All meetings of the Board shall be open to public observation and shall be preceded by reasonable public notice. Notice in the Federal Register shall be deemed to be reasonable public notice for purposes of the preceding sentence. In exceptional circumstances, the Board may close those portions of a meeting, upon a majority vote of its members present and with the vote taken in public session, which are likely to disclose information likely to affect adversely any ongoing peace proceeding or activity or to disclose information or matters exempted from public disclosure pursuant to subsection (c) of
section 552b of title 5.