Defense Secretary Richard B. Cheney has decided that the Air Force's arsenal of short-range, nuclear-tipped missiles should be permanently removed from strategic aircraft on war alert to avoid risk of accidents that could scatter toxic plutonium dust, a senior U.S. defense official said yesterday.

Cheney ordered last June that the missiles be temporarily removed from B-1, B-52 and FB-111 bombers and stored while the Air Force and Energy Department completed a special study of associated safety risks. His action followed statements by senior U.S. weapons scientists that accidental aircraft fires could detonate conventional explosives surrounding the weapons' fissionable cores. The resulting blast would spread cancer-causing plutonium over a wide area.

A senior U.S. defense official said Cheney determined this week that the scientists' concerns were substantiated by the safety studies, which other officials said had been completed and submitted to the secretary's office three months ago. His order will affect hundreds of missiles deployed at 13 bases in nine states.

Bombers on alert are thought to be more susceptible to accidental fires than other bombers because they are loaded, fueled and routinely stationed at the end of a runway for quick takeoff in the event of a crisis. Nuclear bombs, which reach their targets by the force of gravity, will remain deployed on U.S. alert bombers, U.S. officials said.

A replacement for the hazardous missiles is under development.