuries, the U.S. will still honor Energy Secretary Hazel O'Leary's December, 1993, vow to compensate the human guinea pigs--many of them unwitting victims--for unjust treatment. The remaining question: How much to pay? Even generous awards may not quell lawsuits by subjects.