The Energy Department for the first time is denying the Defense Nuclear Facilities Safety Board access to some information on operations of nuclear weapons work.

The U.S. Department of Energy is denying access to some information that the nation’s only independent safety agency for work on nuclear weapons says it needs to oversee operations at sites where the atomic arsenal is built and maintained.

The denials of access to records and meetings are “the most significant” in the history of the Defense Nuclear Facilities Safety Board, according to a letter the board sent Congress in July.