DUE TO THE SERIOUS THREAT OF NUCLEAR BLACKMAIL BY INTERNATIONAL TERRORISTS, THE U.S. DEFENSE DEPARTMENT IS TAKING STEPS TO STRENGTHEN SECURITY FOR OVER 1,500 NUCLEAR WEAPONS-STORAGE SITES MAINTAINED AROUND THE WORLD. ESPECIALLY VULNERABLE, EITHER TO A TERRORIST GRAB OR AN ENEMY GROUND ATTACK, ARE THE MORE THAN 100 SITES IN EUROPE. IN FISCAL 1978, THE DEFENSE DEPARTMENT IS PLANNING TO ADD 20,000 MORE TROOPS TO IMPROVE THE SECURITY OF NUCLEAR WEAPONS-STORAGE SITES, AND ABOUT $400 MILLION IS BEING SPENT ON ANTI-INTRUSION SENSORS, WARNING SYSTEMS, AND HARDENING OF THE SITES. THE NUCLEAR SITES ARE PROTECTED BY STANDARD 9-GAUGE, 2-INCH-SQUARE CHAIN-LINK FENCES WITH UNDERGROUND BRACES AND A BARBED-WIRE Y AT THE TOP. CUTTING THE WIRE OR DIGGING UNDER IT SETS OFF AN ALARM. THE DOUBLE-PERIMETER FENCE IS MONITORED IN SOME PLACES BY GUARDS IN A TOWER AND BY A CONTROL CENTER THAT WATCHES THE DETECTION SYSTEMS. THESE PATROLS ARE AUGMENTED BY QUICK-RESPONSE FORCES WHO ARE CAREFULLY SCREENED AND TRAINED MILITARY POLICE OR SECURITY TROOPS. THESE READY TROOPS CAN BE AUGMENTED, IF NECESSARY, BY COMBAT FORCES FROM NEARBY MILITARY POSTS. SHOULD A TERRORIST GROUP CAPTURE A BOMB, SAFETY DEVICES AND COMBINATION LINKAGES ARE SUCH THAT THE LIKELIHOOD OF DETONATION IS SLIM. IN THE EVENT THAT A NUCLEAR WEAPONS SITE IS CAPTURED, MEANS FOR DESTROYING THE SITE HAVE BEEN PLANNED. IT IS BELIEVED THAT WITH THE IMPROVEMENTS NOW UNDERWAY, BREAK-INS WOULD BE VIRTUALLY IMPOSSIBLE. (RCB)