Amarillo, Texas --

The last of the nation's biggest nuclear bombs, a Cold War relic 600 times more powerful than the atomic bomb dropped on Hiroshima, was dismantled Tuesday in what one energy official called a milestone in President Obama's mission to eliminate nuclear weapons.

Workers in Texas separated the roughly 300 pounds of high explosives inside from the nuclear material, uranium, known as the pit. The work was done outside of public view for security reasons, but explosives from a bomb taken apart earlier were detonated as officials and reporters watched from less than a mile away.

Put into service in 1962, when Cold War tensions peaked during the Cuban Missile Crisis, the B53 weighed 10,000 pounds and was the size of a minivan. Many of the bombs were disassembled in the 1980s, but a significant number remained in the U.S. arsenal until they were retired from the stockpile in 1997.

The B53's disassembly ends the era of big megaton bombs, said Hans Kristensen, a spokesman for the Federation of American Scientists. The biggest nuclear bomb in the nation's arsenal now is the 1.2-megaton B83, he said. The B53 was 9 megatons.