Worried that the nation's aging nuclear arsenal is increasingly fragile, American scientists have begun designing a new generation of nuclear arms meant to be sturdier and more reliable and to have longer lives, federal officials and private experts say. The officials say the program could help shrink the arsenal and the high cost of its maintenance. But critics say it could needlessly resuscitate the complex of factories and laboratories that make nuclear weapons and could possibly ignite a new arms race. So far, the quiet effort involves only $9 million for warhead designers at the nation's three nuclear weapon laboratories,...