The documents have been withdrawn from public access, and the department is planning to assess any damage to national security that may have resulted from their release.
They included information ranging from nuclear weapons design to Navy nuclear propulsion system information to decades-old information on the locations of nuclear weapons storage depots.
The documents were discovered during an ongoing department review of 2 million pages of publicly available records kept by the National Archives and Records Administration.
Congress was notified of the discoveries in a mandated, classified November 2001 report. The Energy Department released an excised version of that report Tuesday.
The identified documents, dating from the early 1950s to the mid-1970s, contained "restricted data" and "formerly restricted data." The Energy Department report said the State and Defense departments inadvertently declassified the material from 1995 to 1998, making it available to the public through the Archives.
"The improper marking of the documents for classification level, classification category and/or downgrading may have contributed to the inadvertent release of the documents," the report said.
Steven Aftergood, a Federation of American Scientists intelligence analyst, however, criticized the Energy review for impeding public access to declassified information that may not deserve to be removed.
Aftergood noted the Energy report said the removed material "may" contain classified nuclear information and observed it revealed decades-old information on locations of nuclear weapons storage depots. Such information, he wrote, might be politically sensitive, but "while formally classified, do[es] not pose a proliferation hazard."
"The fact that we may have had nuclear weapons in Japan or Greece or elsewhere 30 to 50 years ago is hardly a threat to national security today," Aftergood said in an interview.
"The reason this is a difficult policy issue is not all of that information is of that benign character," he said, noting nuclear weapons design information has been found in this and previous reviews that warrants removal from public access.
The congressionally mandated reports were required after it was discovered in the 1990s that a large number of nuclear weapons documents were either unmarked or mislabeled and as a result were declassified and made available through the public archives, Aftergood said.
More selective targeting of information, enabling much less money and time to be spent on combing through documents, would be a better policy, he said.
According to the unclassified report, the retracted documents contained information on such subjects as the mass or dimensions of fissile materials, pits or nuclear assembly systems; high explosives for nuclear weapons; boosting systems; other nuclear weapon design, function, uses or configurations, or nuclear test information; and Navy nuclear propulsion systems.
Such information concerns the early generations of nuclear weapons the country developed in the 1950s and may be of use abroad, the report said.
"Potential adversaries, emerging proliferant nations and terrorist groups aggressively target U.S. nuclear weapon information. Information regarding older nuclear weapons is of significant value since it is often technically less sophisticated. These designs would be most readily used by a would-be nuclear proliferant to obtain its first nuclear weapon," according to the report.
Khidhir Hamza, former director of Iraq's nuclear weapons program, has said copies of declassified U.S. Manhattan Project reports provided an early blueprint for Iraq's nuclear weapons program.
The formerly restricted documents also contained information on nuclear weapons stockpile quantities, storage locations, costs, yields and delivery system accuracy, the report said.
That information "could assist potential adversaries in assessing the strengths of the U.S. nuclear arsenal" and some of its release could violate international agreements and harm diplomatic ties with foreign host countries, the report said.
The information on Navy nuclear propulsion, the report said, could help other countries develop better propulsion systems for their surface ships and submarines.
The British Defense Ministry earlier this week recalled publicly available documents with detailed information on constructing the first British nuclear bomb, British officials said.