WASHINGTON, July 3 — A factory that makes uranium fuel for nuclear reactors had a spill so bad that it kept the plant closed for seven months last year and became one of only three incidents in all of 2006 serious enough for the Nuclear Regulatory Commission to include in an annual report to Congress. After an investigation, the commission changed the terms of the factory’s license and said that the public had 20 days to request a hearing on the changes.

But no member of the public ever did. In fact, no member of the public could find out about the changes. The document describing them, including the notice of hearing rights for anyone who felt adversely affected, was stamped “official use only,” meaning that it was not publicly accessible. “Official use only” is a category below “Secret” and, while documents in that category are not technically classified, they are kept from the public.

The agency would not even have told Congress which factory was involved were it not for the efforts of one of the five commissioners, Gregory B. Jaczko, who named the company, Nuclear Fuel Services, of Erwin, Tenn., in a memo that became part of the public record. His memo said that other public documents would allow an informed person to deduce that the factory belonged to Nuclear Fuel Services.Such secrecy by the Nuclear Regulatory Commission is now coming under attack by influential members of Congress who complain that the agency is withholding numerous documents about the country’s nuclear facilities in the name of national security, but that many withheld documents are not sensitive. The lawmakers say the agency must move to balance its penchant for secrecy with the public’s right to participate in the licensing process and its right to know about potential hazards.

Additional details of the 2006 incident are coming to light now because of a letter sent on Tuesday to the nuclear agency by the House Energy and Commerce Committee. The committee chairman, Representative John D. Dingell, and the chairman of the oversight subcommittee, Representative Bart Stupak, both Democrats from Michigan, complained that the N.R.C. “went far beyond” the need to protect sensitive security information by keeping documents about Nuclear Fuel Services, a private company, from the public. The agency, the congressmen complained, “has removed hundreds of otherwise innocuous documents relating to the N.F.S. plant from public view.”