A model of the atomic bomb dropped on Hiroshima (search) that had been on display at a museum for more than a decade has been removed and replaced for security reasons.

Los Alamos National Laboratory (search) officials in charge of safeguarding national security now view the technology that ended the war against Japan 60 years ago as a potential target for theft.

"The basic issue was that changes in the world created changes in classification that we had to respond to," said John Rhoades, director of the Bradbury Science Museum (search).

Rhoades said he was not able to comment on what aspects of the "Little Boy" bomb model may be classified. "Little Boy" was dropped on Hiroshima on Aug. 6, 1945. "Fat Man" was dropped on Nagasaki three days later.

A new 1,100-pound replica of the Hiroshima bomb was put in place Monday. Rhoades said the new model is more historically accurate but also satisfies security concerns.

The museum also will replace its replica of "Fat Man."