The NOTAM you are thinking of was probably referring to nuclear power
plants.


found it on AOPA's website:

4/0811 ...SPECIAL NOTICE... THIS IS A RESTATEMENT OF A PREVIOUSLY
ISSUED ADVISORY NOTICE. IN THE INTEREST OF NATIONAL SECURITY AND TO
THE EXTENT PRACTICABLE, PILOTS ARE STRONGLY ADVISED TO AVOID THE
AIRSPACE ABOVE, OR IN PROXIMITY TO SUCH SITES AS POWER PLANTS
(NUCLEAR, HYDRO-ELECTRIC, OR COAL), DAMS, REFINERIES, INDUSTRIAL
COMPLEXES, MILITARY FACILITIES AND OTHER SIMILAR FACILITIES. PILOTS
SHOULD NOT CIRCLE AS TO LOITER IN THE VICINITY OVER THESE TYPES OF
FACILITIES. WIE UNTIL UFN